The effect of winter length on survival and duration of dormancy of four sympatric species of Rhagoletis exploiting plants with different fruiting phenology

Variation in Thermal Sensitivity of Diapause Development among Individuals and over Time Predicts Life History Timing in a Univoltine Insect

AbstractPhysiological time is important for understanding the development and seasonal timing of ectothermic animals but has largely been applied to developmental processes that occur during spring and summer, such as morphogenesis. There is a substantial knowledge gap in the relationship between temperature and development during winter, a season that is increasingly impacted by climate change. Most temperate insects overwinter in diapause, a developmental process with little obvious morphological change. We used principles from the physiological time literature to measure and model the thermal sensitivity of diapause development rate in the apple maggot fly Rhagoletis pomonella, a univoltine fly whose diapause duration varies substantially within and among populations. We show that diapause duration can be predicted by modeling a relationship between temperature and development rate that is shifted toward lower temperatures compared with typical models of morphogenic, nondiapause development. However, incorporating interindividual variation and ontogenetic variation in the temperature-to-development rate relationship was critical for accurately predicting fly emergence, as diapause development proceeded more quickly at high temperatures later in diapause. We conclude that the conceptual framework may be flexibly applied to other insects and discuss possible mechanisms of diapause timers and implications for phenology with warming winters.

Coping with global warming: Adult thermal thresholds in four pestiferous Anastrepha species determined under experimental laboratory conditions and development/survival times of immatures and adults under natural field conditions

Climate change, particularly global warming, is disturbing biological processes in unexpected ways and forcing us to re-study/reanalyze the effects of varying temperatures, among them extreme ones, on insect functional traits such as lifespan and fecundity/fertility. Here we experimentally tested, under both laboratory and field conditions, the effects of an extreme range of temperatures (5, 10, 15, 20, 30, 40, and 45 °C, and the naturally varying conditions experienced in the field), on survivorship/lifespan, fecundity, and fertility of four pestiferous fruit fly species exhibiting contrasting life histories and belonging to two phylogenetic groups within the genus Anastrepha: A. ludens, A. obliqua, A. striata, and A. serpentina. In the field, we also measured the length of the entire life cycle (egg to adult), and in one species (A. ludens), the effect on the latter of the host plant (mango and grapefruit). Under laboratory conditions, none of the adults, independent of species, could survive a single day when exposed to a constant temperature of 45 °C, but A. striata and A. serpentina females/males survived at the highly contrasting temperatures of 5 and 40 °C at least 7 days. Maximum longevity was achieved in all species at 15 °C (375, 225, 175 and 160 days in A. ludens, A. serpentina, A. striata and A. obliqua females, respectively). Anastrepha ludens layed many eggs until late in life (368 days) at 15 °C, but none eclosed. Eclosion was only observed in all species at 20 and 30 °C. Under natural conditions, flies lived ca. 100 days less than in the laboratory at 15 °C, likely due to the physiological cost of dealing with the highly varying environmental patterns over 24 h (minimum and maximum temperatures and relative humidity of ca. 10–40 °C, and 22–100%, respectively). In the case of A. ludens, the immature’s developmental time was shorter in mango, but adult survival was longer than in grapefruit. We discuss our results considering the physiological processes regulating the traits measured and tie them to the increasing problem of global warming and its hidden effects on the physiology of insects, as well as the ecological and pest management implications.

Sensitivities to Chill Durations and No-Chill Temperatures Regulating Eclosion Responses Differ Between Rhagoletis zephyria (Diptera: Tephritidae) and its Braconid Parasitoids (Hymenoptera: Braconidae)

Seasonal temperatures select for eclosion timing of temperate insects and their parasitoids. In western North America, the fruit fly Rhagoletis zephyria Snow (Diptera: Tephritidae) is parasitized by the hymenopterous wasps Utetes lectoides (Gahan), an egg parasite, and Opius downesi Gahan, a larval parasite (both Braconidae). Eclosion of wasps should be timed with the presence of susceptible fly stages, but reports indicate U. lectoides ecloses in the absence of flies under no-chill conditions. Based on this, we tested the hypotheses that chill durations and no-chill temperatures both differentially regulate eclosion times of R. zephyria and its parasitic wasps. When fly puparia were chilled at ~3°C for 130-180 d, U. lectoides and O. downesi always eclosed on average later than flies. However, after 180-d chill, flies eclosed on average earlier than after 130- and 150-d chill, whereas eclosion times of U. lectoides and O. downesi were less or not affected by chill duration. When fly puparia were exposed to 20-22°C (no chill), U. lectoides eclosed before flies, with 88.9% of U. lectoides versus only 0.61% of flies eclosing. Taken together, findings show that eclosion times of flies are more sensitive to changes in chill duration than those of wasps. Flies are less sensitive than wasps to no-chill in that most flies do not respond by eclosing after no-chill while most wasps do. Our results suggest that shorter winters and longer summers due to climate change could cause mismatches in eclosion times of flies and wasps, with potentially significant evolutionary consequences.

Host plant stimuli effects on survival and duration of dormancy of Rhagoletis zoqui, Rhagoletis completa (Diptera: Tephritidae) and associated parasitoids

Tephritid fruit flies in the genus Rhagoletis bridge between predictable periods of fruit availability by becoming dormant. To cope with acyclic unpredictable events (e.g., frost, mast seeding, etc), a proportion of the population can undergo prolonged dormancy. In the case of walnut infesting Rhagoletis, host plant-derived cues such as juglone soil concentration vary seasonally in predictable patterns. Here, we examined the effects of host plant parts and derived compounds on emergence rates and dormancy duration of Rhagoletis completa (Cresson), Rhagoletis zoqui (Bush) (Diptera: Tephritidae), and associated parasitoids. Pupae of both species were exposed to walnut leaves, fruit, or fruit and leaves and compared to a control. In a second experiment, R. zoqui were exposed to 10 mg l-1 of juglone applied to pupation medium during four consecutive 4-week time periods under variable combinations of temperature and frequency of exposure. Overall, the presence of fruit resulted in greater overwintering survival of R. completa but had no effect on the duration of dormancy of either fly species. Application of juglone over two consecutive periods produced greater mortality of R. zoqui than the control. Three parasitoid species emerged from R. completa and one from R. zoqui. Duration of dormancy for parasitoids was longer than that of fly hosts. Regardless of treatment, 13.3-18.4% of R. completa pupae and 1.3-2.8% R. zoqui engaged in prolonged (>year) dormancy. Our results indicate that host plant derived cues have little or no effect on survival and duration of dormancy of walnut infesting Rhagoletis, and at the tested concentration juglone is toxic to R. zoqui pupae. Testing the effect of juglone at lower concentrations is necessary to rule out its role as an environmental cue for regulation of dormancy. So far, host plant fruiting phenology appears to play a greater role than host plant derived cues in selecting for fly eclosion times.

The Eclosion of Rhagoletis pomonella (Diptera: Tephritidae) Under Different Chill Durations and Simulated Temperate and Tropical Conditions

The apple maggot fly, Rhagoletis pomonella (Walsh) (Diptera: Tephritidae), is a serious pest of apple in North America that is subject to quarantine measures to prevent its spread to currently pest-free regions, including the tropics. How the fly may survive in warmer climates is unclear. Here, we studied the effects of exposing postchill puparia to simulated temperate and tropical environmental conditions on eclosion of R. pomonella from Washington State, U.S.A. Puparia were chilled for 0-30 wk at 3°C and then held under four postchill conditions: A = 23°C, 16:8 L:D, 40% RH; B = 26°C, 12:12 L:D, 80% RH; C = 26°C, 16:8 L:D, 80% RH; and D = 23°C, 12:12 L:D, 40% RH, with B and D representing tropical conditions and A and C temperate conditions. Within each chill duration, total numbers of flies eclosed were equally high in tropical treatment B and temperate treatment C, while they were lower in treatments A and D. Mean weeks of the first eclosion in treatments B and C were earlier than in treatment D; mean week of peak eclosion and 50% eclosion in treatments A, B, and C were earlier than in treatment D. Eclosion spans in treatments A, B, and D were generally shorter than in treatment C. Results suggest that if introduced into a humid tropical country, R. pomonella puparia from Washington State could produce adult flies, regardless of chill duration or lack of chilling during the pupal stage, but whether flies could establish there would require further study.

Distinct Adult Eclosion Traits of Sibling Species Rhagoletis pomonella and Rhagoletis zephyria (Diptera: Tephritidae) Under Laboratory Conditions

Closely related phytophagous insects that specialize on different host plants may have divergent responses to environmental factors. Rhagoletis pomonella (Walsh) and Rhagoletis zephyria Snow (Diptera: Tephritidae) are sibling, sympatric fly species found in western North America that attack and mate on plants of Rosaceae (~60 taxa) and Caprifoliaceae (three taxa), respectively, likely contributing to partial reproductive isolation. Rhagoletis zephyria evolved from R. pomonella and is native to western North America, whereas R. pomonella was introduced there. Given that key features of the flies' ecology, breeding compatibility, and evolution differ, we predicted that adult eclosion patterns of the two flies from Washington State, USA are also distinct. When puparia were chilled, eclosion of apple- and black hawthorn-origin R. pomonella was significantly more dispersed, with less pronounced peaks, than of snowberry-origin R. zephyria within sympatric and nonsympatric site comparisons. Percentages of chilled puparia that produced adults were ≥67% for both species. However, when puparia were not chilled, from 13.5 to 21.9% of apple-origin R. pomonella versus only 1.2% to 1.9% of R. zephyria eclosed. The distinct differences in eclosion traits of R. pomonella and R. zephyria could be due to greater genetic variation in R. pomonella, associated with its use of a wider range of host plants than R. zephyria.

Assessing water retention and correlation to climate conditions of five plant species in greywater treating green walls

Green walls are becoming a popular infrastructure choice in densely built urban environments, due to their multiple benefits. However, high and vastly variable water requirements of these systems are preventing their further widespread. Only a small number of studies have investigated water needs of green walls, even though this can help to design more optimal systems with increased benefits. Additionally, the knowledge on interactions between plant uptake and climate conditions (temperature and humidity) is lacking. The aim of this study was to understand daily water requirements of five plant species (C. appressa, N. obliterata, L. muscari, M. parvifolium and O. japonicus) used in greywater treating green walls, across different seasons, temperature, and humidity conditions of temperate-oceanic climate (common in parts of Australia, US and Europe). The results showed that during summer, dominant water uptake processes were plant uptake and transpiration, resulting in three to four times higher water needs than during winter, when evaporation is a major effect. Top levels of the multi-level green wall exhibited significantly higher plant activity compared to bottom levels, showing four times greater water uptake. Temperature and humidity changes during winter caused the change in water uptake of plants, pointing to different growing and activity patterns of tested plants. During summer only N. obliterata showed temperature and humidity dependence. Annual plant water uptake and other practical recommendations are given based on the results. Even though this study focused on water requirements of greywater treating green walls, its findings can also inform traditional green wall designs.

Differential Response to Photoperiod by Diverging Rhagoletis pomonella (Diptera: Tephritidae) Populations Exploiting Host Plants with Different Fruiting Phenology

Photoperiod has been found to influence the proportion of non-dormant individuals and the duration of dormancy among North American populations of Rhagoletis pomonella (Walsh) (Diptera: Tephritidae). In particular, long days combined with high temperatures can produce a 100% non-dormant generation. There are several genetically distinct populations of R. pomonella in the highlands of Mexico, two of which occur at the same latitude, at different elevations, that exploit hawthorn plants with different fruiting phenology (early (summer-fall) and late (winter)). Flies exploiting such hosts might use day length in different ways as a cue to match adult emergence with fruit availability. Here, we compared responses of pupae from two Mexican populations of R. pomonella to a 12/12 L/D photoperiod, a long-day photoperiod 17/7 L/D, and continuous illumination. Experiments were performed under warm conditions (27°C). Day length had no effect on the proportion of adults emerged from the early-fruiting hawthorn population, while pupae extracted from late-fruiting hawthorns and exposed to long days emerged as adults in lower proportions and engaged in prolonged dormancy in greater proportions than those exposed to a short day. Photoperiod had no effect on the proportion from both Mexican populations foregoing dormancy. Dormancy duration was affected by photoperiod and was longer than previous reports for North American populations. Host plant fruiting phenology may be the driver of these differences. These findings, coupled with previous reports of genetic and biological differences, suggest that the Eje Volcanico Trans-Mexicano R. pomonella population may actually be a distinct species.

The Effect of Seasonal Humidity on Survival and Duration of Dormancy on Diverging Mexican Rhagoletis pomonella (Diptera: Tephritidae) Populations Inhabiting Different Environments

Phytophagous insects synchronize emergence with plant phenology by engaging in dormancy during periods of host scarcity and environmental stress. Regulation of dormancy is achieved through response to seasonal cues. While temperature and photoperiod are important cues in temperate latitudes, seasonal humidity, such as the onset of rains, can be a reliable cue to for synchronization of emergence and affects survival of overwintering insects. We compared response of Mexican Rhagoletis pomonella (Walsh) (Diptera: Tephritidae) populations inhabiting subtropical environments differing in humidity patterns, to seasonal humidity regimes. Both populations emerged as adults in high proportions and suffered lower mortality under humidity regimes ending with a humid summer, but the effect was more pronounced for the Eje Volcanico Trans Mexicano (EVTM) population, which inhabits a dryer environment and undergoes longer dormancy. While there were no differences among pupae from the Sierra Madre Oriental (SMO) in percent of non-emerged pupae surviving and engaging in prolonged dormancy after a year, EVTM pupae exposed to an initial humid period engaged in prolonged dormancy in higher proportions than those exposed to other regimes. Seasonal humidity had little effect on the duration of dormancy, but EVTM pupae exposed to consecutive dry periods took longer to emerge than those exposed to other regimes. Our results suggest that rather than being used as a token stimulus, humidity affected survival of overwintering R. pomonella, especially at the end of dormancy when energy reserves are depleted and there is an increase in metabolic rate that renders EVTM pupae more susceptible to desiccation.

The Effect of Winter Length on Duration of Dormancy and Survival of Rhagoletis completa (Diptera: Tephritidae) and Associated Parasitoids From Northeastern Mexico

The walnut husk fly Rhagoletis completa (Cresson), native to the Midwestern United States and Mexico, is invasive in California and Europe. It is one of the most important pests of walnuts in areas gathering 30% of the world production. Knowledge of life-history regulation is important for the design of management strategies. Research on dormancy has been performed on invasive populations, and not on populations at the southern extreme of its native range. Here, we examined the effect of winter length on fly and parasitoid emergence, survival, and duration of dormancy. Percent emergence was higher for chill periods at 5°C ranging from 8 to 20 wk. No or insufficient chill resulted in low emergence and a significant proportion of individuals in prolonged dormancy (>1 yr). Duration of dormancy was longer for pupae at constant temperatures and a 4-wk chill period than longer winter durations. Dormancy was longer for Mexican than that reported for U.S. populations, suggesting the existence of a latitudinal cline where populations at southern latitudes have evolved slower metabolic rates. Three parasitoid species were found associated with R. completa (Aganaspis alujai (Wharton and Ovruski) (Hymenoptera: Figitidae), Diachasmimorpha juglandis Muesebeck, and Diachasmimorpha mellea Gahan) (Hymenoptera: Braconidae). Results suggest that rearing of R. completa is possible by subjecting pupae to chill periods between 8 and 20 wk. Overwintering mortality of flies and A. alujai could be further reduced above 5°C. Our findings can contribute for the accurate development of predictive models on invasion potential, development, fly and parasitoid rearing, and biological control.

The effect of winter length on duration of dormancy and survival of specialized herbivorous Rhagoletis fruit flies from high elevation environments with acyclic climatic variability

Dormancy can be defined as a state of suppressed development allowing insects to cope with adverse conditions and plant phenology. Among specialized herbivorous insects exploiting seasonal resources, diapause frequently evolves as a strategy to adjust to predictable plant seasonal cycles. To cope with acyclic and unpredictable climatic events, it has been found for some insects that a proportion of the population undergoes prolonged dormancy. We compared the response of three species in the Rhagoletis cingulata species group exploiting plants differing in fruiting phenology from environments varying in frequency and timing of acyclic climatic catastrophic events (frost during flowering and fruit set) and varying also in the time of the onset of the rainy season. Small proportions (10 months), and large proportions of pupae died without emerging as adults. The number of days elapsed from the end of artificial winter and adult eclosion was longer for R. cingulata exploiting late fruiting Prunus serotina in Northeastern Mexico than for flies recovered from earlier fruiting plants in the central Altiplano. Rhagoletis turpiniae and northeastern R. cingulata pupae suffered high proportions of parasitism. Large proportions of R. cingulata from central Mexico engaging in prolonged dormancy may be explained by the fact that flowering and fruit set for its host, P. serotina var capuli, driven by the timing of maximum precipitation, matches a period of highest probability of frost often resulting in large areas with fruitless trees at unpredictable time intervals. As a consequence of differences in host plant fruiting phenology, central and northeastern Mexican R. cingulata were found to be allochronically isolated. Prolonged dormancy may have resulted in escape from parasitism.