The complex interactions between nutrition, immunity and infection in insects

Chronic exposure to nicotine in diet enhances the lethal effect of an entomopathogenic fungus in the ant Cardiocondyla obscurior

Nicotine is a naturally occurring alkaloid that has acute toxic effects for insects and affects their behaviour even in sublethal amounts. In addition, nicotine is shown to accumulate and pollute environments through the use of commercially produced pesticides and tobacco products. We investigated how nicotine-polluted diets in two different concentrations impacted colony fitness in the ant Cardiocondyla obscurior, compared to a nicotine-free diet. We measured brood production and development, changes in relative abundances of bacterial endosymbionts, and worker survival in combination with a fungal pathogen. Chronic exposure to nicotine caused a concentration-dependent effect in enhancing the lethality of the fungal infection, with higher concentrations causing higher mortality in infected colonies. In the absence of pathogens, nicotine had no effect on worker survival. Furthermore, nicotine did not affect brood production or development, nor clearly affect the abundances of the bacterial endosymbionts. Our results show that nicotine pollution in the environment can negatively affect ant fitness through synergistic effects in combination with a fungal pathogen. Pathogens play a significant part in the decline of insects, and the influence that nicotine pollution may have in exacerbating them should receive more attention.

Strategies to Mitigate the Adverse Impacts of Viral Infections on Honey Bee (Apis mellifera L.) Colonies

Honey bees (Apis mellifera) play a crucial role in global food production through the pollination of various crops. These vital insects are susceptible to a range of viral pathogens that can disrupt their normal behavior and physiology, ultimately affecting colony dynamics and survival. There are diverse viruses that infect honey bees at different life stages, with a year-round prevalence. There are multiple pathways through which viruses can be transmitted among colonies. Notably, there is also a lack of commercial treatments against viral infections in bees, but some promising strategies exist to mitigate their negative effects, including vector control, and the implementation of good beekeeping practices and biosecurity measures. While methods for treating infected colonies have garnered attention, they receive less focus compared to aspects like transmission methods and seasonal prevalence of viruses. This article aims to review the aforementioned strategies in light of the available literature. It presents succinct and practical approaches categorized based on their potential direct or indirect effects on viruses, providing beekeepers and researchers with an overview of both fully established and still-developing methods. Controlling the ectoparasitic Varroa destructor mite population, which significantly impacts viral prevalence and virulence in bees, is crucial for reducing infections. Practical approaches such as selectively breeding honey bee populations resistant to viruses and ensuring proper nutrition are important strategies. Moreover, genetic methods have also been proposed and tested. The article not only emphasizes these methods but also discusses knowledge gaps and suggests future solutions to improve the health and productivity of honey bee colonies.

Crop diversity induces trade-offs in microbial biopesticide susceptibility that could delay pest resistance evolution

Pathogens often exert strong selection on host populations, yet considerable genetic variation for infection defence persists. Environmental heterogeneity may cause fitness trade-offs that prevent fixation of host alleles affecting survival when exposed to pathogens in wild populations. Pathogens are extensively used in biocontrol for crop protection. However, the risks of pest resistance evolution to biocontrol are frequently underappreciated: the key drivers of fitness trade-offs for pathogen resistance remain unclear, both in natural and managed populations. We investigate whether pathogen identity or host diet has a stronger effect on allelic fitness by quantifying genetic variation and covariation for survival in an insect pest across distinct combinations of fungal pathogen infection and plant diet. We demonstrate substantial heritability, indicating considerable risks of biopesticide resistance evolution. Contrary to conventional thinking in host-pathogen biology, we found no strong genetic trade-offs for surviving exposure to two different fungal pathogen species. However, changes in plant diet dramatically altered selection, revealing diet-mediated genetic trade-offs affecting pest survival. Our data suggest that trade-offs in traits not strictly related to infection responses could nevertheless maintain genetic variation in natural and agricultural landscapes.

Timing of starvation determines its effects on susceptibility to bacterial infection in female fruit flies independent of host evolutionary history

An organism's susceptibility to pathogens is contingent on various environmental factors, including the availability of nutrition. Starvation can alter host susceptibility to infections, either directly via depletion of resources essential for proper functioning of the immune system, or indirectly via the various physiological changes it induces within the host body. We tested if the susceptibility of Drosophila melanogaster populations to Enterococcus faecalis infection is interactively affected by (a) whether the hosts are starved before or after the infection, and (b) the evolutionary history of the host. Hosts from laboratory fly populations that have been experimentally evolved to be more resistant to E. faecalis, and their corresponding control populations, were subjected to infection with or without being starved prior to and after being infected. We found that the effect of starvation on susceptibility to E. faecalis changed with the timing of starvation: starvation after infection improved survival of infected hosts, irrespective of how they were treated before infection, while starving only prior to infection (and not after) compromised post-infection survival. The changes in infection susceptibility were uniform in both the evolved and the control populations, suggesting that the effects of starvation are not dependent on pre-existing resistance to the infecting pathogen.

Ecological immunology: do sexual attraction and immunity trade-off through a desaturase?

Given the limited availability of resources in nature, sexual attractiveness may trade off with immunocompetence, as the immunocompetence handicap hypothesis (ICHH) posits. In invertebrates, a direct link between trade-offs through hormonal/molecular effectors in sexual signals and immunity has not been found so far. Here, we assessed how variation in sexual signals affected parasite infection in two sex pheromone selected lines of the moth Chloridea virescens: an attractive line with a low ratio of 16:Ald/Z11-16:Ald and an unattractive line with a high ratio. When infecting these lines with an apicomplexan parasite, we found that the attractive Low line was significantly more susceptible to the parasite infection than the unattractive High line. Since the ratio difference between these two lines is determined by a delta-11-desturase, we hypothesized that this desaturase may have a dual role, i.e., in the quality of the sexual signal as well as an involvement in immune response, comparable to testosterone in vertebrates. However, when we used CRISPR/cas9 to knockout delta-11-desturase in the attractive Low line, we found that the pheromonal phenotype did change to that of the High line, but the infection susceptibility did not. Notably, when checking the genomic location of delta-11-desaturase in the C. virescens, we found that mucin is adjacent to delta-11-desaturase. When comparing the mucin sequences in both lines, we found four nonsynonymous SNPs in the coding sequence, as well as intronic variation between the two lines. These differences suggest that genetic hitchhiking may explain the variation in susceptibility to parasitic infection.

Insect hosts are nutritional landscapes navigated by fungal pathogens

Nutrition can mediate host-pathogen interactions indirectly when specific deficiencies (e.g., iron or glutamine) constrain host immune performance. Nutrition can also directly govern these interactions as invading pathogens colonize finite landscapes of nutritionally variable host tissues that must be optimally foraged during pathogen development. We first used a conceptual framework of nutritional niches to show that insect-pathogenic Metarhizium fungi navigate host landscapes where different tissues vary widely in (protein [P] and carbohydrates [C]). We next tested whether host-specific Metarhizium species have narrower fundamental nutritional niches (FNNs) than host-generalists by measuring pathogen performance across an in vitro nutritional landscape simulating a within-host foraging environment. We then tested how developing pathogens navigate nutritional landscapes by developing a liquid-media approach to track pathogen intake of P and C over time. Host-specificity did not govern FNN dimensions, as the three tested Metarhizium species: (1) grew maximally across C treatments assuming P was present above a lower threshold, and (2) similarly initiated dispersal behaviors and sporulated when either C or P became depleted. However, specialist and generalist pathogens navigated nutritional landscapes differently. The host specialist (M. acridum) first prioritized C intake, but generalists (M. anisopliae, M. robertsii) prioritized P and C according to their availability. The numbers of known hosts may be insufficient to delimit pathogens as specialists or generalists as diverse hosts do not necessarily comprise diverse nutritional landscapes. Instead, the immune responses of hosts and nutritional niche breadth of pathogens are likely co-equal evolutionary drivers of host specificity.

Harnessing Insect Chemosensory and Mechanosensory Receptors Involved in Feeding for Precision Pest Management

Chemosensation and mechanosensation are vital to insects' survival and behavior, shaping critical physiological processes such as feeding, metabolism, mating, and reproduction. During feeding, insects rely on diverse chemosensory and mechanosensory receptors to distinguish between nutritious and harmful substances, enabling them to select suitable food sources while avoiding toxins. These receptors are distributed across various body parts, allowing insects to detect environmental cues about food quality and adjust their behaviors accordingly. A deeper understanding of insect sensory physiology, especially during feeding, not only enhances our knowledge of insect biology but also offers significant opportunities for practical applications. This review highlights recent advancements in research on feeding-related sensory receptors, covering a wide range of insect species, from the model organism Drosophila melanogaster to agricultural and human pests. Additionally, this review examines the potential of targeting insect sensory receptors for precision pest control. Disrupting behaviors such as feeding and reproduction emerges as a promising strategy for pest management. By interfering with these essential behaviors, we can effectively control pest populations while minimizing environmental impacts and promoting ecological balance.