Data from: Differential aphid toxicity to ladybeetles is not a function of host plant or facultative bacterial symbionts

Herbivores often defend themselves from predation by transmitting toxic
plant-produced chemicals to their enemies. Polyphagous herbivores
sometimes exhibit differential toxicity when found on various host plant
species, which is generally assumed to reflect variation in plant
chemistry. Here, however, we provide evidence that host-associated
herbivore lineages can intrinsically differ in their toxic properties.
Lineages of Aphis craccivora originating from black locust (Robinia
pseudoacacia) are unsuitable food for the ladybeetle Harmonia axyridis,
resulting in death of both larvae and adults, whereas aphid lineages
originating from alfalfa (Medicago sativa) support larval development and
adult reproduction. We show that locust-origin aphids remain toxic and
alfalfa-origin aphids remain non-toxic when reared on any of three legume
plants (fava, alfalfa or locust). Furthermore, toxicity is not a function
of the facultative bacterial symbiont Arsenophonus, which is naturally
present in locust-origin aphid lineages and facilitates aphid use of
locust. Experimentally cured locust-origin lineages remain toxic, and an
experimentally transinfected alfalfa-origin lineage remains non-toxic to
H. axyridis. Instead, Arsenophonus plays an indirect role in the
distribution of toxic aphid lineages by facilitating aphid use of locust.
It is the parthenogenetic coinheritance of Arsenophonus and the toxic
trait that observationally correlates locust-feeding with toxicity in A.
craccivora, rather than host plant chemistry per se. Our results clearly
demonstrate that aphid lineages intrinsically vary in their toxic
properties in a way that neither plant chemistry nor bacterial symbionts
can explain. A more inclusive paradigm is needed for understanding
variation in herbivore defence against predators.