Studies on chill coma recovery in the ladybird, Harmonia axyridis: Ontogenetic profile, effect of repeated cold exposures, and capacity to predict winter survival

The supercooling point depression is the leading cold tolerance strategy for the variegated ladybug, [Hippodamia variegata (Goezel)]

The variegated ladybug, Hippodamia variegata is one of the most effective predators of various pests that hibernate as adult beetles. During the overwintering period from April 2021 to March 2022, we examined the supercooling point (SCP), cold tolerance, and physiological adaptations of beetles in Kerman, Iran. The beetles exhibited their greatest cold tolerance (63.4% after 24 h at -5°C) when their SCP was lowest (-23.2°C). Conversely, from April to October 2021, the SCP reached its peak (approximately -13.0°C), while cold tolerance was at its lowest level (6.7% after 24 h at -5°C). Cryoprotectant content (trehalose, glycerol, and glucose) was at its highest level in September (11.15, 10.82, and 6.31 mg/g, respectively). The critical thermal minimum (CTmin) reached its lowest point of -2.2°C in January and February. The lowest point of the lower lethal temperature (LLT) coincided with the lowest level of the SCP and the highest level of cold tolerance (in February, LT50 = -5.3°C, SCP = -23.2°C, and survival = 77.78% at -4°C/24 h). Chill-coma recovery time (CCRT) was examined at five different temperatures and two different exposure durations. The CCRT increased with a decrease in exposure temperature and time (68.0 s at -2°C after 2 h and 102.0 s at -2°C after 4 h). As the majority of the overwintering beetle's mortality occurred at temperatures significantly higher than SCP, the adults of H. variegata are chill-susceptible insects that primarily rely on a depressed supercooling point to cope with unfavorable conditions during the overwintering period.

Increased pupal temperature has reversible effects on thermal performance and irreversible effects on immune system and fecundity in adult ladybirds

The environmental conditions an organism encounters during development vary in their lasting impact on adult phenotypes. In the context of ongoing climate change, it is particularly relevant to understand how high developmental temperatures can impact adult traits, and whether these effects persist or diminish during adulthood. Here, we assessed the effects of pupal temperature (17 °C - normal temperature, 26 °C - increased temperature, or 35 °C - heat wave) on adult Harmonia axyridis thermal stress tolerance, immune function, starvation resistance, and fecundity. The temperature during pupation significantly affected all investigated traits in fresh adults. Heat acclimation decreased adult haemocyte concentration, cold tolerance, and total egg production, and had a positive effect on heat tolerance and starvation resistance. The negative effects of heat acclimation on cold tolerance diminished after seven days. In contrast, heat acclimation had a lasting positive effect on adult heat tolerance. Our results provide a broad assessment of the effects of developmental thermal acclimation on H. axyridis adult phenotypes. The relative plasticity of several adult traits after thermal acclimation may be consequential for the future geographic distribution and local performance of various insect species.

Cold hardiness of the South American tomato pinworm Tuta absoluta (Lepidoptera: Gelechiidae): both larvae and adults are chill-susceptible

BACKGROUND

For many insects, including invasive species, overwintering survival is achieved behaviourally (e.g. through migration) or physiologically by entering diapause, a state of arrested physiological development that may be accompanied with depressed supercooling points (SCPs). Diapause allows in situ adaptation to adverse environmental conditions, providing sufficient parent propagules for insect pest proliferation when optimal conditions resurface. This phenomenon has however not been observed in the invasive South American tomato pinworm Tuta absoluta in its Mediterranean invaded areas. Moreover, no studies have looked at its overwintering survival in sub-Saharan Africa. Here, we thus investigated the cold hardiness of Tuta absoluta larvae and adults to better explain its local overwintering adaptation strategy.

RESULTS

Larval lower lethal temperatures ranged from -1 to -17 °C for 0.5 to 4 h durations. Adults showed lower temperature activity limits than larvae albeit freeze strategy experiments showed neither survived internal freezing. Fasting and dehydration pre-treatment generally depressed SCPs, although asymmetrically, conferring more negative SCPs for larvae. Ramping rates, synonymic to diurnal temperature changes also significantly affected SCPs while, inoculative freezing significantly compromised freezing temperatures in both larvae and adults.

CONCLUSION

Our results suggest that (i) Tuta absoluta larvae and adults are chill-susceptible and may successfully overwinter, (ii) larvae appear more cold hardy than adults and (iii) ecological factors e.g. inoculative freezing, cooling rates, feeding- and hydration-status may affect cold hardiness. These results are important in determining species range limits, population phenology, modelling pest risk status and allows temporal life-stage specific targeting of management strategies.

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.

Effects of the winter temperature regime on survival, body mass loss and post-winter starvation resistance in laboratory-reared and field-collected ladybirds

Ongoing climate change results in increasing temperatures throughout the seasons. The effects of climate change on insect performance are less studied during the winter season than during the growing season. Here, we investigated the effects of various winter temperature regimes (warm, normal and cold) on the winter performance of the invasive ladybird Harmonia axyridis (Coleoptera: Coccinellidae). Winter survival, body mass loss and post-winter starvation resistance were measured for a laboratory-reared population as well as three populations collected from the field prior to overwintering. The warm winter regime increased the survival rate and body mass loss and reduced post-winter starvation resistance compared to those of the ladybirds in the cold winter regime. The effects of the temperature regime were qualitatively similar for the laboratory-reared and field-collected beetles; however, there were significant quantitative differences in all measured overwintering parameters between the laboratory-reared and field-collected populations. The winter survival of the laboratory-reared beetles was much lower than that of the field-collected beetles. The laboratory-reared beetles also lost a larger proportion of their body mass and had reduced post-winter starvation resistance. Winter survival was similar between the females and males, but compared to the males, the females lost a smaller proportion of their body mass and had better post-winter starvation resistance. The pre-overwintering body mass positively affected winter survival and post-winter starvation resistance in both the laboratory-reared and field-collected ladybirds. The significant differences between the laboratory-reared and field-collected individuals indicate that quantitative conclusions derived from studies investigating solely laboratory-reared individuals cannot be directly extrapolated to field situations.

An Examination of Flying Insects in Seven Hospitals in the United Kingdom and Carriage of Bacteria by True Flies (Diptera: Calliphoridae, Dolichopodidae, Fanniidae, Muscidae, Phoridae, Psychodidae, Sphaeroceridae)

Insects are efficient vectors of bacteria and in the hospital environment may have a role in spreading nosocomial infections. This study sampled the flying insect populations of seven hospitals in the United Kingdom and characterized the associated culturome of Diptera, including the antibiotic resistance profile of bacterial isolates. Flying insects were collected in seven U.K. hospitals between the period March 2010 to August 2011. The bacteria carried by Diptera were isolated using culture-based techniques, identified and characterized by antimicrobial susceptibility testing. A total of 19,937 individual insects were collected with Diptera being the most abundant (73.6% of the total), followed by Hemiptera (13.9%), Hymenoptera (4.7%), Lepidoptera (2.9%), and Coleoptera (2%). From Diptera, 82 bacterial strains were identified. The majority of bacteria belonged to the Enterobacteriaceae (42%), followed by Bacillus spp. (24%) and Staphylococcus spp. (19%). Less abundant were bacteria of the genus Clostridium (6%), Streptococcus (5%), and Micrococcus (2%). A total of 68 bacterial strains were characterized for their antibiotic resistance profile; 52.9% demonstrated a resistant phenotype to at least one class of antibiotic. Staphylococcus spp. represented the highest proportion of resistant strains (83.3%), followed by Bacillus spp. (60%) and Enterobacteriaceae (31.3%). Diptera were the predominant flying insects present in the U.K. hospital environments sampled and found to harbor a variety of opportunistic human pathogens with associated antimicrobial resistance profiles. Given the ability of flies to act as mechanical vectors of bacteria, they present a potential to contribute to persistence and spread of antimicrobial-resistant pathogenic bacteria in the hospital environment.

Cold Tolerance of the Tribolium castaneum (Coleoptera: Tenebrionidae), Under Different Thermal Regimes: Impact of Cold Acclimation

The red flour beetle, Tribolium castaneum (Herbst), is a serious pest of stored product worldwide. Cold tolerance or cold hardiness is an important ecophysiological trait related directly to survival, fitness, and distribution of insects. In this study, the effects of four thermal regimes, i.e., control (C), cold acclimation (CA), rapid cold hardening (RCH), and fluctuating-acclimation (FA), were examined for their effects on cold tolerance, supercooling point (SCP), lower lethal temperature (LLT), and chill-coma recovery time (CCRT) of the red flour beetle. In addition, changes in cryoprotectant (trehalose, sorbitol, and myo-inositol) levels were investigated under each thermal treatment. The results documented a substantial enhancement in the SCP, cold hardiness, and cryoprotectant levels of the adults of T. castaneum under CA regimes. The lowest SCP, highest trehalose and myo-inositol contents, and, subsequently, the greatest survival rate were observed in cold-acclimated beetles. In addition, coordination between cryoprotectant level, SCP, and cold tolerance of the pest was observed. The highest and lowest CCRT were observed at control and CA, respectively. In RCH regime with the highest impact, LLT reached the lowest level of -22°C. As most of the mortality of T. castaneum occurred at a temperature above the SCP, so this pest could be considered as a chill-susceptible insect.

Cold acclimation triggers lipidomic and metabolic adjustments in the spotted wing drosophila Drosophila suzukii (Matsumara)

Chronic cold exposure is detrimental to chill susceptible insects that may accumulate chill injuries. To cope with deleterious effects of cold temperature, insects employ a variety of physiological strategies and metabolic adjustments, such as production of cryoprotectants, or remodeling of cellular membranes. Cold tolerance is a key element determining the fundamental niche of species. Because Drosophila suzukii is an invasive fruit pest, originating from East Asia, knowledge about its thermal biology is urgently needed. Physiological mechanisms underlying cold tolerance plasticity remain poorly understood in this species. Here, we explored metabolic and lipidomic modifications associated with the acquisition of cold tolerance in D. suzukii using Omics technologies (LC- and GC-MS/MS). In both cold-acclimated males and females, we observed physiological changes consistent with homeoviscous/homeophasic adaptation of membranes: reshuffling of phospholipid head groups and increasing unsaturation rate of fatty acids. Modification of fatty acids unsaturation were also observed in triacylglycerides, which would likely increase accessibility of lipid reserves. At the metabolic level, we observed clear-cut differentiation of metabolic profiles with cold-acclimated metabotypes showing accumulation of several potential cryoprotectants (sugars and amino acids). Metabolic pathway analyses indicated a remodeling of various processes, including purine metabolism and aminoacyl tRNA biosynthesis. These data provide a large-scale characterization of lipid rearrangements and metabolic pathway modifications in D. suzukii in response to cold acclimation and contribute to characterizing the strategies used by this species to modulate cold tolerance.