Regiella insecticola is a bacterial endosymbiont in insects that exhibits a negative effect on the fitness of hosts. Thus, it is not clear why this costly endosymbiont can persist in host populations. Here, we tested a hypothesis that the infection pattern and negative roles of the endosymbiont were not constant but environmentally dependent. The grain aphids Sitobion avenae , belonging to different genotypes and infected with Regiella or not, were used in this study. We found that S . avenae populations were infected with Regiella , Hamiltonella defensa , Serratia symbiotica and Rickettsia . The predominant endosymbionts in the aphid populations varied with season. Serratia and Rickettsia were predominant from December to February while Regiella predominated from March to May. The vertical transmission of Regiella was poorer at high temperature, but following conditioning for seven generations, the transmission rate improved. Regiella inhibited the production of winged aphids at 25 °C, but it did not affect winged morph production at the higher temperatures of 28 °C and 31 °C. Regiella infection decreased the intrinsic rate of increase ( r m ) of aphids at 25 °C and 28 °C. However, at 31 °C, the effect of Regiella on the r m varied depending on the aphid genotype and density. Thus, the negative effects of this endosymbiont on its host were environmentally dependent.

Polyphagous cotton-melon aphid populations usually comprise cotton- and cucurbit-specialized biotypes. Host-specialized aphids are prone to food shortages. Cucumber, the favourite food of cucurbit-specialized aphids, is usually absent during autumn and winter in Nanjing, China. Therefore, suboptimal host plants act as refuges and govern the population dynamics of this aphid. The species, growth stages and leaf ages of host plants that cotton- and cucurbit-specialized aphids potentially could use were explored in this study. Cotton-specialized aphids were found to use wild chrysanthemum, potato, zucchini, pumpkin and flowering cucumber besides cotton, whilst cucurbit-specialized aphids were able to utilize potato, zucchini, pumpkin and mature cotton besides cucumber. The population dynamics and genotype frequencies of aphids on hibiscus, cotton, zucchini, cucumber and pumpkin showed that cotton-melon aphids on cucumber could transfer onto mature cotton. Aphids on zucchini shared microsatellite genotypes with aphids on cotton and cucumber. The predominant genotype of aphids on cotton was found on hibiscus, but the predominant genotype on cucumber was not found on hibiscus. Host-specialized aphids clearly have refuges during food shortages. Hibiscus is an overwintering host for cotton-specialized aphids but not for cucurbit-aphids. Removing refuges or managing aphids on refuges could potentially be an effective method to control cotton-melon aphids.

Host specialization is an ubiquitous character in aphid populations. Many polyphagous aphid populations usually consist of several subpopulations that have strong fidelity to a specific host or a subset of host range. Host specialization is an evolutional result of food habit of insects. However, genetic basis and molecular mechanism of host specialization are still unclear. In this study, we presented a comparative analysis on global gene expression profiles of three lineages of Aphis gossypii Glover: cotton-specialized (CO), cucurbit-specialized (CU), and CU reared on cowpea (CU-cowpea), using RNA-Seq method. More than 157 million clean reads and 38,398 different unigenes were generated from transcriptomes of these three aphid lineages. The 1,106 down- and 2,835 up-regulated genes were found between CO and CU, and 812 down- and 14,492 up-regulated genes between CU-cowpea and CU. Differentially expressed genes between CO and CU were enriched in sugar metabolism, immune system process, pathogen infection or symbiosis, and salivary secretion. Genes associated with cytochrome P450, major facilitator superfamily, and salivary effector were differentially expressed between CO and CU, which might be involved in determining host specialization. UDP-glycosyltransferases genes were sensitive to host shift. Carboxylesterases and digestion-related protease genes were related to both the host specialization and host shift of aphids. Expression levels of 22 out of 24 genes of CO and CU measured by RT-qPCR method were as similar as the results from RNA-seq method. This study provides a road map for future study on molecular mechanism of host specialization in aphids.

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To escape or alleviate low temperatures in winter, insects have evolved many behavioral and physiological strategies. The purple stem borer, Sesamia inferens (Walker) is currently reported to be expanding their northern distributions and causing damage to summer maize in Xinxiang, China. However, their method of coping with the lower temperature in the new northern breeding area in winter is largely unknown. This paper investigates the overwinter site of S. inferens, and identifies the cold hardiness of larvae collected from a new breeding area in winter and explores a potential distribution based on low temperature threshold and on species distribution model MaxEnt. The results show that the overwintering location of the S. inferens population is more likely to be underground with increasing latitude and the population gradually moved down the corn stalk and drilled completely underground in later winter (February) in the north. The cold hardiness test shows the species is a moderate freeze-tolerant one, and Supercooling Points (SCP), Freezing Points (FP) and the incidence of mortality during the middle of winter (January, SCP:-7.653, FP:-6.596) were significantly lower than early winter (October) or late winter (March). Distribution in the new expansion area was predicted and the survival probability area was below N 35˚for the Air Lower Lethal Temperature (ALLT 50) and below N 40˚for the Underground Lower Lethal Temperature (ULLT 50). The suitable habitat areas for S. inferens with MaxEnt were also below N 40˚. This study suggests the overwinter strategies of S. inferens have led to the colonization of up to a five degree more northerly overwintering latitude.