Rapid evolution to host plant resistance by an invasive herbivore: soybean aphid (Aphis glycines) virulence in North America to aphid resistant cultivars

Effects of previous infestation with Bemisia tabaci MEAM1 on the resistance to Chrysodeixis includens in Bt and non-Bt soybean plants

Transgenic Bt soybean plants have been developed to control insect pests, such as Anticarsia gemmatalis and Chrysodeixis includens. This objective has been achieved successfully; however, recently, some authors claimed that Bt soybean plants have been more susceptible than non-Bt soybean to Bemisia tabaci MEAM1. In addition, it is unknown whether Bt soybean plants infested by B. tabaci become less resistant to target pests. Therefore, this study aimed to evaluate: (i) whether the previous infestation with B. tabaci can compromise Bt and non-Bt soybean resistance to C. includens; (ii) the effects of B. tabaci infestations on Bt and non-Bt soybean plant growth; and (iii) whether B. tabaci feeding reduces contents of chlorophyll and carotenoids of soybean plants. Bt and non-Bt soybean plants pre-infested with B. tabaci showed no changes in resistance to C. includens. Bt soybean plants infested with B. tabaci showed a lower plant height than uninfested plants. Differently, non-Bt soybean plants exhibited no reduction in plant growth due to B. tabaci feeding. Bt soybean plants suffered a reduction in dry matter only under double infestation (B. tabaci and C. includens), while non-Bt soybean plants experienced reduction in dry matter when infested with B. tabaci and C. includens or by C. includens only. B. tabaci feeding did not alter contents of chlorophyll and carotenoids, and perhaps the reduction in plant growth was related to salivary toxins. Concluding, both Bt and non-Bt soybean plants were susceptible to B. tabaci feeding, evidencing necessity of developing soybean cultivars resistant to B. tabaci.

Museum Genomics of an Agricultural Super-Pest, the Colorado Potato Beetle, Leptinotarsa decemlineata (Chrysomelidae), Provides Evidence of Adaptation from Standing Variation

Despite extensive research on agricultural pests, our knowledge about their evolutionary history is often limited. A mechanistic understanding of the demographic changes and modes of adaptation remains an important goal, as it improves our understanding of organismal responses to environmental change and our ability to sustainably manage pest populations. Emerging genomic datasets now allow for characterization of demographic and adaptive processes, but face limits when they are drawn from contemporary samples, especially in the context of strong demographic change, repeated selection, or adaptation involving modest shifts in allele frequency at many loci. Temporal sampling, however, can improve our ability to reconstruct evolutionary events. Here, we leverage museum samples to examine whether population genomic diversity and structure has changed over time, and to identify genomic regions that appear to be under selection. We focus on the Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say 1824; Coleoptera: Chrysomelidae), which is widely regarded as a super-pest due to its rapid, and repeated, evolution to insecticides. By combining whole genome resequencing data from 78 museum samples with modern sampling, we demonstrate that CPB expanded rapidly in the 19th century, leading to a reduction in diversity and limited genetic structure from the Midwest to Northeast United States. Temporal genome scans provide extensive evidence for selection acting in resistant field populations in Wisconsin and New York, including numerous known insecticide resistance genes. We also validate these results by showing that known selective sweeps in modern populations are identified by our genome scan. Perhaps most importantly, temporal analysis indicates selection on standing genetic variation, as we find evidence for parallel evolution in the two geographical regions. Parallel evolution involves a range of phenotypic traits not previously identified as under selection in CPB, such as reproductive and morphological functional pathways that might be important for adaptation to agricultural habitats.

Exploring the Dynamics of Virulent and Avirulent Aphids: A Case for a 'Within Plant' Refuge

The soybean aphid, Aphis glycines (Hemiptera: Aphididae), is an invasive pest that can cause severe yield loss to soybeans in the North Central United States. A tactic to counter this pest is the use of aphid-resistant soybean varieties. However, the frequency of virulent biotypes that can survive on resistant varieties is expected to increase as more farmers use these varieties. Soybean aphids can alter soybean physiology primarily by two mechanisms, feeding facilitation, and the obviation of resistance, favoring subsequent colonization by additional conspecifics. We developed a nonlocal, differential equation population model to explore the dynamics of these biological mechanisms on soybean plants coinfested with virulent and avirulent aphids. We then use demographic parameters from laboratory experiments to perform numerical simulations via the model. We used this model to determine that initial conditions are an important factor in the season-long cooccurrence of both biotypes. The initial population of both biotypes above the resistance threshold or avirulent aphid close to resistance threshold and high virulent aphid population results in coexistence of the aphids throughout the season. These simulations successfully mimicked aphid dynamics observed in the field- and laboratory-based microcosms. The model showed an increase in colonization of virulent aphids increases the likelihood that aphid resistance is suppressed, subsequently increasing the survival of avirulent aphids. This interaction produced an indirect, positive interaction between the biotypes. These results suggest the potential for a 'within plant' refuge that could contribute to the sustainable use of aphid-resistant soybeans.

Aphid resistance is the future for soybean production, and has been since 2004: efforts towards a wider use of host plant resistance in soybean

The soybean aphid (Aphis glycines) is an important pest of soybeans in the Midwestern US. The first aphid resistance genes were identified in the early 21st century and resistant varieties have been commercially available for 10 years, but have been very underutilized. Major seed companies have avoided commercializing aphid resistant soybean varieties for conventional farmers (i.e., not organic), in part because of the discovery of virulent biotypes in North America. The emergence of soybean aphid populations resistant to insecticides creates a greater incentive for the use of host plant resistance. New research on aphid genetics and markers, plant gene expression and in-plant refuges, suggest important avenues for insect resistance management (IRM) which may encourage more widescale commercialization of this valuable pest management tool.

Dynamics of a predator-prey model with generalized Holling type functional response and mutual interference

Mutual interference and prey refuge are important drivers of predator-prey dynamics. The "exponent" or degree of mutual interference has been under much debate in theoretical ecology. In the present work, we investigate the interplay of the mutual interference exponent, and prey refuge, on the behavior of a predator-prey model with a generalized Holling type functional response - considering in particular the "non-smooth" case. This model can also be used to model an infectious disease where a susceptible population, moves to an infected class, after being infected by the disease. We investigate dynamical properties of the system and derive conditions for the occurrence of saddle-node, transcritical and Hopf-bifurcations. A sufficient condition for finite time extinction of the prey species has also been derived. In addition, we investigate the effect of a prey refuge on the population dynamics of the model and derive conditions such that the prey refuge would yield persistence of the population. We provide additional verification of our analytical results via numerical simulations. Our findings are in accordance with classical experimental results in ecology (Gause, 1934), that show that extinction of predator and prey populations is possible in a finite time period - but that bringing in refuge can effectively yield persistence.

Soybean aphid (Hemiptera: Aphididae) response to lambda-cyhalothrin varies with its virulence status to aphid-resistant soybean

BACKGROUND

Soybean aphid, Aphis glycines, is an invasive insect in North America, considered one of the most important pests of soybean. Their management relies heavily on foliar insecticides, but there is growing effort to expand these tools to include aphid-resistant varieties. We explored if the LC50 and LC25 of lambda-cyhalothrin varied between virulent (resistant to Aphis glycines (Rag) soybeans) and avirulent (susceptible to Rag-genes soybeans) populations of soybean aphid with a leaf-dip bioassay. We also investigated the response to the LC25 of lambda-cyhalothrin on adults (F0) and their progeny (F1) for both avirulent and virulent soybean aphid.

RESULTS

The LC50 of the virulent aphid population was significantly higher compared with the LC50 of the avirulent population. The LC25 significantly reduced fecundity of the F0 generation of avirulent soybean aphid, but no significant effect was observed for virulent aphids. In addition, the LC25 significantly shortened the adult pre-oviposition period (APOP) and lengthened the total pre-oviposition period (TPOP) of avirulent aphids, while the mean generation time (T) was significantly increased. For the virulent aphid, sublethal exposure significantly lengthened development time of first and third instars, TPOP, and adult longevity. In addition, all demographic parameters of virulent soybean aphid were significantly affected when they were exposed to the LC25 of lambda-cyhalothrin.

CONCLUSION

Our results demonstrate lambda-cyhalothrin is less toxic to virulent aphids and exposure to the LC25 can trigger hormesis, which may have implications for the long-term management of this pest with this insecticide as well as with aphid-resistant varieties of soybean. © 2019 Society of Chemical Industry.

Generalism in Nature…The Great Misnomer: Aphids and Wasp Parasitoids as Examples

In the present article we discuss why, in our view, the term ‘generalism’ to define the dietary breadth of a species is a misnomer and should be revised by entomologists/ecologists with the more exact title relating to the animal in question’s level of phagy—mono-, oligo, or polyphagy. We discard generalism as a concept because of the indisputable fact that all living organisms fill a unique ecological niche, and that entry and exit from such niches are the acknowledged routes and mechanisms driving ecological divergence and ultimately speciation. The term specialist is probably still useful and we support its continuing usage simply because all species and lower levels of evolutionary diverge are indeed specialists to a large degree. Using aphids and parasitoid wasps as examples, we provide evidence from the literature that even some apparently highly polyphagous agricultural aphid pest species and their wasp parasitoids are probably not as polyphagous as formerly assumed. We suggest that the shifting of plant hosts by herbivorous insects like aphids, whilst having positive benefits in reducing competition, and reducing antagonists by moving the target organism into ‘enemy free space’, produces trade-offs in survival, involving relaxed selection in the case of the manicured agro-ecosystem.

Additive and epistatic interactions between AKR and AIN loci conferring bluegreen aphid resistance and hypersensitivity in Medicago truncatula

Aphids, including the bluegreen aphid (BGA; Acyrthosiphon kondoi), are important pests in agriculture. Two BGA resistance genes have been identified in the model legume Medicago truncatula, namely AKR (Acyrthosiphon kondoi resistance) and AIN (Acyrthosiphon induced necrosis). In this study, progeny derived from a cross between a resistant accession named Jester and a highly susceptible accession named A20 were used to study the interaction between the AKR and AIN loci with respect to BGA performance and plant response to BGA infestation. These studies demonstrated that AKR and AIN have additive effects on the BGA resistance phenotype. However, AKR exerts dominant suppression epistasis on AIN-controlled macroscopic necrotic lesions. Nevertheless, both AKR and AIN condition production of H2O2 at the BGA feeding site. Electrical penetration graph analysis demonstrated that AKR prevents phloem sap ingestion, irrespective of the presence of AIN. Similarly, the jasmonic acid defense signaling pathway is recruited by AKR, irrespective of AIN. This research identifies an enhancement of aphid resistance through gene stacking, and insights into the interaction of distinct resistance genes against insect pests.

Biological and genetic features of introduced aphid populations in agroecosystems

In agroecosystems, introduced aphids that reproduce by obligate parthenogenesis (OP) show strong biased representation of a few genotypes (superclones), whereas species with cyclical parthenogenesis (CP) exhibit the opposite trend with many unique genotypes. We analyzed the biological and genetic features of 23 different aphid species introduced in different geographic areas and climates, finding putative superclones in about 60% of them. We have examined the proximal causes for aphid establishment and spread after their introduction, and found that OP, host availability, and phenotypic plasticity are among the main variables underpinning the ability of aphids to succeed in new geographic areas, which may explain the high potential for invasion in this group of pest insects.