Shifts in reproductive strategy by Hemileuca eglanterina (Lepidoptera: Saturniidae) in response to prescribed burning

Phytophagous insects should allocate reproductive with either resource abundance or host plant quality if they can accurately assess resources for offspring performance. There is often a temporary pulse of nitrates in the soil following fires, which, may lead to a concurrent increase in plant N-content potentially generating an ephemeral landscape of nutritious host plants for phytophagous insects. In a western Oregon wetland restricted metapopulation of sheepmoth, Hemileuca eglanterina, I measured egg clutch size and the relationship to host plant (Rosa nutkana) abundance, in two pairs of adjacent burned and unburned prairie parcels to understand the potential shifts in reproductive strategy (resource abundance vs host quality) as a consequence response of fire. To assess whether there could be a selective advantage to selecting host plants in burned habitat, I additionally measured the survival of larvae reared in a common garden environment with ad libitum burned and unburned host plants.

In the unburned parcels, egg clutch size (30-180 eggs/clutch) was positively correlated with host plant abundance, suggesting that female sheepmoths adjust reproductive effort to match host resource abundance. However, in the burned parcels there was no statistically significant relationship between host plant abundance and clutch size, as females seemingly disengaged in the host resource matching strategy as a response to fire. Furthermore, the lack of resource matching in the burned habitat appeared to be temporary as egg clutch size and host plant abundance were positively correlated in the burned habitat in the post-burn year. Survival of larvae feed ad libitum host plant foliage was substantially greater (~ 20% through the 3^rd instar) in larvae offered only burned host plants when uncharacteristically warm spring temperatures and disease (likely nuclear polyhedrosis virus) killed all larvae feed unburned host plants. Water foliar leaf content of unburned plants was slightly greater than that of burned host plants fed to the caged larvae but N-content was greater for the burned plants. It appears that female sheepmoth reproductive strategy shift in response to fire may be an adaptive response to periodic fires that are thought to play a role in the maintenance of western Oregon grasslands.