Shifts in reproductive strategy by Hemileuca eglanterina (Lepidoptera: Saturniidae): Adaptive response to host plant quality following a prescribed burn

Background/Question/Methods

Phytophagous insects should allocate reproductive with either resource abundance or host plant quality if they can accurately assess resources for offspring performance.  Fire can shift the physiological state and patterns of host plant growth, potentially providing either advantageous or detrimental conditions for insect herbivores.  In a rare wetland prairie ecosystem restricted to western Oregon, USA, I studied the reproductive responses of a locally rare, capital breeding moth, Hemileuca eglanterina.  Because H. eglanterina oviposit in only a few large clutches, and progeny fitness is determined entirely by larval resources (adults have degenerate mouth parts), there is likely positive selection favoring the accurate assessment of host plant abundance and quality.  I measured egg clutch size, frequency of oviposition, and relationship to host plant abundance, in a wetland prairie site for two years within burned and adjacent non-burned prairie to determine whether H. eglanterina adjusts clutch size to match host plant (Rosa nutkana) resources.  To verify whether host plants in the prescribed burn were superior in quality to unburned plants, I evenly split larvae from 6 egg clutches laid on unburned plants and raised them in cages where they were fed (ad libitum) host plants from burned/unburned areas and followed larval survival.

Results/Conclusions

In the unburned prairie, egg clutches (30-180 eggs/clutch) were positively correlated with host plant (Rosa nutkana) abundance in both years, suggesting that females assess and regulate egg clutch size with respect to local host plant abundance.  However, there was no relationship between host plant abundance and clutch size in the burned area in the burn year, despite more clutches with greater mean clutch sizes, smaller and less abundant host plants, and oviposition bias for burned plants on the treatment edge.  In the following year, burned area clutch sizes were positively correlated with host plant abundance suggesting a transient switch in reproductive strategy in response to fire.  A similar pattern between clutch size and host plant abundance was recorded from a different prescribed burn and unburned area 4 years later.  Unfortunately, purifying selection, from acute temperature stress and a pathogen (likely nuclear polyhedrosis virus) killed 100% of the caged larvae feeding on unburned plants.  However, ≈ 20% of the caged larvae feeding on burned plants survived, suggesting that the temporary switch in reproductive strategy and clutch size regulation by the Hemileuca eglanterina population may be an adaptive response to fire where host plant quality is favored over host plant quantity.