Aphid lineage rather than host plant determines Aphis craccivora toxicity to ladybeetles

Ladybeetles often use aphids as a food source, but some aphids can be toxic to ladybeetles. Some aphid species, such as the cowpea aphid, Aphis craccivora, show variable levels of toxicity. Previously, this variability was presumed to be the result of aphid host plant; aphid populations collected from some host plants (e.g., black locust, Robinia pseudoacacia) were consistently toxic, whereas populations collected from other host plants were not. An alternative hypothesis is that differential aphid toxicity is a result of aphid lineage, rather than being attained from the host plant. To test this hypothesis, I conducted a series of feeding trials using A. craccivora colonies that originated from two different plants: black locust and alfalfa (Medicago sativa).  Colonies were separately maintained on fava (Vicia faba) for a year prior to experimentation, to remove potential maternal effects. Aphids were fed to first instar Harmonia axyridis, an invasive ladybeetle species. Alfalfa-origin aphids supported ladybeetle development, with 86% surviving to adulthood. Locust-origin aphids, however, were lethal, with an average ladybeetle survival time of 4.0±0.1 days. Because both aphid lineages fed on the same host plant, this result shows that toxicity is a function of aphid lineage, rather than host plant.  I conducted similar experiments using the native ladybeetle Coleomegilla maculata, and found that C. maculata  could survive to adulthood on both locust- and alfalfa-origin aphids, although the locust-origin aphids resulted in significantly slower development and slightly smaller adults.  Locust-origin craccivora may therefore play an important role in conserving subdominant native lady beetles species.