Chill coma recovery of Ceratitis capitata adults across the Northern Hemisphere

Overwintering potential of the Mediterranean fruit fly (Diptera: Tephritidae) in Austria

The Mediterranean fruit fly (medfly), Ceratitis capitata (Wiedemann), one of the most important invasive pests of fresh fruits and vegetables from the coastal Mediterranean habitats, is expanding its current geographic distribution to cooler more temperate areas of Europe. Every year since 2010 the fly is detected in the area of Vienna, Austria. However, whether it can establish permanent populations is not known. In this current paper, the capacity of C. capitata to overwinter in Vienna, Austria (48.1° northern latitude) was studied over 2 consecutive winter seasons (2020-2022). Overwintering trials with different life stages (larva, pupa, and adult) of C. capitata were performed in the open field and in the protected environment of a basement without a heating system. Control flies were kept under constant conditions in a climate chamber (25 °C, 60% RH, 14:10 L:D). Our data showed that no life stage of the Mediterranean fruit fly was able to survive the Austrian winter in the open field. However, in the protected environment C. capitata outlived the winter months in all studied life stages at least in small numbers and several surviving females were able to lay eggs at the time of the following fruiting season. Implications of these findings for the ongoing geographic range expansion of the pest in temperate European countries are discussed.

Mediterranean fruit fly population phenological patterns are strongly affected by elevation and host presence

The Mediterranean fruit fly (medfly) (Ceratitis capitata, Diptera: Tephritidae), is an extremely polyphagous pest that threatens the fruit production and trading industry worldwide. Monitoring C. capitata populations and analysing its dynamics and phenology is considered of outmost importance for designing and implementing sound management approaches. The aim of this study was to investigate the factors regulating the population dynamics of the C. capitata in a coastal and semi-mountainous area. We focused on effects of topography (e.g. elevation), host presence and seasonal patterns of ripening on the phenological patterns considering data collected in 2008. The experimental area is characterized by mixed fruit orchards, and Mediterranean climate with mild winters. Two trap types were used for population monitoring. The female targeted McPhail type and the male targeted Jackson type. Traps were placed in farms located at different elevations and landscape morphology (coastal and semi-mountainous areas). The main crops included citrus, apples, peaches, plums, pears, figs, quinces and apricots. Adult captures were first recorded in May, peaked in mid-summer and mid-autumn and almost ceased at the end of the season (January 2008). Captures in the coastal areas preceded that of highlands by 15 days. Most of the adults detected during the fruit ripening of late stone fruit cultivars (first peak) and citrus (second peak). The probability of capturing the first adults preceded almost three weeks the peak of adult captures either considering the elevation or host focus analyses. The results provide valuable information on the seasonal population trend of C. capitata in mixed fruit Mediterranean orchards and can support the set-up of IPM systems in areas with various landscapes and different hosts throughout the fruit growing season.

Evidence that recent climatic changes have expanded the potential geographical range of the Mediterranean fruit fly

The species distributions migration poleward and into higher altitudes in a warming climate is especially concerning for economically important insect pest species, as their introduction can potentially occur in places previously considered unsuitable for year-round survival. We explore the expansion of the climatically suitable areas for a horticultural pest, the Mediterranean fruit fly (medfly) Ceratitis capitata (Diptera, Tephritidae), with an emphasis on Europe and California. We reviewed and refined a published CLIMEX model for C. capitata, taking into consideration new records in marginal locations, with a particular focus on Europe. To assess the model fit and to aid in interpreting the meaning of the new European distribution records, we used a time series climate dataset to explore the temporal patterns of climate suitability for C. capitata from 1970 to 2019. At selected bellwether sites in Europe, we found statistically significant trends in increasing climate suitability, as well as a substantial northward expansion in the modelled potential range. In California, we also found a significant trend of northward and altitudinal expansion of areas suitable for C. capitata establishment. These results provide further evidence of climate change impacts on species distributions and the need for innovative responses to increased invasion threats.

Latitudinal variation in survival and immature development of Ceratitis capitata populations reared in two key overwintering hosts

Ceratitis capitata, a major agricultural pest, is currently expanding its geographic distribution to northern, temperate areas of Europe. Its seasonal biology and invasion success depend on temperature, humidity and host availability. In coastal warmer Mediterranean regions and cooler temperature areas, bitter oranges and apples serve as overwintering hosts during the larval stage. We assessed the overwintering capacity of C. capitata populations obtained from different areas of the northern hemisphere by studying the survival and development rates of immature stages in both fruits under laboratory conditions. Eggs from each population were artificially inserted in the flesh of the two hosts and kept at 15, 20, or 25 °C until pupation and adult emergence. Climatic analysis of the area of the population origin showed combined effects of latitude, host and macroclimatic variables on immature survival and development rates. Egg to adult survival rates and developmental duration were longer in apples than in bitter oranges. For populations originated from southern-warmer areas, egg to adult developmental duration was prolonged and adult emergence reduced at 15 °C compared to those populations obtained from northern regions. Our findings reveal varying plastic responses of medfly populations to different overwintering hosts and temperatures highlighting the differential overwintering potential as larvae within fruits. This study contributes towards better understanding the medfly invasion dynamics in temperate areas of Northern Europe and other parts of the globe with similar climates.

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.

Larval nutritional-stress and tolerance to extreme temperatures in the peach fruit fly, Bactrocera zonata (Diptera: Tephritidae)

Within the factors affecting insect tolerance to extreme environmental conditions, insect nutrition, particularly of immature stages, has received insufficient attention. In the present study, we address this gap by investigating the effects of larval nutrition on heat and cold tolerance of adult Bactrocera zonata - an invasive, polyphagous fruit fly pest. We manipulated the nutritional content in the larval diet by varying the amount of added yeast (2-10% by weight), while maintaining a constant sucrose content. Adults derived from the different larval diets were tested for their tolerance to extreme heat and cold stress. Restricting the amount of yeast reduced the efficacy of the larval diet (i.e. number of pupae produced per g of diet) as well as pupal and adult fresh weight, both being significantly lower for yeast-poor diets. Additionally, yeast restriction during the larval stage (2% yeast diet) significantly reduced the amount of protein but not lipid reserves of newly emerged males and females. Adults maintained after emergence on granulated sugar and water for 10 days were significantly more tolerant to extreme heat (i.e. knock-down time at 42 ^oC) when reared as larvae on yeast-rich diets (8% and 10% yeast) compared to counterparts developing on a diet containing 2% yeast. Nevertheless, the composition of the larval diet did not significantly affect adult survival following acute cold stress (exposure to -3°C for 2 hrs.). These results are corroborated by previous findings on Drosophilid flies. Possible mechanisms leading to nutrition-based heat-tolerance in flies are discussed.

Differential Cold Tolerance on Immature Stages of Geographically Divergent Ceratitis capitata Populations

Cold tolerance of adult medflies has been extensively studied but the effect of subfreezing temperatures on the immature stages remains poorly investigated, especially as far as different populations are regarded. In this study, we estimated the acute cold stress response of three geographically divergent Mediterranean fruit fly populations originating from Greece (Crete, Volos) and Croatia (Dubrovnik) by exposing immature stages (eggs, larvae, pupae) to subfreezing temperatures. We first determined the LT50 for each immature stage following one hour of exposure to different temperatures. Then eggs, larvae and pupae of the different populations were exposed to their respective LT50 for one hour (LT50 = -11 °C, LT50 = -4.4 °C, LT50 = -5 °C for eggs, larvae and pupae, respectively). Our results demonstrate that populations responded differently depending on their developmental stage. The population of Dubrovnik was the most cold-susceptible at the egg stage, whereas in that of Crete it was at the larval and pupal stage. The population of Volos was the most cold-tolerant at all developmental stages. The egg stage was the most cold-tolerant, followed by pupae and finally the 3rd instar wandering larvae. This study contributes towards understanding the cold stress response of this serious pest and provides data for important parameters that determine its successful establishment to unfavorable environments with an emphasis on range expansion to the northern, more temperate regions of Europe.

Effect of thermal acclimation on the tolerance of the peach fruit fly (Bactrocera zonata: Tephritidae) to heat and cold stress

Understanding the thermal biology of insects is of increasing importance for predicting their geographic distribution, particularly in light of current and future global temperature increases. Within the limits set by genetic makeup, thermal tolerance is affected by the physiological conditioning of individuals (e.g., through acclimation). Considering this phenotypic plasticity may add to accurately estimating changes to the distribution of insects under a changing climate. We studied the effect of thermal acclimation on cold and heat tolerance of the peach fruit fly (Bactrocera zonata) - an invasive, polyphagous pest that is currently expanding through Africa and the Middle East. Females and males were acclimated at 20, 25 and 30 °C for up to 19 days following adult emergence. The critical thermal minimum (CTmin) and maximum (CTmax) were subsequently recorded as well adult survival following acute exposure to chilling (0 or -3 °C for 2 h). Additionally, we determined the survival of pupae subjected for 2 h to temperatures ranging from -12 °C to 5 °C. We demonstrate that acclimation at 30 °C resulted in significantly higher CTmax and CTmin values (higher heat resistance and lower cold resistance, respectively). Additionally, adult recovery following exposure to -3 °C was significantly reduced following acclimation at 30 °C, and this effect was significantly higher for females. Pupal mortality increased with the decrease in temperature, reaching LT50 and LT95 values following exposure to -0.32 °C and -6.88 °C, respectively. Finally, we found that the survival of pupae subjected to 0 and 2 °C steadily increased with pupal age. Our findings substantiate a physiological foundation for understanding the current geographic range of B. zonata. We assume that acclimation at 30 °C affected the thermal tolerance of the flies partly through modulating feeding and metabolism. Tolerance to chilling during the pupal stage probably changed according to temperature-sensitive processes occurring during metamorphosis, rendering younger pupae more sensitive to chilling.

Acute cold stress and supercooling capacity of Mediterranean fruit fly populations across the Northern Hemisphere (Middle East and Europe)

The Mediterranean fruit fly, Ceratitis capitata (Diptera: Tephritidae), holds an impressive record of successful invasion events promoted by globalization in fruit trade and human mobility. In addition, C. capitata is gradually expanding its geographic distribution to cooler temperate areas of the Northern Hemisphere. Cold tolerance of C. capitata seems to be a crucial feature that promotes population establishment and hence invasion success. To elucidate the interplay between the invasion process in the northern hemisphere and cold tolerance of geographically isolated populations of C. capitata, we determined (a) the response to acute cold stress survival of adults, and (b) the supercooling capacity (SCP) of immature stages and adults. To assess the phenotypic plasticity in these populations, the effect of acclimation to low temperatures on acute cold stress survival in adults was also examined. The results revealed that survival after acute cold stress was positively related to low temperature acclimation, except for females originating from Thessaloniki (northern Greece). Adults from the warmer environment of South Arava (Israel) were less tolerant after acute cold stress compared with those from Heraklion (Crete, Greece) and Thessaloniki. Plastic responses to cold acclimation were population specific, with the South Arava population being more plastic compared to the two Greek populations. For SCP, the results revealed that there is little to no correlation between SCP and climate variables of the areas where C. capitata populations originated. SCP was much lower than the lowest temperature individuals are likely to experience in their respective habitats. These results set the stage for asking questions regarding the evolutionary adaptive processes that facilitate range expansions of C. capitata into cooler temperate areas of Europe.

Winter survival of adults of two geographically distant populations of Aedes albopictus in a microclimatic environment of Athens, Greece

Overwintering capacity is a major step towards establishment of invasive mosquitoes from the tropics in temperate zone areas and one of the main elements determining next seasons' population size that regulates disease transmission of competent invasive vector species. The Asian tiger mosquito, Aedes albopictus (Skuse) (Diptera: Culiciidae) is an aggressive invasive species that has greatly expanded its geographical boundaries over the last few decades. The species' ability to induce photoperiodic-based egg diapause allows its overwintering in temperate regions, which favors its establishment in higher latitudes. In warmer temperate areas winter survival can be accomplished in the adult stage as well especially in human-made shelters. Aedes albopictus is already showing signs of adaptation to colder climates which may result in disease transmission in new areas. Although the Asian tiger mosquito has invaded Greece since 2003-4, little is known regarding its overwintering capacity in the country, especially as far as adults are regarded. We studied the survival of Ae. albopictus adults during winter in a protected shelter in Athens, the capital city of Greece. The study involved two geographically isolated populations originating from Chania (Crete, most southern part of Greece), and Palaio Faliro (Athens, Central Greece). We exposed different cohorts of F3 adults of the above two populations that emerged from field collected eggs to "winter condition" from November 2018 to beginning of January 2019. in a protected microclimatic environment. Adult mortality was recorded systematically until the death of the last individual in the cohort. Results demonstrated evidence of winter survival of adults for both populations. Longevity of both females and males of the Palaio Faliro population was longer than that of the Chania population for almost all installation dates. Survival curves, regardless of the date of installation, were steeper for females of the Chania population compared to those from Palaio Faliro. In conclusion, overwinter survival of both populations in the adult stage highlights the need for future studies, considering local and microclimatic condition that vary significantly between regions. Winter climatic conditions play vital role in adult survival of this highly important public health species concerning distribution limits and knowledge on winter survival of Ae. albopictus transmitted viruses.