Effects of Extreme Low Winter Temperatures on the Overwintering Survival of the Introduced Larval Parasitoids Spathius galinae and Tetrastichus planipennisi: Implications for Biological Control of Emerald Ash Borer in North America

Introduction to the Collection: Climate Change, Insect Pests, and Beneficial Arthropods in Production Systems

Climate change is expected to alter pressure from insect pests and the abundance and effectiveness of insect pollinators across diverse agriculture and forestry systems. In response to warming, insects are undergoing or are projected to undergo shifts in their geographic ranges, voltinism, abundance, and phenology. Drivers include direct effects on the focal insects and indirect effects mediated by their interactions with species at higher or lower trophic levels. These climate-driven effects are complex and variable, sometimes increasing pest pressure or reducing pollination and sometimes with opposite effects depending on climatic baseline conditions and the interplay of these drivers. This special collection includes several papers illustrative of these biological effects on pests and pollinators. In addition, in response to or anticipating climate change, producers are modifying production systems by introducing more or different crops into rotations or as cover crops or intercrops or changing crop varieties, with potentially substantial effects on associated insect communities, an aspect of climate change that is relatively understudied. This collection includes several papers illustrating these indirect production system-level effects. Together, biological and management-related effects on insects comprise the necessary scope for anticipating and responding to the effects of climate change on insects in agriculture and forest systems.

Fruit Fly in a Challenging Environment: Impact of Short-Term Temperature Stress on the Survival, Development, Reproduction, and Trehalose Metabolism of Bactrocera dorsalis (Diptera: Tephritidae)

An understanding of physiological damage and population development caused by uncomfortable temperature plays an important role in pest control. In order to clarify the adaptability of different temperatures and physiological response mechanism of B. dorsalis, we focused on the adaptation ability of this pest to environmental stress from physiological and ecological viewpoints. In this study, we explored the relationship between population parameters and glucose, glycogen, trehalose, and trehalose-6-phosphate synthase responses to high and low temperatures. Compared with the control group, temperature stress delayed the development duration of all stages, and the survival rates and longevity decreased gradually as temperature decreased to 0 °C and increased to 36 °C. Furthermore, with low temperature decrease from 10 °C to 0 °C, the average fecundity per female increased at 10 °C but decreased later. Reproduction of the species was negatively affected during high-temperature stresses, reaching the lowest value at 36 °C. In addition to significantly affecting biological characteristics, temperature stress influenced physiological changes of B. dorsalis in cold and heat tolerance. When temperature deviated significantly from the norm, the levels of substances associated with temperature resistance were altered: glucose, trehalose, and TPS levels increased, but glycogen levels decreased. These results suggest that temperature stresses exert a detrimental effect on the populations' survival, but the metabolism of trehalose and glycogen may enhance the pest's temperature resistance.

Gradually Increasing the Temperature Reduces the Diapause Termination Time of Trichogramma dendrolimi While Increasing Parasitoid Performance

Trichogramma dendrolimi Matsumura is widely used as a biological control agent of many lepidopteran pests. Diapause has been used as an effective method to preserve the Trichogramma products during mass rearing production. However, it currently takes at least 70 days to break diapause, and we tested whether gradually increasing the temperature instead of using constant temperature could reduce the time of diapause termination and offer a higher flexibility to Trichogramma producers. The diapause termination rates of individuals kept at different conditions were measured, and five groups for which diapause termination rate reached the 95% were selected to test five biological parameters, including the number of eggs parasitized, the parasitism and emergence rates, the female sex ratio, the wing deformation rate, and the parasitoid longevity. Compared to the currently used procedure (70 days at 3 °C), treatments with at least two different temperatures resulted in higher parasitism and emergence rates while keeping the other parameters constant. The treatment that consisted of at least two different temperatures preceded by only 55 days of induction period had the highest population trend index, meaning that the population under these conditions grows more rapidly. Our results demonstrate that gradually increasing temperature allows T. dendrolimi to complete diapause earlier than at present while increasing its potential pest control capacity and providing additional flexibility in mass production of T. dendrolimi.

Five Visual and Olfactory Target Genes for RNAi in Agrilus Planipennis

RNA interference (RNAi) is a widely used technique for gene function researches and recently pest controls. It had been applied in emerald ash borer (EAB Agrilus planipennis) larvae and adults, and achieved significant interference effects, whether by ingesting or microinjecting. Feeding in the phloem and cambial regions, the larvae of A. planipennis are difficult to be controlled by conventional insecticides, so adult stage is the critical stage for EAB control. However, the target genes of adult stage of A. planipennis need to be further screened. Here, we preliminarily screened five potential target genes of vision and olfaction for RNAi in A. planipennis. Three odorant binding proteins (OBPs) and three opsins, which expressed significantly different between newly emerged and sexually mature EABs (OBP5, OBP7, OBP10, LW opsin 1 and UV opsin 2) or highly in sexually mature male EAB (UV opsin 3), were selected as targets to design primers for gene silencing. After dsRNA injection, the gene expression levels were determined by real-time quantitative PCR. We found that the expression levels of five genes were significantly down-regulated, during the 4 days after dsRNA injection. Among these genes, the expression of LW opsin 1 was down-regulated the most, causing a reduction of 99.1% compared with the control treated with EGFP dsRNA, followed by UV opsin 3 (97.4%), UV opsin 2 (97.0%), OBP7 (96.2%), and OBP10 (88.7%). This study provides a basis for further RNAi-based new controlling method development of A. planipennis at adult stage.

Duration of Low Temperature Exposure Affects Egg Hatching of the Colorado Potato Beetle and Emergence of Overwintering Adults

Simple Summary

The Colorado potato beetle is a pest of Solanaceae in China, more than 30 species of Solanaceae as host plants of the Colorado potato beetle, and this beetle can cause yield losses to potato farms. Temperature is one of the main factors that affect the growth and survival of insects. In recent years, due to global climate change, sudden drops in short-term temperatures are also frequent, which will affect insects. This paper studies the influence of short-term low temperature on the Colorado potato beetle. In laboratory experiments, we evaluated the effect of short-term low temperature on the Colorado potato beetle eggs, and the emerged date, the number of Colorado potato beetles, were analyzed in combination with the low temperature conditions. Our results show that low temperature had adverse effects on the development of Colorado potato beetle eggs and the emerged of adults Colorado potato beetles, and the longer the low temperature treatment time, the greater the impact.

Abstract

The Colorado potato beetle is a serious pest of Solanaceae in China. In early summer, cold spells in later spring may occur for brief periods in the field environmental conditions, and temperatures often deviate far below the normal temperature for short periods, such as sudden short-term low temperature, may affect the development of Colorado potato beetle eggs. This paper studies the effects of low temperature stress at 8 °C for 0 d, 1 d, 3 d, 5 d, 7 d, and 10 d on the development of Colorado potato beetle eggs. Our results show that egg survival is significantly affected by short-term low temperature exposure. The percentage of eggs hatched is significantly affected by different treatment times (p = 0.000)—the percentage of eggs hatched decreases with increased treatment time, and Colorado potato beetles will extend the wintering time of their soil to resist the effects of lower temperatures. Thus, exposure of Colorado potato beetles to a short-term low temperature affects their emergence and population growth; this study could provide information for the occurrence, monitoring, and early warning of Colorado potato beetle during short-term temperature.

Optimal Conditions for Diapause Survival of Aprostocetus fukutai, an Egg Parasitoid for Biological Control of Anoplophora chinensis

Simple Summary

Diapause is a critical state of an insect’s life cycle when it undergoes arrestment of growth and/or reproduction to survive adverse environmental conditions and/or food shortage. Aprostocetus fukutai is a specialist egg parasitoid of the citrus longhorned beetle, Anoplophora chinensis, a high-risk invasive woodboring pest. The parasitoid overwinters as diapausing mature larva in the host egg and emerges in early summer in synchrony with the egg-laying of A. chinensis. Here, we determined the optimal conditions for diapause survival of this parasitoid. We showed that the parasitoid had a low (36.7%) diapause survival rate inside host eggs laid on potted plants due to desiccation or tree wound defense response under semi-natural conditions. Under laboratory rearing conditions, when parasitized host eggs were extracted from wood, the parasitoid did not survive at low humidity (44% RH). Survival rate increased with humidity, reaching the highest at 100% RH. Survival rate also increased with increasing chilling period temperature from 2 to 12.5 °C. Post-diapause developmental time decreased with increased humidity or temperature, but the reproductive fitness of the parasitoid was not significantly affected by the temperature regimes. Overall, high humidity (100% RH) and mild temperatures (12.5 °C) are the most suitable survival conditions for the diapausing parasitoid and thus should be used in laboratory rearing.

Abstract

Aprostocetus fukutai is a specialist egg parasitoid of the citrus longhorned beetle Anoplophora chinensis, a high-risk invasive pest of hardwood trees. The parasitoid overwinters as diapausing mature larvae within the host egg and emerges in early summer in synchrony with the egg-laying peak of A. chinensis. This study investigated the parasitoid’s diapause survival in parasitized host eggs that either remained in potted trees under semi-natural conditions in southern France or were removed from the wood and held at four different humidities (44, 75, 85–93 and 100% RH) at 11 °C or four different temperature regimes (2, 5, 10 and 12.5 °C) at 100% RH in the laboratory. The temperature regimes reflect overwintering temperatures across the parasitoid’s geographical distribution in its native range. Results show that the parasitoid resumed its development to the adult stage at normal rearing conditions (22 °C, 100% RH, 14L:10D) after 6- or 7-months cold chilling at both the semi-natural and laboratory conditions. It had a low survival rate (36.7%) on potted plants due to desiccation or tree wound defense response. No parasitoids survived at 44% RH, but survival rate increased with humidity, reaching the highest (93.7%) at 100% RH. Survival rate also increased from 21.0% at 2 °C to 82.8% at 12.5 °C. Post-diapause developmental time decreased with increased humidity or temperature. There was no difference in the lifetime fecundity of emerged females from 2 and 12.5 °C. These results suggest that 100% RH and 12.5 °C are the most suitable diapause conditions for laboratory rearing of this parasitoid.

Host Overwintering Phenology and Climate Change Influence the Establishment of Tetrastichus planipennisi Yang (Hymenoptera: Eulophidae), a Larval Parasitoid Introduced for Biocontrol of the Emerald Ash Borer

Emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), is one of the most serious forest pests in the United States. Ongoing research indicates that establishment of larval parasitoids depends upon the season-long availability of host stages susceptible to parasitism. We monitored emerald ash borer overwintering stages at 90 sites across 22 states to: 1) produce a model of the percentage of emerald ash borer overwintering as non-J larvae; 2) link that model to establishment of Tetrastichus planipennisi; and 3) explore changes to our model under climate change scenarios. Accumulated growing degree days (GDD) is an important predictor of the proportion of emerald ash borer overwintering as non-J larvae (1-4 instar larvae under the bark; available to parasitoids emerging in spring) versus J-larvae (fourth-instar larvae in pupal chambers in the outer wood; unavailable to parasitoids). From north to south, the availability of non-J emerald ash borer larvae in the spring decreases as accumulated GDD increases. In areas where the model predicted >46-75%, >30-46%, >13-30%, or ≤13% of emerald ash borer overwintering as non-J larvae, the probability of establishment of T. planipennisi was 92%, 67%, 57%, and 21%, respectively. We determined that 13% of emerald ash borer overwintering as non-J larvae was the lowest threshold for expected T. planipennisi establishment. Additional modeling predicts that under climate change, establishment of T. planipennisi will be most affected in the Central United States, with areas that are currently suitable becoming unsuitable. Our results provide a useful tool for the emerald ash borer biological control program on how to economically and successfully deploy emerald ash borer biological control agents.