Seasonal changes in supercooling points and glycerol content in overwintering larvae of the asiatic rice borer from rice and water-oat plants

Morphological and biochemical responses of a neotropical pest insect to low temperatures

As traditionally cold areas become warmer due to climate change, temperature could no longer be a barrier to the establishment of non-native insects. This is particularly relevant for pest insects from warm and tropical areas, mainly those with some tolerance to moderately low temperatures, which could expand their range into these new locations. From this perspective, in this work we studied the morphological and biochemical responses of the Neotropical pest Paysandisia archon to low temperatures, as part of a possible strategy to colonize new areas. To that end, wild larvae were exposed for 7 days to either low (1 and 5 °C) or ambient (23 °C) temperatures. We then quantified the inner and outer morphological changes, by X-Ray Computer Tomography and Digital Holographic Microscopy, as well as the accumulation of metabolites acting as potential endogenous cryoprotectants, by Spectrophotometry. We found that Paysandisia archon developed a cold-induced response based on different aspects. On the one hand, morphological changes occurred with a significant reduction both in fluids susceptible to freezing and fat body, together with the thickening, hardening and increased roughness of the integument. On the other hand, we found an increase in the hemolymph concentration of cryoprotective substances such as glucose (6-fold) and glycerol (2-fold), while trehalose remained unchanged. Surprisingly, this species did not show any evidence of cold-induced response unless the environmental temperature was remarkably low (1 °C). These results could be useful to improve models predicting the possible spread of such a pest, which should incorporate parameters related to its resistance to low temperatures.

Low temperature shock and chill-coma consequences for the red flour beetle (Tribolium castaneum) and the rice weevil (Sitophilus oryzae)

Insects face several (environmental) abiotic stressors, including low temperature, which cause the failure of neuromuscular function. Such exposure leads insects toa reversible comatose state termed chill-coma, but the consequences of this state for the organism biology were little explored. Here, the consequences of the chill-coma phase were investigated in two of the main stored product pest species - the red flour beetle Tribolium castaneum (larvae and adults) and the rice weevil Sitophilus oryzae (adults). For this purpose, a series of low-temperature shocks were used to estimate the chill-coma recovery time (CCRT), survival, nutrition and weight gain/growth of T. castaneum (larvae and adults) and S. oryzae, as well as the development of T. castaneum life stages. The relatively long CCRT was characteristic of beetle larvae, at different low-temperature shocks, and CCRT increased with decreasing temperatures and increasing exposure intervals for both pest species. The survival was little affected by the low-temperature shocks applied, but such shocks affected insect feeding and growth. Tribolium castaneum larvae was more sensitive than adults of both insect species. Moreover, the relative consumption and weight gain of S. oryzae adults were lower than those of T. castaneum adults and mainly larvae, while feeding deterrence was not affected by low temperature shocks, unlike food conversion efficiency. Low-temperature shocks, even under short duration at some temperatures, significantly delayed development. The lower the temperature and the higher the exposure period, the more delayed the development. Thus, the physiological costs of chill-coma are translated into life-history consequences, with potential implications for the management of this insect pest species in stored products and even more so on red flour beetles and rice weevils.

Comparative transcriptomics of the pheromone glands provides new insights into the differentiation of sex pheromone between two host populations of Chilo suppressalis

Reproductive isolation between different host populations is often based on intraspecific sex pheromone differences. The mechanisms underlying these differences have not been thoroughly elucidated to date. Previous studies suggested that Chilo suppressalis has differentiated into rice and water-oat host populations, and these two populations manifest clear differences in sex pheromone titer and mating rhythm. Hence, this moth is an ideal model to investigate the endogenous mechanisms of intraspecific reproductive isolation. Here, we identified a series of putative genes associated with sex pheromone biosynthesis based on the C. suppressalis pheromone gland transcriptome data. Transcripts of most genes were at higher level in the rice population. Then we obtained 11 pivotal differentially expressed genes (DEGs). The expression levels of these DEGs exhibited a distinct increase in the rice population. Moreover, we also observed the expression rhythm of these DEGs is discrepant between two host populations. Our study offers a new understanding to elucidate the mechanisms of intraspecific reproductive isolation.

Gene flow across host-associated populations of the rice stem borer Chilo suppressalis Walker (Lepidoptera: Crambidae): implications for Bt resistance management in rice

BACKGROUND

The rice stem borer, Chilo suppressalis, is a serious pest of rice, but also damages an aquatic vegetable, water oats (Zizania latifolia Turcz.). The time at which mating occurs is different between populations of rice stem borer associated with rice and those associated with water-oats, which suggests that undetected cryptic species may be associated with these plant hosts. If true, this would have significant management implications. This study is the first empirical test of this idea, using population genetic tools from two sampling cohorts. We genotyped 320 rice stem borer individuals from 2014, collected from rice and water-oats across five locations (where they exist in sympatry), using seven microsatellite loci.

RESULTS

We found no genetic structuring associated with host plant species. On water oats, some rice stem borers were found that had a similar mating time to the rice population, so in 2016, a second cohort of samples was screened by their timing of mating to get 'pure rice feeders' and 'pure water oats feeders'. These samples were genotyped with microsatellites, mitochondrial DNA (mtDNA) (COI and COII), and a nuclear gene (EF1-α). Our mtDNA data suggest a relatively low amount of population subdivision associated with plant host, but the microsatellite data revealed no such genetic structure, and we were only able to identify one haplotype of EF1-α.

CONCLUSIONS

Our results indicate gene flow between rice and water oats populations of rice stem borer, indicating that water oats will likely provide a refuge for resistance management of Bt rice. © 2019 Society of Chemical Industry.

Host population related variations in circadian clock gene sequences and expression patterns in Chilo suppressalis

The rice stem borer, Chilo suppressalis Walker, is one of the most important global agricultural pests. C. suppressalis has distinct rice and water-oat host populations. Asynchrony in sexual activity is thought to be the main factor maintaining reproductive segregation between these populations, particularly the obvious difference in the circadian rhythm of female calling activity between populations. However, the mechanism responsible for this difference in the timing of female calling is poorly understood. The circadian clock is an essential regulator of daily behavioral rhythms in insects, including female calling. We investigated the variation in circadian clock genes of the rice and water-oat populations of C. suppressalis. We did this by comparing deduced amino acid sequences and the expression patterns of seven circadian clock genes (clock, cycle, period, timeless, timeout, cryptochrome1, and cryptochrome2) between females from each population. We found that the two populations had different variants of the timeout and cryptochrome1 genes and differed in the expression of period, timeless and timeout. This suggests that population-related variation in the circadian clock genes period, timeless, timeout and cryptochrome1 could be responsible for the different circadian rhythms of female calling in these host population of C. suppressalis. These results provide new insights into the molecular mechanisms underlying asynchronous sexual activity in insect populations and suggest new topics for future research on the origins and maintenance of population differentiation in insects.

Divergence in larval diapause induction between the rice and water-oat populations of the striped stem borer, Chilo suppressalis (Walker) (Lepidoptera: Crambidae)

Differences in diapause traits can result in the seasonal reproductive isolation of host plant-associated insect populations and thereby facilitate the population divergence. The striped stem borer, Chilo suppressalis, has two host plant-associated populations: rice population and water-oat population. Several studies have found evidence that seasonal reproductive isolation between these populations is at least partially due to interpopulation differences in diapause. However, there still lack unambiguous evidence comparing characteristics of diapause induction for both populations. We compared the photoperiodic response and the age of peak photoperiod sensitivity of these populations and used RNA-Seq to compare the molecular response of diapause induction between populations. The photoperiodic response of the two populations differed at 25 °C; the critical night length of larvae from the rice population was 11 h and 20 min, whereas no obvious critical night length was in those from the water-oat population. In rice population, larvae were most sensitive to photoperiod at 9-12 days of age, whereas in water-oat population, larvae were the most sensitive to photoperiod at 9-10 days of age. The RNA-Seq results indicated that there were several differences in the molecular response of diapause induction and small overlap in differentially expressed genes (DEGs) between populations. Furthermore, GO analysis indicated that both rice and water-oat population's DEGs were significantly enriched in heme and iron binding. Besides, water-oat population's DEGs were significantly enriched in metabolizing nutrients but rice population's DEGs do not. Thus, our results described differences in diapause induction between rice and water-oat populations of C. suppressalis which could affect the timing of diapause and thereby contribute to the seasonal reproductive isolation of these host plant-associated populations. In conclusion, this work suggests that difference in diapause induction could promote the population divergence in insects associated with different host plants.

Rapid Cold Hardening Confers a Transient Increase in Low Temperature Survival in Diapausing Chilo suppressalis Larvae

The striped stem borer, Chilo suppressalis (Walker), overwinters as a diapausing larva. The diapausing larvae were tested for a rapid cold hardening (RCH) response and its role in the insect’s survival of sub-zero temperatures. When laboratory-reared diapausing larvae were transferred directly from the rearing temperature of 25 °C to −14 °C and maintained there for 2 h, 21% survived. Acclimation of diapausing larvae for 4 h at 5 °C before their exposure for 2 h to −14 °C increased survival to approximately 41%, indicating an RCH response. Durability of RCH effects on low temperature survival was less than 1 h. Although transient in the test, the increased survival acquired through rapid cold hardening may play a role in preparing the diapausing larvae for abrupt temperature drops in the field that would otherwise be lethal.

Cold Tolerance and Supercooling Capacity of the Redbanded Stink Bug (Hemiptera: Pentatomidae)

The redbanded stink bug Piezodorus guildinii (Westwood) (Hemiptera: Pentatomidae) is an invasive stink bug species in the United States. First documented as a soybean pest in Louisiana in the year 2000, this species continues to spread in the Mid-South region of the United States. We designed laboratory and field studies to investigate supercooling points, lethal exposure time (LT), critical thermal minimum (CTmin), and winter mortality of this species. The mean supercooling points (SCP) ± SE of adult field collected P. guildinii ranged from -8.3 ± 0.2°C (highest) in March to -11.0 ± 0.2°C (lowest) in January. Significant differences in SCP occurred over the months and between sexes with significant interactions between month and sex. The CTmin was significantly different between adults and nymphs (third, fourth, and fifth instars combined). LT50 and LT90 were evaluated at subzero temperatures of 0°C, -2°C, and -5°C. There were significant differences in LT50 and LT90 among the temperature treatments. Winter survival significantly differed between the two study years and decreased with progression of winter months.

Difference in diel mating time contributes to assortative mating between host plant-associated populations of Chilo suppressalis

Behavioral isolation in animals can be mediated by inherent mating preferences and assortative traits, such as divergence in the diel timing of mating activity. Although divergence in the diel mating time could, in principle, promote the reproductive isolation of sympatric, conspecific populations, there is currently no unequivocal evidence of this. We conducted different mate-choice experiments to investigate the contribution of differences in diel mating activity to the reproductive isolation of the rice and water-oat populations of Chilo suppressalis. The results show that inter-population difference in diel mating activity contributes to assortative mating in these populations. In the rice population, most mating activity occurred during the first half of the scotophase, whereas in the water-oat population virtually all mating activity was confined to the second half of the scotophase. However, when the photoperiod of individuals from the water-oat population was altered to more closely align their mating activity with that of the rice population, mate choice was random. We conclude that inter-population differences in diel mating time contribute to assortative mating, and thereby the partial reproductive isolation, of these host-associated populations of C. suppressalis.

Do differences in life-history traits and the timing of peak mating activity between host-associated populations of Chilo suppressalis have a genetic basis?

The development of host races, genetically distinct populations of the same species with different hosts, is considered to be the initial stage of ecological speciation. Ecological and biological differences consistent with host race formation have been reported between water-oat and rice-associated populations of Chilo suppressalis. In order to confirm whether these differences have a genetic basis, we conducted experiments to determine the extent to which various life-history traits and the time of peak mating activity of these populations were influenced by the species of host plant larvae were raised on. Individuals from each population were reared for three consecutive generations on either water-oat fruit pulp or rice seedlings. Descendants of both populations had higher larval survival rates, shorter larval developmental periods, higher pupal weight, and longer adult forewings, when reared on water-oats than when reared on rice. The time of peak of mating activity differed between the descendants of each population, irrespective of whether they were raised on water-oats or rice. These results indicate that although some life-history traits of host-associated populations of C. suppressalis are influenced by the host plant larvae are raised on, time of peak mating activity is not. Because it is a stable, objective, phenotypic trait, further research on difference in the time of peak mating activity between host-associated populations of C. suppressalis should be conducted to clarify the mechanism responsible for host race formation in this species.

Seasonal and geographical variation in diapause and cold hardiness of the Asian corn borer, Ostrinia furnacalis

Asian corn borer, Ostrinia furnacalis (Guenée), is a key corn pest in the Asian-Western Pacific countries. It overwinters as full-grown larvae in plant stalks or in a spun-silk covering located in the plant debris in the temperate regions of China. Supercooling point (SCP) and survival rate after low sub-zero temperature treatment were assessed for field-collected populations in the laboratory using a cool bath with a 1°C/min cooling rate until -40°C. Mean SCPs were varied among geographical populations, with a significant decline from -22.7°C of Haikou, the multivoltine tropical population in the south, to -28.5°C of Gongzhuling, the univoltine temperate population in the northeast of China. In addition, there was more than 1°C difference in SCP between Gongzhuling univoltine and bivoltine populations that were from the same geographic origin. Mean SCPs of the Guangzhou population fluctuated over the year, with significantly lower SCPs in winter than in other seasons, which correlated with a significantly higher proportion of diapausing larvae in winter than in other seasons. Over 41% of overwintering larvae from the northeast population could withstand to be supercooled for a few minutes to the low sub-zero temperature of -40°C, but only 6.7% of their southern counterparts did so. The findings from this study suggest that O. furnacalis mostly takes advantage of freeze avoidance as diapausing larvae for overwintering in the southern region, whereas it exhibits freeze tolerance in diapause in the northeastern region.

Effects of seasonal acclimation on cold tolerance and biochemical status of the carob moth, Ectomyelois ceratoniae Zeller, last instar larvae

The carob moth, Ectomyelois ceratoniae, a pest of Punica granatum, overwinters as a larva. In this study, physiological changes, water content, cold hardiness and supercooling points (SCPs) in relation to ambient temperature in the overwintering period (October to March) and changes of these factors between diapausing (February) and non-diapausing (September) larvae were studied. Pupae that were derived from diapausing larvae (April) and from non-diapausing larvae (August) were also compared. Total body sugar, lipid and protein contents increased with decrease in the temperature and reached the highest levels (12.82, 1.99 and 6.11 mg g-1 body weight, respectively) in February, but glycogen content decreased and reached the lowest level (1.12 mg g-1 body weight) in February. There were significant differences in the levels of these compounds between diapausing and non-diapausing larvae, and pupae that were derived from diapausing and non-diapausing larvae. Trehalose and myo-inositol contents increased during diapause and reached the highest levels (0.50 and 0.07 mg g-1 body weight, respectively) in February. There were significant differences in the levels of these compounds between diapausing and non-diapausing larvae, but the differences between pupae that were derived from diapausing and non-diapausing larvae were not significant. The SCP of diapausing larvae (-17.3 °C) was significantly lower than in the non-diapausing larvae (-12.0 °C). SCP decreased gradually in autumn and reached the lowest level in the middle of winter. Changes of cold hardiness were inversely proportional to SCP changes. The lowest levels of water (65%) and weight (43.13 mg) were recorded in January and March, respectively. Most probably, lipids play a role as energy reserve, and low-molecular weight carbohydrates and polyols provide cryoprotection for overwintering larvae of the carob moth. Since the overwintering larvae die at temperatures above the SCP, the carob moth larvae were found to be a chill-intolerant insect.

Cross generation plasticity in cold hardiness is associated with diapause, but not the non-diapause developmental pathway, in the blowfly, Calliphora vicina

Predicting insect responses to global climate change involves understanding cross generation effects of temperature. The majority of temperate insects overwinter in a state of diapause, a pre-emptive response to winter conditions associated with increased cold hardiness. Diapause is often induced following maternal adult detection of an environmental cue signifying the onset of winter, whilst diapause is initiated in a subsequent life-stage/generation. Continued global warming will expose adults to higher late-autumn temperatures, whilst diapause life-stages will still experience prolonged winter-cold. The cross generation effect of temperature was investigated by acclimating adult Calliphora vicina, to present day (15°C) and future (20°C) late-autumn conditions and assessing cold-hardiness in diapause (D15 and D20) and non-diapause (ND15 and ND 20) progeny. A cross generation plasticity in cold hardiness was associated with D but not ND larvae. D15 larvae exhibited an enhanced ability to suppress the internal freezing (SCP = -18.9±0.9°C) compared to D20 (-15.3±0.8°C), and displayed a greater tolerance of prolonged exposure to -4°C (LT50 26.0± 1.0 days and 11.4±1.1 days, respectively) and -8°C (5.1±1.1 days and 3.0 ±1.1 days, respectively). These changes were associated with a reduced glucose content in D15 (2.4±0.3 g mg-1) compared to D20 (3.0±0.3 g mg-1) larvae. In conclusion, C. vicina adults exposed to warmer autumn conditions during diapause induction will produce larvae with a reduced cold hardiness capacity, which could negatively impact winter survival. Given that maternal regulation of diapause is common among temperate insects this could be a widespread phenomenon.

Study on the physiology of diapause, cold hardiness and supercooling point of overwintering pupae of the pistachio fruit hull borer, Arimania comaroffi

The pistachio fruit hull borer, Arimania comaroffi (Ragonot) (Lepidoptera: Pyralidae), is a key pest of pistachio orchards in Iran. This pest passes the winter as diapausing pupae. In this study, some physiological changes in relation to environmental temperature were investigated in field collected pupae. The relationship between supercooling point, cold hardiness and physiological changes of a wild population of this pest was also investigated. The glycogen content decreased with decrease in environmental temperature. Decrease in glycogen content was proportional to increase in total body sugar, trehalose, myo-inositol and sorbitol contents. In January with mean ambient temperature of 5.4°C, glycogen (5 mg/g fresh body weight) content was at the lowest level whereas total body sugar (10.3 mg/g fresh body weight), trehalose (8.6 mg/g fresh body weight), myo-inositol (5.3 mg/g fresh body weight) and sorbitol (2.6 mg/g fresh body weight) were at the highest levels. Total body sugar, trehalose, myo-inositol and sorbitol contents increased as mean temperature decreased from 22.7°C in October to 5.4°C in January. Total body lipid decreased during overwintering and reached to the lowest level at the end of March. Supercooling points were decreased from October to January and reached to the lowest level (-16°C) in January with minimum ambient temperature of -10°C. Survival at low temperature after 24 h was also greatest in January with 72% survival at -10°C, 39% survival at -15°C and 0% survival at -20°C. Increase in temperature from February onward, was proportional with increase in supercooling points and decrease in survival rate. Regardless of sampling date, all pupae died after 24 h at -20°C, whereas none pupae died after 24 h at -5°C. This indicates that this insect is freeze-intolerant.

Overwintering physiology and microhabitat use of Phyllocnistis populiella (Lepidoptera: Gracilliariidae) in interior Alaska

We investigated the overwintering physiology and behavior of Phyllocnistis populiella Chambers, the aspen leaf miner, which has caused severe and widespread damage to aspen in Alaska over the past 10 yr. Active P. populiella moths caught in spring and summer supercooled to an average temperature of -16°C, whereas dormant moths excavated from hibernacula in the leaf litter during fall and winter supercooled to an average of -32°C. None of the moths survived freezing in the laboratory. Counts of overwintering moths in leaf litter across microhabitats in interior Alaska demonstrated that moths occurred at significantly higher density beneath white spruce trees than beneath the aspen host, several other hardwood species, or in open areas among trees. During winter, the temperature 1-2 cm below the surface of the leaf litter beneath white spruce trees was on average 7-9°C colder than beneath aspen trees, and we estimate that during at least one period of the winter the temperature under some white spruce trees may have been cold enough to cause mortality. However, the leaf litter under white spruce trees was significantly drier than the litter from other microhabitats, which may assist P. populiella moths to avoid inoculative freezing because of physical contact with ice. We conclude that in interior Alaska, P. populiella overwinter in a supercooled state within leaf litter mainly under nonhost trees, and may prefer relatively dry microhabitats over moister ones at the expense of lower environmental temperature.

Transgenes for insect resistance reduce herbivory and enhance fecundity in advanced generations of crop-weed hybrids of rice

Gene flow from transgenic crops allows novel traits to spread to sexually compatible weeds. Traits such as resistance to insects may enhance the fitness of weeds, but few studies have tested for these effects under natural field conditions. We created F₂ and F₃ crop–weed hybrid lineages of genetically engineered rice (Oryza sativa) using lines with two transgene constructs, cowpea trypsin inhibitor (CpTI) and a Bt transgene linked to CpTI (Bt/CpTI). Experiments conducted in Fuzhou, China, demonstrated that CpTI alone did not significantly affect fecundity, although it reduced herbivory. In contrast, under certain conditions, Bt/CpTI conferred up to 79% less insect damage and 47% greater fecundity relative to nontransgenic controls, and a 44% increase in fecundity relative to the weedy parent. A small fitness cost was detected in F₃ progeny with Bt/CpTI when grown under low insect pressure and direct competition with transgene-negative controls. We conclude that Bt/CpTI transgenes may introgress into co-occurring weedy rice populations and contribute to greater seed production when target insects are abundant. However, the net fitness benefits that are associated with Bt/CpTI could be ephemeral if insect pressure is lacking, for example, because of widespread planting of Bt cultivars that suppress target insect populations.

Relationships between body weight of overwintering larvae and supercooling capacity; diapause intensity and post-diapause reproductive potential in Chilo suppressalis Walker

The rice stem borer, Chilo suppressalis Walker, overwinters in China as a larva in facultative diapause. The instars and body weights of overwintering larvae vary widely. In this paper, the relationships between body weight and supercooling capacity, diapause intensity and post-diapause reproductive potential of overwintering larvae collected in late-stage rice field were examined. There was a significant positive correlation between body weight, instar, and head capsule width, thus the overwintering larvae were divided into five groups based on body weight (I, up to 35.0mg; II, 35.1-57.0mg; III, 57.1-79.0mg; IV, 79.1-101.0mg; and V, over 101.1mg) for further analysis. The body water content of the lighter group (I) was significantly higher than that of the heavier groups (IV-V). However, the mean supercooling point decreased with an increase of the mean larval body weight in five groups; mean supercooling point of group I was significantly lower than that of group V, except in January 2009. After transfer of overwintering larvae to 15, 20 and 25°C on different dates, smaller individuals pupated slightly faster than larger ones at the same temperature, suggesting that diapause was less intense in smaller overwintering larvae. On 19 March 2009 there was a strong positive correlation between larval body weight and the weight of 3 day-old pupae, and the number of eggs carried by 2 day-old adult females at 15, 20 and 25°C. The average number of eggs carried by 2 day-old adult females differed significantly among different groups. The average number of eggs carried by 2 day-old adult females in group V was significantly greater than those of other groups, and that of group I was significantly lower than those of other groups, suggesting that post-diapause reproductive potential was determined, to a certain extent, by body weight of the overwintering larvae.

Host status of wheat and corn for Chilo suppressalis (Lepidoptera: Crambidae)

Host plant specificity depends on recognition of the host and the ability to discriminate it from nonhost plants. Chilo suppressalis (Walker) (Lepidoptera: Crambidae), an important insect pest of rice, is considered to be polyphagous, although few papers have ever reported infestation of C. suppressalis on most of the recorded hosts. The present investigation was designed to test whether two important gramineous crops, wheat and corn, are host plants of C. suppressalis based on the host-plant finding process in cage and Y-tube olfactometer tests, oviposition and egg hatching, and larval feeding and survival. In the cage tests, gravid C. suppressalis females did not differentiate rice plants from wheat or corn plants when only visual cues were involved, but were more attracted to rice plants when only olfactory cues or both vision and olfaction were present. The Y-tube olfactometer tests further confirmed that the females did not prefer wheat or corn plants, and revealed that they responded equally to clean air and odors from wheat or corn plants. Under no-choice and choice condition alike, the females laid eggs on a lower proportion of wheat and corn plants and egg number and hatching rate were significantly reduced on wheat and/or corn plants than on rice plants. Larval feeding was not observed in wheat and lower in corn than in rice plants, and no pupae or surviving larvae were collected from wheat and corn plants. The results suggest that wheat and corn are not host plants of C. suppressalis. These findings are discussed in context of host-finding process in C. suppressalis and management of resistance to transgenic Bt rice.

Cold tolerance of first-instar nymphs of the Australian plague locust, Chortoicetes terminifera

The cold tolerance of first-instar nymphs of the Australian plague locust, Chortoicetes terminifera, was examined using measures of total body water content, supercooling point and mortality for a range of sub-zero temperature exposure regimes. The supercooling points for starved and fed nymphs were -13.1+/-0.9 and -12.6+/-1.6 degrees C, and freezing caused complete mortality. Above these temperatures, nymphs were cold tolerant to different degrees based on whether they were starved or given access to food and water for 24h prior to exposure. The rate of cooling also had a significant effect on mortality. Very rapid cooling to -7 degrees C caused 84 and 87% mortality for starved and fed nymphs respectively, but this significantly decreased for starved nymphs if temperature declined by more ecologically realistic rates of 0.5 and 0.1 degrees C min(-1). These results are indicative of a rapid cold hardening response and are discussed in terms of the likely effects of cold nights and frost on first-instar nymphal survival in the field.