Herbivore abundance is independent of weather? A 20‐year study of a galling aphid Baizongia pistaciae (Homoptera: Aphidoidea)

Abstract

AbstractThe gall abundance of Baizongia pistaciae (L.) (Pemphiginae: Fordini) on marked Pistacia palaestina trees (Anacardiaceae) was monitored for 20 years at two natural sites in Israel (Carmel, in the center of the host distribution, and Beit Guvrin, near its southern limit), and in the botanical gardens of Tel Aviv University. Gall abundance varied between zero and 500 galls/tree and fluctuated from year to year. The analysis aimed at a diagnosis of the underlying causes of the temporal changes in gall abundance. Weather variables were expected to synchronize gall abundance on most trees on the same site. There was little indication of synchrony among trees in most years. Correlations of mean gall abundance with weather variables were low (<0.4, explaining <16% of the variation) and almost none were significant. The temporal pattern of gall abundance was analyzed for every tree separately (the “autecological” paradigm). There was no convincing evidence of self‐regulation (density‐dependence) in gall abundance on any tree, although the rate of change in abundance tended to decrease with increasing density on some trees. At one site (Carmel), significantly more abundance peaks (1 SD above the mean) occurred in “wet” years (precipitation above average) than in “dry” years. This pattern was not observed at the other sites. The biannual fluctuations in abundance, observed in the first 10 years of the study, were far less clear when the 20‐year sequence was analyzed. If the causative mechanism is tree stress – as assumed previously – then the biannual pattern may not be expected except in years of favorable conditions, when gall abundance is high enough to drain the tree of resources and cause a lower gall abundance the following year. The life cycle of galling aphids is very complex. A clone enclosed in a gall on a tree alternates with a subterranean population on the roots of herbaceous plants, with two migratory phases in between. Moreover, a sequence of parthenogenetic generations is interrupted by a single event of sexual reproduction. In B. pistaciae, this cycle takes two years to complete. Regulating mechanisms of gall abundance at the different stages may work in different directions, canceling each other out. I suggest that direct effects of weather on the aphids at any stage of their complex life cycle may be obscured when the net result (gall abundance) is studied. Weather effects may be detectable only when it is extremely favorable or unfavorable for the aphids (or the host plant).