Larval Exposure to Parasitic Varroa destructor Mites Triggers Specific Immune Responses in Different Honey Bee Castes and Species

Innate immune systems are key defenses of animals and particularly important in species that lack the sophisti-cated adaptive immune systems as found in vertebrates. Here, we were interested to quantify variation in innate immune responses of insects in hosts that differ in their parasite susceptibility. To do this, we studied immune responses in honey bees, which can host a remarkable number of different parasites, which are major contribu-tors of declining bee health and colony losses. The most significant parasite of honey bees is the mite Varroa destructor, which has infested the majority of global honey bee populations, and its control remains a major challenge for beekeepers. However, a number of nonmanaged honey bees seem able to control Varroa infections, for example, the Eastern honey bee Apis cerana cerana or the African honey bee Apis mellifera scutellata. These bees therefore make interesting study subjects to identify un-derlaying resistance traits, for example, by comparing them to more susceptible bee genotypes such as Western honey bees (A. mellifera ligustica). We conducted a series of interlinked experiments and started with behavioral assays to compare the attractiveness of bee larvae to mites using different honey bee genotypes and castes. We found that 6-day-old larvae are always most attractive to mites, independently of genotype or castes. In a next step, we compared volatile profiles of the most attractive larvae to test whether they could be used by mites for host se-lection. We found that the abundance of volatile com-pounds differed between larval ages, but we also found significant differences between genotypes and castes. To further study the expected underlaying physiological dif-ferences between potentially resistant and susceptible host larvae, we compared the larval hemolymph pro-teomes of the three honey bee genotypes and two castes in response to mite exposure. We identified consistent upregulation of immune and stress-related genes in Var-roa-exposed larvae, which differed between genotypes and castes. Tolerant honey bee castes and genotypes were characterized by stronger or more distinct immune esponses. In summary, we provide first insights into the complex involvement of the innate immune system of tolerant honey bees against mite infestations, which could be used for future breeding purposes.

Automated summary

There is variation in innate immune responses of insects in hosts that differ in their parasite susceptibility. Some honey bee genotypes and populations show stronger immune responses against Varroa infestations, which is important for future breeding purposes.