Oviposition behavior: Host plant matters more than geographic origin

Background/Question/Methods: The process of local adaptation is a putative mechanism that ultimately enables ecological speciation, which posits that populations adapt to different environments or resources, generating divergence in taxa. We are studying a novel system of weevils (Rhyssomatus spp.) that includes populations that seem to be differentially adapted to feed on stems versus seedpods of milkweed (Asclepias spp.) as a result of adult oviposition behavior. Is this difference in oviposition behavior the result of host specialization facilitated by local adaptation to milkweed? We collected adult weevils from 18 populations in the Northeastern USA. At each location, we recorded oviposition behavior on stems and on seedpods if available. We then took these weevils to a greenhouse and performed bioassays on Asclepias syriaca and Asclepias incarnata to determine if weevils from different locations changed their oviposition behavior depending on the host plant and plant tissue available (stems vs. seedpods).

Results/Conclusions: In the field, we found that weevils manipulate the stems of milkweed plants in different ways depending on location. Weevils in the south of the range (Virginia, USA) had different ovipositon behaviors to those in the north (Ontario, Canada). Weevils from the south pierce rings surrounding the stems before oviposition (i.e., girdling) and their larvae are found in seedpods more often than in northern populations. Some of these manipulations, such as girdling of the apical meristem, produce stem death at an early stage of plant development. While in other cases where this behavior does not occur, the plant is less damaged by the developing larva inside the stem. However, these differences were not observed in the bioassays, and location of origin did not affect whether the weevil girdles or not. Instead, plant species (i.e., A. incarnata or A. syriaca) had an effect on girdling behavior.  Weevils in the bioassays girdled more often on A. incarnata than on A. syriaca (P>0.05).  Girdling behavior was not observed when flowers or seedpods were present. This study suggests that weevils might be able to adjust their oviposition behavior depending on host plant and resource availability. It is still unclear whether this difference in behavior is the result of local adaptation to milkweed patches or phenotypic plasticity facilitated by different environments.