Control of Tribolium confusum J. Du Val by diatomaceous earth (Protect-IT™) on stored groundnut ( Arachis hypogaea ) and Aspergillus flavus link spore dispersal

Abstract

Environmental and human health problems associated with the use of synthetic pesticides have prompted the demand for non-polluting, biologically specific insecticides. The current study assessed the losses caused by Tribolium confusum and its control by diatomaceous earth and the effect on Aspergillus flavus spore dispersal during storage of groundnuts. When losses due to Tribolium confusum were assessed over a period of 60 days, it was found that an increase in insect population in stored groundnuts resulted in increased weight loss of stored groundnuts. The true weight loss due to insect feeding was 0.60 g per 400 g of stored groundnuts. When diatomaceous earth (DE) was applied to groundnuts followed by the introduction of insects in a compartment (A), increased mortality of insects with increased diatomaceous earth concentration was observed. For a concentration range of 0-2.5 g DE/kg groundnut, 2.5 g/kg treatment was the most effective (only 5 surviving T. confusum adults out of 50 were recovered in samples treated with 2.5 g/kg compared to 38 adults in the control samples). Larval emergence from groundnuts treated with DE also declined with increased diatomaceous earth concentration. When groundnuts were inoculated with A. flavus spores, followed by DE application and T. confusum introduction into compartment A, the transfer of spores between inoculated groundnut samples in compartment A and uninoculated samples in compartment B was reduced. The mean A. flavus spore concentration recovered from initially sterile compartment B was1.08x103; while it was 45 in the control and in samples treated with 2.5 g/kg dosage respectively. There was a significant difference in the mean numbers of spores recovered from groundnuts in different compartments (A, B) (H = 13.99, df = 4 and P = 0.007). Thus, from this study, losses due to T. confusum on groundnuts and fungal spore transfer in storage by this insect can be minimized by the application of diatomaceous earth (Protect-IT™) to stored groundnuts.

Full text

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CONTROL OF TRIBOLIUM CONFUSUM J . DU VAL BY DIATOMACEOUS
EARTH (PROTECT-ITTM) ON STORED GROUNDNUT (ARACHIS
HYPOGAEA) AND ASPERGILLUS FLAVUS LINK SPORE DISPERSAL

Mohale S1, Allotey J *1 and BA Siame1



J oseph Allotey























*Corresponding author email: alloteyj@mopipi.ub.bw

1University of Botswana, Department of Biological Sciences, Pr. Bag UB00704
Gaborone, Botswana.

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ABSTRACT

Environmental and human health problems associated with the use of synthetic
pesticides have prompted the demand for non-polluting, biologically specific
insecticides. The current study assessed the losses caused by Tribolium confusum and
its control by diatomaceous earth and the effect on Aspergillus flavus spore dispersal
during storage of groundnuts. When losses due to Tribolium confusum were assessed
over a period of 60 days, it was found that an increase in insect population in stored
groundnuts resulted in increased weight loss of stored groundnuts. The true weight
loss due to insect feeding was 0.60 g per 400 g of stored groundnuts. When
diatomaceous earth (DE) was applied to groundnuts followed by the introduction of
insects in a compartment (A), increased mortality of insects with increased
diatomaceous earth concentration was observed. For a concentration range of 0-2.5 g
DE/kg groundnut, 2.5 g/kg treatment was the most effective (only 5 surviving T.
confusum adults out of 50 were recovered in samples treated with 2.5 g/kg compared
to 38 adults in the control samples). Larval emergence from groundnuts treated with
DE also declined with increased diatomaceous earth concentration. When groundnuts
were inoculated with A. flavus spores, followed by DE application and T. confusum
introduction into compartment A, the transfer of spores between inoculated groundnut
samples in compartment A and uninoculated samples in compartment B was reduced.
The mean A. flavus spore concentration recovered from initially sterile compartment
B was1.08x103; while it was 45 in the control and in samples treated with 2.5 g/kg
dosage respectively. There was a significant difference in the mean numbers of spores
recovered from groundnuts in different compartments (A, B) (H = 13.99, df = 4 and P
= 0.007). Thus, from this study, losses due to T. confusum on groundnuts and fungal
spore transfer in storage by this insect can be minimized by the application of
diatomaceous earth (Protect-It) to stored groundnuts.

K ey words: Groundnut, Tribolium confusum, Diatomaceous earth


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INTRODUCTION

Storage of agricultural produce is part of the post-harvest system through which food
material passes on its way from field to consumer. It is generally accepted that 5-15%
of the total weight of all cereals, oilseeds, and pulses is lost after harvest [1]. There is
a continuous need to protect the stored products against deterioration, especially loss
of quality and weight during storage. Quantitative and qualitative losses of stored
grains may result from the feeding and waste production by insects, mites, rodents
and birds or from the growth of microorganisms all of which are influenced by
environmental conditions. Insects, mites and fungi may cause hydrolysis and
oxidation and decrease the level of certain nutrients in stored products or even form
toxic substances such as mycotoxins. Mould contamination in stored groundnuts has
been found to be closely associated with insect infestations [2, 3, 4, 5]. Controlling
insects during storage may also contribute towards reducing spread of fungal spores
within the storage system. The mean % incidence of Aspergillus flavus in insect
damaged samples of wheat was much higher (87%) than in insect-free samples (25%)
[6]. In damaged wheat kernels, A. flavus growth was observed inside the holes caused
by the insects [6]. This report is in agreement with the findings on the migration of the
mite Tyrophagus putrescentiae Schrank and its ability to disperse the toxigenic
fungus A. flavus from artificially contaminated lots of maize grains [5]. Fungal growth
was more marked in compartments containing grains inoculated with both mites and
fungi than in compartments containing grains inoculated with fungi only [5]. This is
indicative of the fact that mites constantly move within the storage site, and hence
improving the dispersal of fungi and other microorganisms that are carried around on
their body surfaces or delivered on their faeces.

When investigating the impact of grain pests on seed deterioration and aflatoxins
production, fungal growth was observed on legs, antennae, at the joints and even
over the whole body surface of insects [4]. In the same study, a decrease in insects,
Tribolium castaneum Herbst and Sitophilus oryzae L., in A. flavus infected maize was
reported [4]. The decline was attributed to the ability of A. flavus to modify a
favourable insect growth environment into an unfavourable one. Various storage
fungi, including Aspergilli, have been found to be valuable dietary supplements to
insects found in storage [7, 8]. The insect mortalities may be due to aflatoxins in the
contaminated grains [4].

On the contrary insects on mouldy grain may be able to metabolize toxic fungal
metabolites [9]. For example, the fungus feeding Carpohilus hemipterus L. is
unaffected by 25 ppm of aflatoxin B1 in the diet and is ten times more efficient in
metabolizing the Fusarium-produced 4-monoacetxyscirpenol than non-fungus-
feeding caterpillars. Insect damaged seeds tend to have higher levels of aflatoxins
than undamaged seeds. The level of aflatoxin B1 on insect-damaged wheat samples
varied from 113 to 1248 µg/kg while the two insect-free samples contained 85 and
113 µg/kg [6].


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The use of pesticides is one of the ways of controlling storage insect pests. However,
the choice of pesticides for storage pest control is very limited because of the strict
requirements imposed for the safe use of synthetic insecticides on or near food. The
continuous use of chemical pesticides for control of stored-grain pests has resulted in
serious problems such as insecticide resistance [10]. Chemicals used for the control of
stored product pests, or as protectants, need also to be compared with the suitability
and effectiveness of alternative methods of control. Non-chemical methods are
attractive since they neither leave chemical residues in the commodity nor do they
cause resistance in insects. In recent years, awareness of the consequences of
environmental pollution, the increasing cost of storage insecticides and the growing
problem of insect resistance, has led pest management specialists to reappraise inert
dusts. Inert dusts include silica aerogels, kaolin, sand and diatomaceous earths
(diatomite).

Diatomaceous earths (DE) are the fossilized remains of diatoms, composed mainly of
amorphous hydrated silica, but also other minerals including aluminium, iron oxide,
magnesium, sodium and lime. Source of these dusts are either of marine or fresh-
water origin; the former are said to be more effective [11, 12, 13]. Diatomaceous
earths of marine origin are effective against storage insects at about 0.1% (w/w), but
commercially available products are often enhanced by other compounds, e.g.
ammonium fluorosilicate. Products of fresh-water origin require at least twice the
dosage to produce similar levels of insect control [11].

Unlike conventional contact insecticides, DEs like other inert dusts function through
their physical properties and are, therefore, generally slower acting [11]. Insect
mortality is induced primarily as a result of desiccation. Water loss is a consequence
of the destruction of the wax cuticle [12, 13]. Although DEs, having small dense
particles of silicon dioxide, were said to abrade the cuticle, they also function by
adsorbing the wax [14]. The action of DEs is not dependent on metabolic pathways
and therefore, it has been postulated that insects will not be selected genetically by the
action of these dusts, leading to physiological resistance. Nevertheless, it may be
possible for insects to develop a behavioural response to the dust and avoid contact
[11].

Another advantage of DEs over conventional insecticides is their low mammalian
toxicity. In the USA, diatomaceous earths are ‘Generally Recognised as Safe’ by the
US Food and Drug Administration and are registered for use as food additives [14].

Studies on the effectiveness of diatomaceous earths have been reported. Insecto, a
diatomaceous earth formulation containing 86.7% armophous silica (silicon dioxide),
3% crystalline silica; and 10% food grade additives was shown to effect total
mortality of 1st instars of T. castaneum when applied to maize, Zea mays L. at 0.5 and
1 g/kg rates. At the same rates of Insecto, mortality of Oryzaephilus surinamensis L.
and Plodia interpunctella Hübner 1st instars was 96-97 and 86-97% respectively [15].
Complete suppression in emergence of T. castaneum and P. interpunctella adults at 1
g/kg Insecto application rate and 98% emergence of O. surinamensis have been

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reported [15]. The efficacy of Protect-It (Mississauga, Ontario, Canada) formulation
of diatomaceous earth (DE) to control internal infestations of rice weevil and maize
weevil has been reported [13]. The mortality of S. oryzae and Sitophilus zeamais
Motschulsky emerging from kernels in wheat treated with DE was always greater than
controls, and ranged from 56 to 90% at 220C and was > 90% at 27 and 320C [13].
Diatomaceous earth product Fossil Shield® was shown to be lethal to adult Tenebrio
molitor L. and Tribolium confusum J. du Val [16]. Fossil Shield® was lethal to 1st
instar larvae of P. interpunctella, but not lethal to older larval stages, while two-week
old larvae of T. confusum were more sensitive to diatomaceous earth than P.
interpunctella at the same age [13]. Contact with diatomaceous earth caused adult
Sitophilus granarius L. to lose weight and reduced their water content, suggesting
disruption of ‘the water barrier’ [16]. Increased mortality of T. castaneum and T.
confusum with increased temperature and exposure interval to diatomaceous earth
(Protect-It) has been reported [12]. On- farm trials in Buhera and Binga districts in
Zimbabwe demonstrated that maize can effectively be protected from insect damage
by admixture with either Protect-It or Dryacide at 1 g/kg of grain [17]. The
application diatomaceous earth (Protect-It) to commodities already infested with
internal feeders, such as S. oryzae and S. zeamais, could help eliminate or suppress the
infestation [13].

Work on the use of diatomaceous earth to control stored product insect pests and
hence suppress fungal spore dispersal is non-existent in Botswana. This fact forms the
basis of the research reported here.

MATERIALS AND METHODS

Groundnut samples
Fifty kilograms of shelled, sound, and mature kernels of groundnuts, variety Nyanda
were purchased from Botswana Agricultural Marketing Board and used in this study.

Rearing of Insects
One hundred (100) of Tribolium confusum adult insects selected previously as the
most predominant species in groundnuts were introduced into 6 aquarium boxes (30 x
20 x 20 cm) containing 800g of standard medium (maize/sorghum/glycerine, 8:8:1,
w/w/w) [18], for oviposition. The rearing boxes were covered with muslin cloth for
aeration. The cultures of T. confusum boxes were kept at room temperature range
17.5–26.5oC and 45–72% relative humidity. The ambient readings were taken using a
thermohygrograph (Isuzu, Seisakusho Co. Ltd, Japan) in the Insectary of the
Department Biological Sciences of the University of Botswana. After four weeks, the
introduced adults T. confusum were removed using an aspirator to allow for the
development of only the newly hatched larvae. Newly emerged adults (one month
old) were used in subsequent experiments.

Experimental set-up
Plastic lunch boxes (220 x 110 x 65 mm) with two compartments and a single cover
for both compartments were used in the study. The covers were perforated using a

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