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x + 112 pp. Although Rodolia and other ladybirds have been successfully used to control pest coccids they have not proved effective in classical biological control programmes against aphids. A better understanding of the foraging behaviour of ladybirds and a more realistic theory of insect predator-prey dynamics are beginning to reveal the reason for this. Aphidophagous ladybirds exploit patches of aphid prey for feeding and reproduction. As suitable nurseries for their offspring patches of aphid prey generally only persist for about the same period of time as it takes the larvae of these ladybirds to complete their development. This is the case even in the absence of natural enemies. Thus aphids become scarce within a patch just when the food requirements of the ladybirds are greatest. Optimal foraging theory predicts that ladybirds should lay a few eggs early in the development of a patch and empirical data indicates that ladybirds appear to forage optimally. There have been several studies on the cues used by ladybirds when selecting patches of prey for oviposition. This review will consider how the responses shown by ladybirds may have shaped what has become known as the "egg window", how cannibalism may regulate the number of ladybirds within a patch, and the consequences of this for classical biological control.
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Biology, Ecology and Behaviour of Aphidophagous Insects
DIXON, A.F.G. & J.-L. HEMPTINNE 2003. Ladybirds and the biological control of
aphid populations. Pp. 1-10 in A.O. SOARES, M.A. VENTURA, V. GARCIA & J.-L.
HEMPTINNE (Eds) 2003. Proceedings of the 8th International Symposium on
Ecology of Aphidophaga: Biology, Ecology and Behaviour of Aphidophagous
Insects. Arquipélago. Life and Marine Sciences. Supplement 5: x + 112 pp.
Although Rodolia and other ladybirds have been successfully used to control pest coccids
they have not proved effective in classical biological control programmes against aphids. A
better understanding of the foraging behaviour of ladybirds and a more realistic theory of
insect predator- prey dynamics are beginning to reveal the reason for this.
Aphidophagous ladybirds exploit patches of aphid prey for feeding and reproduction. As
suitable nurseries for their offspring patches of aphid prey generally only persist for about
the same period of time as it takes the larvae of these ladybirds to complete their
development. This is the case even in the absence of natural enemies. Thus aphids become
scarce within a patch just when the food requirements of the ladybirds are greatest. Optimal
foraging theory predicts that ladybirds should lay a few eggs early in the development of a
patch and empirical data indicates that ladybirds appear to forage optimally.
There have been several studies on the cues used by ladybirds when selecting patches of
prey for oviposition. This review will consider how the responses shown by ladybirds may
have shaped what has become known as the "egg window", how cannibalism may regulate
the number of ladybirds within a patch, and the consequences of this for classical biological
Anthony F.G. Dixon (e-mail:, School of Biological Sciences,
University of East Anglia, Norwich, NR4 7TJ, UK; Jean-Louis Hemptinne, Ecole Nationale
de Formation Agronomique, B.P. 87, FR-31326 Castanet-Tolosan, France.
In classical insect predator-prey population
dynamics organisms in two trophic levels
interact; prey and predator (Fig. 1A).
A plant through it's morphology and
chemistry can directly affect the well being of
herbivores, and they similarly can affect
predators. That is, in addition to their effects on
one another's abundance a plant can have a direct
effect on a herbivore, which can have a direct
effect on a predator, and vice versa. In addition to
these direct effects there is a growing literature
that claims predators and parasitoids are attracted
by volatiles emitted by plants under attack by
herbivores. This is regarded as a mutualism, in
which the effectiveness of the searching
behaviour of the natural enemy is enhanced and
the herbivore pressure on the plant reduced
RICE et al. 1980; Fig 1B). Predators are
considered to be part of a plant's defence. When
attacked by herbivores some plants emit volatiles
that are attractive to natural enemies, which has
resulted in them being likened to "body guards"
and the use of emotive phraseology like " the
enemy of my enemy is my ally" (D
ABELIS 1988; SABELIS et al. 2001). That
ladybirds respond to these volatiles is supported
by technically elegant studies in which gas
chromatography of plant volatiles was directly
coupled with recordings from the olfactory organs
of a ladybird. Herbivore damaged plants emit (Z)-
jasmone, which is attractive to adult Coccinella
septempunctata (B
IRKETT et al. 2000; NINKOVIC
et al. 2001). The central tenet of the mutualism
hypothesis is that herbivore-induced plant
volatiles enable natural enemies to more easily
find their prey and so reduce herbivore pressure.
8th International Symposium on Ecology of Aphidophaga
University of the Azores, Ponta Delgada, 1-6 September 2002
Claims that such signals are so used by
parasitoids was scrutinized by VINSON (1999) and
found no field evidence for this.
Fig. 1. The direct and indirect effects on one another
of plants, herbivores and predators in classical insect
population dynamics (A) and plant predator
mutualisms (B).
Although there is no doubting that the
volatiles (synomones - DICKE & SABELIS 1988)
released by plants when attacked by herbivores
are attractive to predators and parasitoids, the way
in which they affect their searching behavior and
the distance over which they operate still needs to
be resolved. Discussions of this problem (e.g.
JANSSEN et al. 2002) tend to follow PRICE et al.
(1980) and only consider the adaptive
significance of herbivore-induced plant volatiles
in terms of plant fitness. It is generally assumed it
is advantageous for natural enemies to respond to
such signals. However, it is pertinent to ask -
What advantages would a predator gain by
responding to these signals? Here we consider
only ladybird beetles, but the principles are likely
to apply to all natural enemies.
It seems likely that the quantity of volatile
material released by a plant depends on the
intensity of herbivore attack, i.e., density-
dependent. If this is true then aphid-infested
plants are likely to be at their most attractive for
ladybirds when aphids are most abundant.
However, at this stage in the infestation it is
highly likely that ladybird larvae will already be
present. Therefore, responding to a strong cue
that a plant is under attack by aphids is not
necessarily advantageous. In addition, as not all
aphids are equally suitable as prey for ladybirds
ANA et al. 2002) it is relevant to ask: - Is the
synomone emanating from a plant specific for a
particular species of aphid or a general response
to aphid infestation? Similarly, is the synomone
produced by a plant in response to being eaten by
lepidopterous larvae different from that produced
when infested with aphids? Therefore, in addition
to determining whether the odour originates
directly from the prey (prey pheromone
hypothesis) or indirectly - after feeding by the
prey - from the plant (plant synomone hypothesis)
there is an urgent need to determine whether the
signals are prey specific and how they affect
predators' searching behaviour. It is well
documented that bark beetles aggregate in
response to volatiles produced by trees and
attractant pheromones produced by the beetles,
and so overcome the host's defences by a mass
attack, but avoid heavily attacked trees, when the
beetles present produce deterrent pheromones
(WOOD 1982; RAFFA 2001). That is, if chemical
signaling by plants significantly influences
ladybird foraging then it is likely the signal is
complex, as in bark beetles.
Alternatively one can ignore plants when
considering predator-prey interactions, which is
the case in most mathematical models of
population dynamics. These have been widely
used to predict the behavior of predator-prey
systems, in particular their stability and the
outcome of introducing natural enemies on the
abundance of pests (B
EDDINGTON et al. 1976,
1978; HASSELL 1978; MURDOCH 1994). In spite
of the great and long-standing interest in these
models, there has been little success in using
them to account for why insect predators,
compared to parasitoids, have generally not been
very effective in suppressing the numbers of their
prey (D
EBACH 1964).
Our studies on the way insect predators, and
ladybirds in particular, forage, led to an
appreciation of the ecological significance of the
difference in mobility of juvenile and adult
insects; the latter can fly while the former cannot
(Fig. 2). That larvae generally stay within a prey
patch while adults may not was incorporated into
a model. Patch in this sense means the space that
a larva can explore by walking, usually one or
only a few adjacent plants, or even only part of an
individual plant as in the case of trees. Three
factors are likely to determine the reproductive
strategy of ladybirds to a much greater extent than
availability of food, which is the usual
Biology, Ecology and Behaviour of Aphidophagous Insects
assumption of models of predator-prey systems:
(1) Ladybird developmental time is much longer
than that of its aphid prey and comparable with
the average duration of a patch of prey (Fig. 3;
1991). Thus it is maladaptive for a ladybird to lay
eggs in an old prey patch, as they are unlikely to
complete their development before the aphids
disappear. (2) As shown by KINDLMANN &
DIXON (1993), there should be a selective
advantage in optimizing the number of eggs laid
in a patch. This is because - as stated above -
ladybird developmental time is similar to the
duration of a patch of aphids. If many eggs are
laid, the ladybird larvae may reduce the rate of
increase of the aphids, cause an earlier decline in
aphid abundance, and thus food may become
scarce well before the larvae complete their
development (Fig. 3). (3) Cannibalism is common
in aphidophagous ladybirds and in nature often
reduces juvenile survival dramatically, as only
about 1% of the eggs laid in a patch survive
IXON 2000). Cannibalism may be selected for
(see below) and even sibling cannibalism may
have a selective advantage, if prey becomes
scarce (OSAWA 1992). To avoid cannibalism,
adults should avoid patches of aphids where
ladybird larvae are already present.
Fig. 2. Aphidophagous ladybirds quickly leave patches
where aphids are scarce (A) but oviposit in patches
where prey is abundant (B). The larvae (D) that hatch
from the eggs (C) are confined to the patch, and have to
pursue and subdue the aphids they need for their
Fig. 3.
Graphical presentation of the components of the ladybird-aphid interaction: temporal changes in the
abundance of aphids and relative developmental time of the ladybird, and the outcome if (A) the eggs are laid late,
(B) a few eggs are laid early, or (C) many eggs are laid early.
8th International Symposium on Ecology of Aphidophaga
University of the Azores, Ponta Delgada, 1-6 September 2002
Assuming that the proportion of conspecifics
in the diet of ladybirds is proportional to their
relative abundance then if prey abundance is kept
constant the incidence of cannibalism increases
with increase in predator abundance. That is,
cannibalism is likely to act as a density dependent
mortality factor. Alternatively if the number of
predators is kept constant and that of their prey is
varied the incidence of cannibalism decreases
with increase in the abundance of prey (Fig. 4).
Fig. 4.
The predicted (A) increase in cannibalism with
increase in predator density, 50 and 150, and (B) the
decrease in cannibalism with increase in aphid density
when predator density is kept constant assuming that:
f(x, y) = ay/(x+y), where x is the number of prey, y is
the number of ladybirds and a is a scaling constant.
This is referred to as the "meet and eat"
hypothesis and accounts for the incidence of
cannibalism in time (D
IXON 2000). However, it is
just as plausible that the latter is due to the
occurrence in time of certain vulnerable stages -
eggs/hatchling larvae and pre-pupae/pupae, which
are unable to avoid or defend themselves against
active larvae. Whatever the reason for the
temporal incidence of cannibalism the outcome is
the same: cannibalism is proportional to the
relative abundance of the predator and therefore
likely to be density dependent.
Consideration of the above leads to the
prediction that there should be a strong selection
for ladybirds to lay eggs only in patches in the
early stages of development and avoid those
containing conspecific larvae (KINDLMANN &
DIXON 1993; DOSTALKOVA et al. 2002). Thus in
assessing the potential effectiveness of a predator
for biological control one should take into
account that selection maximizes predator fitness,
not its effectiveness as a biocontrol agent
(KINDLMANN & DIXON 1999a). In aphidophagous
ladybirds the major determinant of their
reproductive strategy is that their prey develops
much faster then they do (DIXON et al. 1995;
DIXON 1999b). Therefore, the potential fitness of
an adult depends mainly on the future trends in
resource availability for its larvae, which unlike
the adult are confined to a patch (Fig. 2). This
leads to the following predictions. In arthropod
predator-prey systems in which the predator has a
long generation time relative to that of its prey
(ladybird/aphid systems), predator reproduction
should be correlated with the age of a prey patch
rather than the numbers of prey present, and top-
down regulation is unlikely. However, in
ladybird/ coccid systems, where both prey and
predator have similar developmental times,
ladybird reproduction is likely to be correlated
with prey abundance and top-down regulation is
possible (K
addition there is evidence that specificity may
also be an important attribute of a biological
control agent. The coccidophagous ladybirds that
feed on Margarodidae, the group of coccids that
includes Icerya, are generally more specific than
those that feed on other groups of coccids. In
terms of successful control ladybirds have been
used 20 times more successfully to control
Margarodidae than other groups of coccids
IXON 2000).
What evidence is there that selection maximizes
predator fitness? Below is presented the results of
Biology, Ecology and Behaviour of Aphidophagous Insects
In the presence of conspecific larvae and/or
their tracks gravid females of Adalia bipunctata,
Coccinella septempunctata, Cycloneda limbifer,
Harmonia axyridis, and Semiadalia
undecimnotata become very active and if
prevented from leaving the area refrain from
laying eggs for a few hours (H
1992; DOUMBIA et al. 1998; YASUDA et al. 2000;
ŮŽIČKA 2001b). Similar responses are observed
when females of A. bipunctata are placed on
plants in the field experimentally infested with
aphids and contaminated with larval tracks.
(Fréchette, unpublished). Although some species
of ladybird respond to the tracks left by larvae of
other species the response is generally statistically
insignificant and much weaker than that to
conspecific larvae or their tracks (H
al. 1992; Y
ASUDA et al. 2000; RŮŽIČKA 1997b,
2001a, b). This is expected because the greatest
threat to the survival of a ladybird in its preferred
habitat, where it is likely to be the most abundant
ladybird, are individuals of the same species. In
addition, ladybirds appear to be well defended
chemically against intraguild predation
2000). The deterrent effect of larval tracks is
density dependent and mediated via a pheromone
present in the tracks. In the case of A. bipunctata
the cue consists of a cocktail of alkanes, which
spread easily on the hydrophilic cuticle of plants
and so leave a large signal. In addition the
oviposition-deterring pheromone is very stable
lasting for at least 10 days (DOUMBIA et al. 1998;
EMPTINNE et al. 2001).
studies undertaken to assess this in the case of
aphidophagous ladybirds. In particular, this will
be done by examining the evidence for an egg
window, mechanisms for avoiding cannibalism
and the proposed consequences for aphid
Egg Window
Experimental and field studies indicate there is a
density below which ladybirds are unlikely to lay
eggs (D
1978). In addition, in the field ladybirds tend to
lay their eggs well before aphid populations peak
in abundance (Fig. 5; HEMPTINNE et al. 1992).
That is, there is a window in the development of a
patch of aphids when ladybirds are most likely to
lay their eggs. The opening of the window is
possibly determined by the minimum density of
aphids required for the survival of the first instar
larvae (DIXON 1959). The closing of the window
appears to be initiated by adults responding to the
presence of conspecific larvae (HEMPTINNE et al.
In summary, there is good field evidence that
aphidophagous ladybirds, as predicted by theory,
lay their eggs early in the development of patches
of aphids, and laboratory and field experiments
reveal the possible mechanisms by which this is
Fig. 5.
Distribution in time, relative to peak aphid
abundance of the laying of eggs by Adalia bipunctata
on lime trees. Development of aphid populations
expressed in weeks before and after the recorded peak
in aphid abundance in each year. (After H
al. 1992)
Cannibalism is widely recorded for
aphidophagous ladybirds, but rarely mentioned in
the literature on coccidophagous species. Theory
predicts that it should occur when the relative
abundance of ladybirds is high and/or is
8th International Symposium on Ecology of Aphidophaga
University of the Azores, Ponta Delgada, 1-6 September 2002
associated with an asymmetry between cannibal
and victim. The victim is usually at a vulnerable
stage in its development (AGARWALA & DIXON
1992), i.e., in the egg or pupal stage, or is smaller
or about to moult or pupate. That is, cannibalism
should be highest in the egg and pupal stages, and
in the fourth instar larval stage when prey is
likely to be scarce, and decrease with increase in
aphid abundance (Fig. 4). Life table studies done
on field populations and laboratory studies (Fig.
6) support these predictions (A
DIXON 1992; YASUDA & SHINYA 1997).
Fig. 6.
The incidence of cannibalism in the laboratory
of clutches of eggs (A) and larvae (B) of Adalia
bipunctata in relation to aphid abundance (After
In the grain beetle Tribolium there are strains
that show either a high or a low level of
cannibalism, which is genetically determined
(STEVENS 1992). This has also been shown for H.
axyridis (W
AGNER et al. 1999). Thus, selection
should favour an optimum level of cannibalism in
a given environment. That is, a species may be
more or less cannibalistic than one would expect
on the basis of the predicted frequency of
encounters between conspecifics outlined above.
Is there any evidence for this? Clearly some
species are more difficult to rear collectively
because they show higher levels of cannibalism
than other species (unpublished results). A recent
study of cannibalism in the aphidophagous
ladybird H. axyridis indicates it prefers to eat
conspecifics (G
AGNÉ et al. 2002). Thus
cannibalism would appear to have been selected
for in the individuals of H. axyridis used in this
Not only does the high probability of egg
cannibalism make it advantageous for ladybirds
to avoid ovipositing in patches of prey already
occupied by conspecific larvae field, but evidence
indicates that cannibalism, as predicted by theory,
serves subsequently to regulate the numbers of
ladybird larvae within a patch (Fig. 7).
Fig. 7.
The relationship between egg cannibalism and
the number of eggs of Adalia bipunctata per unit area
of lime foliage in relation to aphid abundance in the
field (After M
ILLS 1982)
That is, cannibalism is strongly density
dependent and capable of regulating the
abundance of ladybird larvae within patches
In summary, there is good field evidence that
cannibalism is widespread and an important
mortality factor potentially capable of regulating
the abundance of aphidophagous ladybird larvae
in a patch.
Aphid abundance
The prediction that ladybirds that forage
optimally have little affect on aphid abundance
INDLMANN & DIXON 1993) is the most
contentious. The implied altruism on the part of
the ladybirds and criticism of biological control
practice has greatly impeded the general
acceptance of this supposedly counterintuitive
Biology, Ecology and Behaviour of Aphidophagous Insects
idea. There is good evidence that ladybirds forage
in a way similar to that predicted by optimal
foraging theory and they achieve this by
behaviour that is clearly adaptive at the individual
level. The fact that cannibalism is adaptive and
strongly density dependent indicates that ladybird
numbers are likely to be strongly auto-regulated.
Therefore, the prediction that ladybirds should
have little affect on aphid abundance is in reality
also not counterintuitive.
Unlike in other studies (e.g. ELLIOT &
KIECKHEFER 2000) the shrubs were not caged, so
the patches in effect were open to both
immigration and emigration of both aphids and
ladybirds as in natural ecosystems. That is, as
predicted by theory these predators do not have a
negative effect on the peak numbers of aphids in
In summary, although well based theoretically
and supported by a rigorous field experiment, the
prediction that aphidophagous ladybirds have
little affect on aphid abundance is likely to be
subject to further critical experimentation before
it is generally accepted.
This prediction was tested by monitoring the
numbers of the aphid, Aphis gossypii, on 34 two
metre high shrubs of Hibiscus syriacus in the
field. All the eggs of Coccinella septempunctata
brucki were removed from 8 of the shrubs, all
those of Harmonia axyridis from another 8, all
the eggs of both ladybirds from another 12 and no
eggs were removed from the remaining 6 shrubs
(control). Sticky bands were placed around the
base of the stem of each shrub to prevent the
immigration of larvae on to the shrubs from
surrounding plants. The results were very variable
but clearly indicate that the presence of
aphidophagous predators on the shrubs did not
significantly affect the peak number of aphids
(Fig. 8).
Although the idea of a mutualism between plants
and ladybirds is an attractive one there are no
compelling theoretical reasons for, or field
evidence of, such a relationship. Classical
predator-prey models do not account for why
insect predators are generally less effective in
suppressing the abundance of pests than
parasitoids. A model that includes the essential
features of the foraging behavior of larvae and
adults and the reproductive behavior of adult
ladybirds predicts the patterns observed in the
field. The major determinant of abundance in this
system is the relative developmental times of the
predator and prey - generation time ratio (GTR)
hypothesis. If that of the predator is considerably
longer than that of the prey, as in aphid/ladybird
systems, than top down regulation of prey
abundance is unlikely, whereas when it is of
similar length, as in coccid/ladybird systems, then
top down regulation is possible. The cues used by
aphidophagous ladybirds to assess the quality of
patches of prey have been identified and
rigorously assessed. That is, in the last ten years
there has been a great advance in our
understanding of the patterns and processes in
ladybird-prey interactions.
Fig. 8.
The peak number of Aphis gossypii on Hibiscus
shrubs in the field when aphid numbers were monitored
in the presence of all the naturally occurring natural
enemies (control), and when all the Harmonia axyridis
(H.a) or Coccinella septempuntata brucki (C.s.) or both
species of ladybird (H.a. + C.s.) were removed at the
egg stage.
The GTR model should apply to all insect
predators. However, as far as aphidophaga are
concerned it makes a prediction: those that have
longer generation times than aphids should
behave similarly to ladybirds. Although this has
not been studied intensively many are known to
8th International Symposium on Ecology of Aphidophaga
University of the Azores, Ponta Delgada, 1-6 September 2002
be cannibalistic and show similar reproductive
behaviour. For example, the adults of some
cecidomyids, chrysopids and syrphids are
deterred from ovipositing by the presence of
conspecific larvae or their tracks (H
al. 1993; R
ŮŽIČKA 1994, 1996, 1997a, 1998;
RŮŽIČKA & HAVELKA 1998). Thus, it is likely
that the GTR hypothesis holds for all insect
predators. At present the best support for this
comes from studies on aphidophagous insects.
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Accepted 31 May 2003.
... Coccinellids feed primarily on aphids and only opportunistically upon other prey (Hodek & Honěk, 1996). As a consequence, the release of the larvae usually does not result in the establishment of self-perpetuating populations (Powell & Pell, 2007) since coccinellids lay eggs in patches with aphid colonies at an early stage of development, and their developmental time is similar to the average duration of an aphid colony (Dixon & Hemptinne, 2003). Mirids are broad generalists and polyvoltine species and their density is not only influenced by the density of a specific prey (Harmon & Andow, 2004). ...
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Macrolophus pygmaeusRambur (Hemiptera: Miridae) and Adalia bipunctata(L.) (Coleoptera: Coccinellidae) are two predatory insects commonly used as biological control agents. In order to determine the incidence with which both species attack and eat each other [Intraguild predation (IGP)], the direction and symmetry of the interaction between A. bipunctataand M. pygmaeus were characterized. In addition, whether the intensity of IGP between these two predators increased when the number of extraguild prey Acyrthosiphon pisum(Harris) (Hemiptera: Aphididae) decreased, was also determined. Unidirectional sensu stricto IGP between A. bipunctataand M. pygmaeus was recorded: when IGP occurred, A. bipunctatawas always the IG predator that killed and ate M. pygmaeus, the IG prey. However, the intensity of IGP was a function of the abundance of the extraguild prey, A. pisum, since IGP increased when the number of extraguild prey decreased. These results are discussed in terms of theoretical models that predict stability and the outcome of using natural enemies to control pests.
An operability level coefficient (OLC), defined as the ratio of product permeation and product formation rates, and related to the inverse combination of the Damköhler number and the Peclet number (1/DaPe), is suggested as a useful tool for estimating performances of membrane reactors (MRs) operating as separators in equilibrium-limited reactions. The OLCs for product hydrogen formation in previously reported MRs for methane dry-reforming (MDR), methane steam-reforming (MSR), methanol steam-reforming (MeOHSR), and ethanol steam-reforming (EtOHSR) were correlated with conversion and yield enhancements. For values of OLCs ranging from 0.03 to 0.78, a clear universal trend for increasing conversions and hydrogen yields with increasing OLC was observed for these different types of reforming reactions. The OLC curve calculated from a numerical simulation without adjustable parameters was found to closely approximate experimental data obtained from the MRs, and was shown not to depend on the assumed kinetics. This study confirms that hydrogen selectivity (from the ratio of single-gas permeances) has a substantial influence on conversion and hydrogen yield enhancements in a MR, and demonstrates that a hydrogen selectivity of 100 is sufficient to achieve high performance in a MR.
This contribution presents a theoretical study of a multitubular packed-bed membrane reactor for the ethane to ethylene oxidative dehydrogenation reaction over a highly active and selective Ni–Nb–O mixed oxide catalyst. This theoretical study takes into account the radial composition and temperature profiles using a two-dimensional pseudo-homogeneous model on the reaction side. The feasibility and convenience of using this novel design, as well as the influence of the main operating variables on the reactor performance, are analyzed.The introduction of the membrane leads to lower oxygen partial pressures inside the catalyst tubes, which results in an improved selectivity to ethylene (lower heat generation rates) and high effective heat transfer area per unit volume. The multitubular membrane reactor enables significant ethylene productions per tube and milder temperature profiles than a conventional wall-cooled fixed-bed reactor. Operating conditions have to be carefully adjusted to avoid undesired oxygen accumulation inside the tubes. The presence of small amounts of oxygen at the reactor inlet significantly improves the ethylene production rates.
The aim of this work is to investigate the performance and energy efficiency achieved by an integrated system based on two different ethanol fuel processor configurations: a Conventional Reactor (CR) and a Membrane Reactor (MR). The CR-based configuration system consists of an ethanol reformer followed by two water-gas shift reactors operating at high and low temperatures. The final hydrogen purification is carried out by a preferential oxidizer in order to reduce the CO concentration before feeding the polymer electrolyte membrane fuel cell (PEMFC). A multi-tubular MR process using thin Pd–Ag tubes has also been considered, where the water-gas shift reaction and the hydrogen separation take place simultaneously.The analysis showed that the MR process configuration possesses a simpler system design with a minor advantage in terms of energy efficiency (30%) compared with the conventional system (27%). Moreover, a detailed parametric analysis concerning the effects of water-to-ethanol molar ratio, reaction pressure, reformer and MR temperature, sweep-gas molar ratio and MR configuration on the achieved performance (hydrogen yield) and energy efficiency of the system has also been done.The importance of optimizing integrated systems is shown since the optimal operating conditions from a global efficiency analysis point of view are in general distinct when compared with those obtained when focusing on the reformer reactor or individual process units alone.
This review examines the recent literature on the oxidative dehydrogenation (ODH) of ethane and propane, which aims for the synthesis of the corresponding alkenes. The following aspects are discussed: (a) the main features affecting the catalytic properties of systems based on supported vanadium oxide and molybdenum oxide; (b) the characteristics of catalysts producing outstanding olefin yields; (c) advantages in selectivity gained by means of either special reactor configurations or non-conventional conduction of the reaction; (d) the contribution of homogeneous reactions to the formation of olefins during the oxidation of alkanes.
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The effect of the oviposition deterring pheromone (ODP) in the larval tracks of conspecific and heterospecific ladybirds on oviposition in Harmonia axyridis Pallas was studied in semi natural conditions. Gravid females of H. axyridis were deterred from ovipositing on plants contaminated with conspecific larval tracks, but not on those with heterospecific tracks. H. axyridis females spent significantly less time on plants contaminated with conspecific ODP than on those with heterospecific ODP. This behaviour may account for why fewer eggs were laid on plants contaminated with conspecific ODP.
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Coccinella septempunctata was approximately 20% more reluctant to eat the eggs of Adalia bipunctata than the reverse. In addition, fourth instar larvae of C. septempunctata failed to complete their development on a diet of A. bipunctata eggs and only 30% of those of A. bipunctata completed their development on a diet of C. septempunctata eggs, and the survivors took nearly 2 times as long as those fed aphids. This is an indication that the costs of intraguild predation might outweigh the benefits.
1. A study has been made of the searching behaviour of the predatory coccinellid beetle Adalia decempunctata (L.). 2. Healthy eggs in a batch may hatch successively over a period of several hours. Larvae do not eat their contemporaries. 3. Of the eggs which fail to hatch when the majority of the batch does so, few would normally hatch successfully, and many are eaten by larvae which have already emerged. 4. Searching larvae are negatively geotactic and positively phototactic. 5. When searching a leaf larvae spend most time on the rim and veins. 6. Well-fed larvae tend to search an area more thoroughly than starved individuals. Starved first-instar larvae search an area more thoroughly than starved second-, third- and fourth-instar larvae do. 7. The tendency of starving fourth-instar larvae to revisit the apex of a cylinder, where there are no aphids, decreases as the period of starvation increases. 8. As larvae increase in age their efficiency in capturing the aphid Microlophium evansi (Theobald) increases. Larvae are more efficient in capturing young aphids. Efficiency of adults lies between that of the second- and third-instar larvae. 9. In each successive instar, the time taken to consume aphids of a particular instar decreases. Fourth-instar larvae take twelve times longer to consume an adult aphid than they do a first-instar aphid. 10. Having captured one aphid, a first-instar coccinellid larva's chance of capturing a second aphid is greater than was its chance of capturing the first. 11. The number of aphids required to be provided daily to the larvae, so that 50% survive, increases with each successive instar. 12. With increasing aphid population density, there is an increase in the rate of development and percentage survival of larvae until there are signs of satiety when approximately fifteen third-instar aphids are provided each day. 13. In each successive instar, unfed larvae are capable of covering a greater distance before succumbing to starvation. Larvae which consume very few aphids each day traverse an area greater than that traversed by those which received rather more. 14. For survival of 50% of the individuals, first-instar larvae require a prey population density many times greater than that required by fourth-instar larvae. The actual value is mainly dependent upon the age distribution of the aphid population which is being attacked. 15. Adults are less active and lay more eggs when well fed. 16. In the field, egg batches of A. decempunctata are usually found either on leaves infested with aphids or close to infested ones. 17. Over the range of numbers of aphids provided each day in the laboratory, there is no linear relation between the rate of increase of A. decempunctata and the numbers of aphids provided each day. 18. The searching ability and rate of increase of A. decempunctata are considered in relation to Nicholson and Volterra's assumptions, which appear oversimplified.
From a historical point of view, herbivores have been managed by either increasing a plant's defensive chemicals or through the release or encouragement of biological control agents. More recently, pesticides have become an important management tool. In each case the herbivore is conceptually an enemy that has been managed by manipulation of the plant on one side and by the use of biological control efforts or pesticides on the other, the herbivore being placed between these management strategies. The evolution of the plant and its insect herbivores has been described as an adversarial relationship. The herbivore-parasitoid relationship can be described in the same way. The importance of considering carnivores in understanding the evolution of the plant-herbivore interaction was proposed in the early 1980s. About the same time, a suggestion was made that it is important to consider plants in any effort to understand the herbivore-parasitoid relationship. However, the herbivore has remained in the middle. I suggest here that the specialist plant-herbivore-parasitoid interaction has evolved as a result of a mutualistic tritrophic interaction. As background I present a brief historical look at the management of herbivores followed by an overview of the interactions between the plant and herbivore, the herbivore and parasitoid, and the plant and parasitoid. Next, I consider other interactions between plants and insects where the plants appear to manipulate the insects. Finally, I discuss the mutualistic plant-parasitoid-herbivore interaction as a co-evolved tritrophic system where both the specialist parasitoid and the specialist herbivore have co-evolved together with the plant. The 3 components of the proposed system either interact with each other in a positive way or the potentially negative interactions are neutralized, selection favoring the more successful positive interactions. The proposal is predicated on plants with specialist herbivores and their specialist parasitoids. If this system exists, there is the opportunity for the evolution of subversion, deception, and cheating among the 3 components.
The potential fitness of an adult insect predator depends mainly on the future trends in resource availability throughout the period of development of its larvae, because they, unlike the adult, are confined to a patch. Thus, adult oviposition strategy is likely to be determined by the bottlenecks in resources that occur during the period of its offsprings' development. Therefore, the longer the developmental time of a predator, the smaller the degree of depletion of the patch by that predator species. As a consequence, the relative efficiency of predators as biological control agents is negatively correlated with their generation time relative to that of their prey.
Both theoretical and empirical evidence indicate that in systems where insect predators have longer developmental times than their prey the predators have little impact on the abundance of their prey. In assessing the ‘effectiveness’ of a predator for biological control one should take into account that selection maximizes predator fitness, not its effctiveness as a biocontrol agent. Therefore, predators that have a long developmental time relative to their prey are unlikely to be the best biocontrol agents. If these results can be generalized to other predator–prey systems, then it is clear that an understanding of predator–prey dynamics can only be achieved by studying predators.
The objectives of this study were to determine if coccinellids adjusted their distribution within spring wheat fields in response to spatial variation cereal aphid density in the fields and to describe the patterns of cereal aphid population growth that resulted. Field experiments were completed in which the physical dimensions of patches infested with cereal aphids, cereal aphid density, and access to patches by coccinellids were varied. Aphid infestations consisted of naturally occurring densities (natural patches) and much greater densities created by supplementing patches with aphids (supplemented patches). Coccinellids were denied access to some supplemented patches (exclusion patches) but allowed unlimited access to others. Densities of adult Hippodamia convergens and Coccinella septempunctata were correlated with aphid density in patches whereas density of Coleomegilla maculata was not. Aggregation by coccinellids was independent of patch area. The realized aphid population growth rate (r) was lower in supplemented than natural patches in all four trials but was significantly lower in only one trial. The lower r in supplemented patches was not exclusively caused by coccinellid predation, and emigration of aphids from patches probably also contributed. r was significantly greater in exclusion patches than supplemented and natural patches, indicating that coccinellids markedly reduced aphid numbers in patches even when aphid density was extremely high.
Life tables for two predatory ladybirds,Coccinella septempunctata andHarmonia axyridis, were constructed in two years in which prey abundance differed. The prey aphid,Aphis gossypii, was abundant on its primary host from late May to late June. Females ofH. axyridis andC. septempumctata laid their eggs during the increase and peak in aphid abundance. The oviposition period ofC. septempunctata started before and was shorter than that ofH. axyridis. In both species, intraguild predation and/or cannibalism of the egg and fourth instar stages, but rarely of other developmental stages, were observed. Fourth instar larvae ofH. axyridis had to complete their development when aphids were scarce more frequently than those ofC. septempunctata. The highest frequency of intra- and interspecific predation was of fourth instarH. axyridis larvae. In terms of intraguild predation, the larvae ofH. axyridis preyed on larvae ofC. septempunctata, but the reverse was not observed. Percentage survival from egg to adult inH. axyridis was higher than inC. septempunctata in both years, and least in both species in the year of low aphid abundance. These results suggest that prey abundance influenced the frequency of cannibalism and intraguild predation, which were important in structuring this guild of ladybirds.