Document: HOW-3-51-33

Holly-leafminers on two continents: What makes an outbreak species?

EBER, S.* ¹, H.P.SMITH ¹, R.K.DIDHAM ² and H.V.CORNELL ²

Imperial College at Silwood Park, Ascot, Berks SL5 7PY, United Kingdom. ¹
University of Delaware, Newark, DE 19716, USA ²

Abstract:
It is by no means clear how eruptive insect herbivores differ in their control mechanisms compared to regulated species. We compared two closely related holly leaf-miner species, one of which exhibits outbreaks. Phytomyza ilicicola in the eastern US and P. ilicis in the UK share many ecological traits and have similar life cycles, but P. ilicicola reaches infestation levels of well over one mine per leaf, while P. ilicis is tightly regulated at much lower densities. Using data from the literature and from our own field studies, we tried to find the reasons for these contrasting dynamics. P. ilicicola in the US varied widely in mortality and infestation levels, reaching local densities of over ten mines per leaf. In contrast, P. ilicis in the UK showed high mortality and low infestation at all sites. P. ilicicola oviposits into the leaf lamina and experiences weak larval competition only at high densities. P. ilicis oviposits only in the midrib, which leads to high mortality of young larvae before mine formation. Multiply-mined leaves were thus very common in P.ilicicola but were rare in P. ilicis. Differences in the parasitoid complexes of the two systems accounted for further differences in survival to adulthood. The main (larval) parasitoid, which was found to impose high, density-dependent mortality on P. ilicis is missing on P. ilicicola. It is replaced by an egg-pupal parasitoid, which varies in its impact at different sites. Multiple emergence of adults from multiply-mined leaves is therefore widespread in P. ilicicola, but does not occur in P. ilicis. The differences in oviposition behavior and in the parasitoid complexes are likely to allow P. ilicicola to outbreak when habitat conditions are favorable, while P. ilicis is always tightly regulated.

Keywords: outbreak, leaf-miner, Phytomyza, holly, density-dependence, parasitoid, infestation

This abstract is being presented at: 9:00 AM in session:
Oral Session #26: Invertebrate Herbivore - Plant Interactions.