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Survival and mobility of American cockroaches, Periplaneta americana L, that originated from a wild population were compared in semi-natural and laboratory cool-temperature conditions. In a non-heated building all American cockroaches died when air temperatures were ≤0°C despite having access to wood mulch substrate that remained above freezing. Under constant temperatures of 8, 9, and 10°C, approximately 40% of cockroaches died within 72 hours. Mobility was defined as the ability of a cockroach to right itself when flipped over. At all tested temperatures the percentage of individuals that became immobile increased with time. Survival and mobility increased with temperature. Data showed that adult American cockroaches were not able to survive several days at ≤10°C, suggesting a potential cultural control method for tropical cockroaches. © 2018 Southwestern Entomological Society. All rights reserved.
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American Cockroach Response to Cold
Author(s): David L. Bradt III, W. Wyatt Hoback and B. M. Kard
Source: Southwestern Entomologist, 43(2):335-342.
Published By: Society of Southwestern Entomologists
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American Cockroach1 Response to Cold Temperatures
David L. Bradt III2, W. Wyatt Hoback3*, and B. M. Kard3
Abstract. Survival and mobility of American cockroaches, Periplaneta americana L,
that originated from a wild population were compared in semi-natural and laboratory
cool-temperature conditions. In a non-heated building all American cockroaches
died when air temperatures were 0°C despite having access to wood mulch
substrate that remained above freezing. Under constant temperatures of 8, 9, and
10°C, approximately 40% of cockroaches died within 72 hours. Mobility was
defined as the ability of a cockroach to right itself when flipped over. At all tested
temperatures the percentage of individuals that became immobile increased with
time. Survival and mobility increased with temperature. Data showed that adult
American cockroaches were not able to survive several days at 10°C, suggesting
a potential cultural control method for tropical cockroaches.
The American cockroach, Periplaneta americana L, is a common pest of
structures and an exotic species associated with humans throughout the world
(Sinclair et al. 2015). Native to Africa it was introduced into North America in the
1600s and was given its common name when described by Linnaeus (1758) using
his binomial system. It can cause asthma in children in urban areas (Oudin et al.
2016) and is a vector of food-borne bacterial diseases (Pai et al. 2005). It is
commonly found outdoors in the southern United States and is rarely observed
away from heated dwellings in more temperate zones, including Oklahoma. This
may be because climate-controlled human dwellings provide micro-climates similar
to outdoor temperatures in the tropics (Cornwell 1968).
In temperate zones, insects overwinter by two strategies: freeze avoidance
by producing compounds allowing supercooling, and freeze tolerance by controlling
ice formation in their bodies (Sinclair et al. 2003). Using a third strategy, other
insects survive by seeking favorable environments including human dwellings
where they may remain active, or diapause to overwinter. Many pest cockroaches,
including the American cockroach, seem to use the third strategy, especially in
temperate areas although data generally are lacking on temperature thresholds for
survival (Hamman and Turney 1987).
The American cockroach has been studied as a model organism for insect
physiology (Bell 1981) and to document its life history and methods for chemical
control (Cochran 1999). Several authors anecdotally report that the American
1Blattodea: Blattidae
2255th Medical Detachment (Preventive Medicine), Joint Base Lewis-McChord, WA 98433
3Department of Entomology and Plant Pathology, 127 Noble Research Center, Oklahoma State
University, Stillwater, OK 74078-3033
*Corresponding author:
cockroach dies when exposed to cold temperature. Cold receptors on its antennae
have been identified (Loftus 1966).
When unable to avoid cold, insects become immobile at a species-specific
temperature threshold referred to as the 'chill-coma temperature' (Mellanby 1939).
American cockroaches quickly become immobile when exposed to freezing
temperatures. However, Staszak and Mutchmor (1973) reported recovery from
cold-induced coma after 1 hour at sub-zero temperatures. Recently, a closely
related species, Periplaneta japonica Karny, was found in New York. This species
survives temperatures below freezing for several days (Tanaka and Tanaka 1997).
In addition, studies have shown that some insects need to be acclimated to survive
freezing temperatures, and many species with access to areas sheltered from cold
survive previously reported lethal temperatures (Danks 2012). These reports
suggest the need for more research to determine lethal temperatures and recovery
from exposure to sub-lethal cold by the American cockroach.
In contrast with the American cockroach, some native USA cockroaches are
not pests of human dwellings. There are 12 endemic species in the genus
Parcoblatta (Eliyahu et al. 2011). These cockroaches can be abundant, accounting
for the greatest amount of arthropod biomass in some pine forests in the
southeastern United States (Horn and Hanula 2008). Little is known of the effects
of temperature on their biology, although Blatchley (1920) reported that Parcoblatta
seemed not affected by cold and had the same level of activity on a cold January
day as on a hot June day. Beyond this anecdotal evidence all life stages are found
in early spring, suggesting the ability to survive freezing environmental
temperatures (Horn and Hanula 2002).
Cold tolerance, survival, and response to decreasing temperature were
evaluated for the American cockroach in a laboratory. Two native cockroaches,
Parcoblatta fulvescens Saussure and Zehntner and P. virginica von Wattenwyl were
also tested for response to cold conditions.
Materials and Methods
American cockroaches were obtained from Kansas State University,
Manhattan. Using a vacant greenhouse with open windows and doors, survival of
the American cockroach was tested in a semi-sheltered environment. Adult
cockroaches were placed individually into plastic 1,000-ml containers filled with
damp wood mulch for shelter, given a moist cotton ball for water, and dry dog food.
In total, 85 adult cockroaches were placed in the greenhouse. Temperatures were
recorded with Hobo® data loggers (Bourne, MA) and cockroaches evaluated after
10 days. Check cockroaches were kept at approximately 20°C and 50% relative
humidity in a controlled-environment, indoor insect-rearing facility.
Preliminary tests were done from -4 to 27°C to determine American
cockroach survival across a range of temperatures. Groups of cockroaches were
exposed to constant temperature for 12 hours and mobility and mortality were
recorded. Almost all cockroaches died at 5°C, whereas 8 and 10°C allowed
survival but induced loss of mobility. Therefore, behavior was assayed at these
Three growth chambers set at 8, 9, or 10°C were used. Cockroaches (N =
12) were placed individually with a piece of moist paper towel and dry dog food into
glass containers. Cockroach groups (N = 12) were evaluated at 24, 48, and 72
hours, and behaviors quantified. After 72 hours in growth chambers, all individuals
were moved to room temperature to determine survival. Check cockroaches were
kept at 20°C.
For comparison, native wood cockroaches, P. virginica and P. fulvescens,
were tested for survival at 4°C and for mobility at 8, 9, and 10°C. Check American
cockroaches were kept at 20°C.
In greenhouse conditions with open windows and doors, American
cockroaches were evaluated after 10 days. During this period 100% of the
greenhouse cockroaches died, while all protected check cockroaches kept indoors
at ~23°C survived. Air temperatures in the open greenhouse between 20 November
and 6 December 2014 are shown in Fig. 1. Temperatures ranged from -4 to 41°C.
Fig. 1. Air temperatures in a non-heated open greenhouse during 2014.
Under growth chamber conditions, 10 to 40% of American cockroaches died
within 72 hours. No significant differences in cockroach survival were observed
among the three temperatures tested although 40% died after 3 days at 8°C (Fig. 2).
All check and comparative wood cockroaches survived.
Cockroaches were characterized as being either normal and upright, on their
backs (inverted) but responsive, or inverted and dead. Exposure to cold for 24 and
48 hours caused most American cockroaches to lose their ability to maintain upright
posture (Fig. 3a). An increasing percentage became inverted over time (Fig. 3b).
Cockroaches that lost the ability to maintain upright posture or were inverted did not
recover when monitored for 24 hours after exposure.
Fig. 2. Mean percent mortality (±1 SE) of American cockroaches during three
exposure periods at each of three temperatures.
P. virginica and P. fulvescens were tested in the same growth chamber
conditions as American cockroaches. After 2 weeks they did not appear to be
affected by cold temperatures as they maintained normal activities including walking,
response to stimuli, feeding, and antennal cleaning similar to those observed at
room temperature.
Data show that American cockroaches struggled to survive at moderately
cool temperatures (10°C) and suggest this cockroach depends on heated buildings
in much of its introduced range across the United States. These findings suggest
using cold temperature as a management strategy. Uninhabited buildings could be
allowed to reach cold temperatures during winter months as a potential method to
reduce populations. Future studies should determine the appropriate temperatures
and length of time needed to facilitate cockroach mortality.
Immobility in response to cold also has been observed for insects such as
Drosophila. Before chill-coma occurs, Drosophila spp. lose their ability to right
themselves (Block 1990; David et al. 1998). Similarly, American cockroaches lost
Time (hours)
% Dead
mobility and rolled onto their backs when exposed to chill-coma temperatures as
warm as 9.3°C (Anderson and Mutchmor 1968). The criterion for chill-coma is
measured by whether or not an insect responds to being prodded. Chill-coma is an
established method to immobilize, easily manipulate, and transport cockroaches
(Cornwell 1976) and is reversible if the insects do not suffer chilling-injury (Gibert
and Huey 2001). Previous studies show that American cockroaches can recover
after being maintained in a cold-induced coma for 1 hour (Staszak and Mutchmor
This study assessed only survival of adult American cockroaches.
Refrigerated American cockroach oöthecae take longer to hatch than those at room
temperature (Tee and Lee 2013). This suggests that egg cases of American
cockroaches could survive mild winter temperatures. However, additional study is
required to determine the length of time and effects of cold temperatures.
Periplaneta japonica survived freezing temperatures for several days
including walking unharmed on top of snow (Tanaka and Tanaka 1997). Future
studies should compare its temperature tolerance with P. americana. It could also
be determined if Japanese cockroaches seek shelter in buildings or if they survive
in more natural, exposed environments.
Fig. 3a. Mean (±1 SE) number of American cockroaches mobile and responsive
when exposed to different temperatures for different times. *All immobile but not all
dead (Immobile cockroaches did not recover within 24 hours).
Fig. 3a. Mean (±1 SE) number of American cockroaches mobile and responsive
when exposed to different temperatures for different times. *All immobile but not all
dead (Immobile cockroaches did not recover within 24 hours).
Time (hours)
% Mobile
* * *
Fig. 3b. Mean (±1 SE) number of American cockroaches inverted but not dead
when exposed to different temperatures for different times. *All inverted but not all
dead. Inverted cockroaches did not recover within 24 hours.
To apply the strategy of cold temperatures to control American cockroaches,
additional studies in non-inhabited buildings that evaluate thermal refuges would be
helpful. A similar study on Gromphadorhina portensa (Schaum), another tropical
cockroach, found that in response to cold stress it increased the amount of protein
in its fat body, an example of "cold hardening" that improved survival (Chowanski et
al. 2015). Considering results of the open building experiment and growth chamber
studies, it may be possible to induce cold-hardening in American cockroaches,
perhaps by using shorter exposure periods than those tested here. However,
permanent chill-coma injury was 100% after 72 hours at 8 to 10°C. Future studies
could evaluate inducement of cold hardiness in American cockroaches.
Thanks to John and Heidi Niblack and the Niblack Scholars Program for
funding, and to Andrine Shufran and Kiffnie Holt for providing cockroaches.
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... According to the current literature, chill-comas are completely reversible and organisms subjected to a chill-coma can fully recover, granted any chill-related injuries are not sustained. Significant studies that tested cold exposure and chill-comas include Staszak and Mutchmor (1973) and Bradt et al. (2018), both of which considered the effects of multiple low temperatures and increasing exposure time to these temperatures on the American cockroach ( Periplaneta americana ), a known tropical species. In these studies, it was discovered that chill-coma recovery in cockroaches takes longer in colder temperatures, and that percent mortality increased at low temperatures over increasing exposure intervals (Staszak andMutchmore 1973, Bradt et al. 2018). ...
... Additionally, we predicted that B. dubia will display 100% mortality after 48 hours of freezing. Our predictions were generated from Bradt et al. (2018), who found that 40% mortality occurred at an exposure time of 72 hours in 8˚C conditions. Given that we are using a lower temperature (0˚C), we predicted that full mortality should occur at a sooner time of exposure. ...
... Blaptica dubia cockroaches displayed increased levels of chill tolerance. When compared to the american cockroach, Periplaneta americana, B. dubia has the ability to resume normal physiological function after prolonged exposure to 0℃ while P. americana was found to have near complete mortality when exposed to temperatures lower than 5℃ for 12 hours (Bradt et al. 2018). This indicates there is some adaptation to withstand chill temperatures in B. dubia , the mechanism of this adaptation was not discovered in this study, but would be a worthy avenue of study going forward. ...
Full-text available
Experiment on the ability to tolerate chill coma in the tropical cockroach species Blaptica dubia, and the ability to recover from exposure to freezing temperatures.
... Compared to heat stress, cellular damage was more extensive after cold shock. According to Bradt et al. (2018) adult American cockroaches cannot survive several days at ≤ 10 0 c which confirms the chilling effect on the insect [23]. ...
... Compared to heat stress, cellular damage was more extensive after cold shock. According to Bradt et al. (2018) adult American cockroaches cannot survive several days at ≤ 10 0 c which confirms the chilling effect on the insect [23]. ...
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American cockroaches (Periplaneta americana) are considered as potential vectors and possible reservoirs of pathogenic organisms. They respond to any infection by mounting immune responses mediated by morphologically and functionally distinct haemocytes. To understand the effect of abiotic stress like heat, cold and biotic stress induced by infection on American cockroahes, microscopic study of haemocytes was carried out from adult P. americana (Linnaeus) (Insecta: Blattodea: Blattidae). In the present study, morphological alterations of haemocytes were observed at high temperature (45°C for 12 hours), low temperature (4°C for 12 hours) and following inoculation with E. coli bacteria compared to the control haemocytes. Stress induced necrotic changes of haemocytes like loss of cellular integrity, vacuolization, cell swelling, blebbing, margination of chromatin, membrane rupture were recorded in all the three types of stress conditions. The most affected haemocytes were Plasmatocytes, Granulocytes and Prohaemocytes. Unusual nuclear separation in a cap like bleb formation and margination were observed at low temperature. Cellular aggregation and phagocytic extensions of Granulocytes and Plasmatocytes were visible after induced infection. This is the first study where the changes in the primitive immune cells of American cockroaches have been reported in response to abiotic and biotic stress. Hemocyte structural modifications and changes observed in the present study have potential implications in the study of stress response in P. americana in the extreme environment and in destruction of pathogens. Cellular aggregation and phagocytosis are characteristics defence reactions which are not found in thermal stressed cells. Although vacuolization, blebbing and chromatin margination were common alterations in response to all types of stresses applied in this study.
... There were a positive correlation between daily temperature and hunting in outdoor gatherings [26] . In unheated structures, all American cockroaches died in air temperatures of 0.0 °C despite reaching sawdust, and 40% of cockroaches died within 72 hours at 10 °C [27] . ...
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Article History: "Blattella germanica" was the most prevalence species captured (84.4%), followed by the American cockroach "Periplaneta americana (14.6%)", and the brown cockroach "Periplaneta brunnea (0.99%)". The German and American cockroaches were found in all areas of the hospitals, while the brown cockroaches were only caught in the kitchen, outpatient clinics, and medical supply storages of the hospitals. The population density of cockroaches in the hospitals recorded a higher number of cockroaches/10 m 2 (94.7±11.1) and the infestation rate (30.9%) in kitchens compared to other places in 2014; while the highest number of cockroaches/10 m 2 (21.6±0.2) and the infestation rate (43.0%) were recorded in outpatient clinics compared to other places in 2019. The results also showed a very significant correlation between the cockroach population density and the temperature (R = 0.897, P = 0.006); but it was inversely correlated with relative humidity (R = 0.433, P = 0.332) in 2014; while in 2019 a non-significant positive correlation was observed between population density and both the temperature and the relative humidity. It was clear that the cockroach population density was reduced in 2019 compared to 2014 (F = 54.17, P < 0.01), possibly due to application of various mechanisms to control cockroaches besides raising hygiene level in the hospitals.
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Objectives To describe the home environment in terms of housing conditions and their association with child health in a disadvantaged immigrant population. Design A cross-sectional observational study. Setting Enrolment took place during 2010–2011 in Rosengård, Malmö, Sweden. Participants Children aged 0–13 years in 2 study neighbourhoods were recruited from local health records and from schools. 359 children participated, with a participation rate of 40%. Data on health, lifestyle and apartment characteristics from questionnaire-led interviews with the mothers of the children were obtained together with data from home inspections carried out by trained health communicators. Outcome measures Logistic regression analysis was used to estimate ORs for various health outcomes, adjusted for demographic information and lifestyle factors. Results The housing conditions were very poor, especially in one of the study neighbourhoods where 67% of the apartments had been sanitised of cockroaches, 27% were infested with cockroaches and 40% had a visible mould. The association between housing conditions and health was mostly inconclusive, but there were statistically significant associations between current asthma and dampness (OR=4.1, 95% CI 1.7 to 9.9), between asthma medication and dampness (OR=2.8, 95% CI 1.2 to 6.4), and between mould and headache (OR=4.2, 95% CI 1.2 to 14.8). The presence of cockroaches was associated with emergency care visits, with colds, with headache and with difficulty falling asleep, and worse general health was associated with mould and presence of cockroaches. Conclusions The associations between dampness and asthma, and the association between mould and headache, are in line with current knowledge. The presence of cockroaches seemed to be associated with various outcomes, including those related to mental well-being, which is less described in the literature. The results of the present study are hypothesis generating and provide strong incentives for future studies in this study population.
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Wood cockroaches (Blattaria: Blattellidae) are important prey of the red-cockaded woodpecker, Picoides borealis Wilson (Piciformes: Picidae), an endangered species inhabiting pine (Pinus spp.) forests in the southern United States. These woodpeckers forage on the boles of live pine trees, but their prey consists of a high proportion of wood cockroaches, Parcoblatta spp., that are more commonly associated with dead plant material. Consequently, we sampled large woody debris, logs and standing dead trees (snags), in a South Carolina pine forest to determine densities of wood cockroaches in these habitats. Nearly 80% of the 662 wood cockroaches we collected from woody debris were found in snags. However, when we estimated the number of wood cockroaches per hectare, we found that the two habitats contained approximately equal numbers because logs are more abundant than snags. The broad wood cockroach, Parcoblatta lata Brunner, was the most common cockroach on live pine boles constituting 46% of the wood cockroaches. Males were present from late April to late July in field studies suggesting that P. lata has only one generation per year, which is consistent with laboratory studies in which males lived an average of 91.3 d. Female P. lata lived almost twice as long (158.2 d) and produced an average of 12.6 oothecae/female (SE = 3.4) or ≈517 offspring/female. Although P. lata were common on boles of live trees, our results show that snags and logs also are important habitats of these wood cockroaches in pine forests.
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The minimum time of heating (48, 50, and 53 °C) required to kill Periplaneta americana (L.) (Dictyoptera: Blattidae) oothecae aged 1, 2, 3 and 4 weeks was determined and their suitability as hosts after different storage conditions (live, heat killed and cold stored for 0.5, 1, 2, 3, and 4 months at 4 °C) for rearing parasitoids Aprostocetus hagenowii (Ratzeburg) and Evania appendigaster (L.) (Hymenoptera: Eulophidae; Evaniidae) was investigated. Heating at 48 °C for 45 min killed 100% of the oothecae of all ages. Both parasitoids had low emergence in live 4-week-old oothecae but emergence improved after oothecae were heat killed. For A. hagenowii, an interaction effect between ootheca age and storage duration was found for percentage emergence. Emergence rates were reduced in cold-stored 3- and 4-week-old oothecae (1–4 and 3–4 months, respectively) but not in 1–2-week-old oothecae. The number of progeny produced in 1–2-week-old and 3–4-week-old oothecae decreased significantly after cold storage for 3–4 and 0.5–4 months, respectively. In all conditions, the development time was 34.5–39.9 d and the progeny sex ratio was female-biased (88.2–94.3%). E. appendigaster had low parasitism and emergence rates (0–20%) in cold-stored oothecae and took longer to develop in heat-killed and cold-stored oothecae than in live oothecae. In all conditions, the proportion of females was 0.619–0.821. These results illustrate the effects of ootheca age on host treatment and storage and can be used to develop efficient ways to use oothecae for rearing oothecal parasitoids.
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Arthropods, as poikilotherms, adapt to cold environments in a variety of ways that include extension of locomotory activity to low temperatures, enhancement of metabolic rate and maintenance of a positive energy balance whenever possible. The ecological implications for many such animals are extension of the life cycle and a requirement for an individual to overwinter several times. Prolonged sub-zero temperatures increase the risk of tissue freezing, and two main strategies have been evolved, first avoidance of freezing by supercooling, and secondly, tolerance of extracellular ice. In the first strategy, freezing is invariably lethal and extensive supercooling (to -30 ^circC and below) occurs through elimination or masking of potential ice nucleators in the body and accumulation of cryoprotective substances such as polyhydric alcohols and sugars. Such species are termed freezing intolerant. The second strategy, freezing tolerance, is uncommon in arthropods and other invertebrates, and usually occurs in a single life stage of a species. Freezing of liquid in the extracellular compartment is promoted by proteinaceous ice nucleators. Freezing is therefore protective, and the lethal temperature is well below the supercooling point in freezing tolerant individuals, whereas in most freezing intolerant species it is close to or at the supercooling point. Proteins also act as antifreezes in insects of both strategies, producing a thermal hysteresis by lowering the freezing point of haemolymph in a non-colligative fashion while not affecting the melting point temperature. Recent studies and developments in arthropod cold tolerance are discussed against this background, and a broader approach than hitherto is advocated, which integrates ecological information with physiological data.
Cold tolerance of overwintering nymphs of a cockroach, Periplaneta japonica, was examined in terms of the supercooling capacity and lower lethal temperature. The supercooling point of overwintering nymphs fell in a relatively narrow range of temperature from -6 to -9°C and no correlation was observed between the supercooling point and body size. In the temperature range from -5 to -8°C, a significant proportion of cockroaches could tolerate a 12 hr period of tissue freezing. The freeze tolerance capacity differed between nymphal instars, but the supercooling capacity was similar for all nymphs. In a freezing trial at -6 and -7°C, none of the first instar nymphs recovered after tissue freezing, whereas many mid (from 3rd to 5th) and final (8th) instar nymphs survived freezing. Glucose, myo-inositol, scyllo-inositol and trehalose were found in overwintering nymphs, but neither the array nor the content except for trehalose differed among the nymphal instars. Unexpectedly, the concentration of trehalose was negatively correlated to freeze tolerance. Winter survival of this cockroach may be based on both the freeze tolerance and microhabitat selection.
Insect performance is limited by the temperature of the environment, and in temperate, polar, and alpine regions, the majority of insects must face the challenge of exposure to low temperatures. The physiological response to cold exposure shapes the ability of insects to survive and thrive in these environments, and can be measured, without great technical difficulty, for both basic and applied research. For example, understanding insect cold tolerance allows us to predict the establishment and spread of insect pests and biological control agents. Additionally, the discipline provides the tools for drawing physiological comparisons among groups in wider studies that may not be focused primarily on the ability of insects to survive the cold. Thus, the study of insect cold tolerance is of a broad interest, and several reviews have addressed the theories and advances in the field. Here, however, we aim to clarify and provide rationale for common practices used to study cold tolerance, as a guide for newcomers to the field, students, and those wishing to incorporate cold tolerance into a broader study. We cover the 'tried and true' measures of insect cold tolerance, the equipment necessary for these measurement, and summarize the ecological and biological significance of each. Finally, we suggest a framework and workflow for measuring cold tolerance and low temperature performance in insects.
Three species of cockroaches, Periplaneta americana, Leucophaea maderae and Blaberus craniifer, were acclimated at different temperatures. P. americana and L. maderae were acclimated at 10 or 12, 20, and 30°C whereas B. craniifer was acclimated at 12 and 30°C. Action potential extinction temperatures were determined as the intact nerve cord and isolated metathoracic ganglia were cooled. These values were compared to the chill-coma temperatures of the acclimated animals. It was found that all three species acclimate to temperature as evidenced by their different chill-coma temperatures. It has also been shown that both the intact nerve cord and isolated ganglion have electrical activity below the chill-coma temperature. This indicates that a failure of the nerve cord is not a direct cause of chill-coma in the three species. Possible causes for the onset of chill-coma are discussed.
A review of insect adaptations for resistance to cold and for life-cycle timing reveals the complexity of the adaptations and their relationships to features of the environment. Cold hardiness is a complex and dynamic state that differs widely among species. Surviving cold depends on habitat choice, relationships with ice and water, and synthesis of a variety of cryoprotectant molecules. Many aspects are time-dependent and are integrated with other factors such as taxonomic affinity, resource availability, natural enemies, and diapause. Timing adaptations reflect the fact that all environments change over many different time frames, from days to thousands of years. Environments differ in severity and in the extent, nature, variability, and predictability of change, as well as in how reliably cues indicate probable conditions in the future. These differences are reflected by a wide range of insect life-cycle systems, life-cycle delays, levels of responsiveness to various environmental signals, genetic systems, and circadian responses. In particular, the degree of environmental change, its predictability on different time frames, and whether it can be monitored effectively dictate the balance between fixed and flexible timing responses. These same environmental features have to be characterized to understand cold hardiness, but this has not yet been done. Therefore, the following key questions must be answered in order to put cold hardiness into the necessary ecological context: How much do conditions change? How consistent is the change? How reliable are environmental signals?
Drosophila melanogaster adults, grown at 21°C, were distributed in groups of 50 after a light anaesthesia. Culture vials with flies were later submitted to a cold treatment at 0°C. All adults entered a chill coma; the recovery time was measured at ambient temperature.2. Recovery time was strongly influenced by recovery temperature, with shorter values between 20-25°C.3. Recovery time increased almost linearly with duration of cold treatment.4. Recovery time was consistently larger for males than for females. It was highly variable among groups and increased with flies’ age.5. Variability among flies of the same group was always very high, with CVs often over 25%.6. Chill coma and its recovery seem to imply a modification of the nervous system, analogous in several aspects to what is observed with usual anaesthetics such as CO2.