Seasonal and landscape effects on the biotic resistance of forest communities to experimental insect introductions

Abstract

Natural enemies play an important role in the regulation of many forest insect populations. The hypothesis that these organisms also reduce invasion potential is one element of a concept known as biotic resistance. While many studies have shown that the abundances of natural enemies are affected by landscape structure and diversity, seasonality, and host tree species, this study tests the hypothesis that these factors affect the biotic resistance of forest communities to invasion by a non‐native tussock moth. At 20 sites on Vancouver Island, Canada, spanning a range of natural to urban forests, small populations of the rusty tussock moth, Orgyia antiqua, an exotic polyphagous tussock moth, were introduced. Introductions were repeated at three different periods in the year, on coniferous and deciduous host trees, and included both late‐instar larvae and pupae. The survival of these small populations was monitored in relation to four landscape variables measured around each site. Spring introductions had significantly lower mortality rates than either early or mid‐summer introductions. There was little difference in predation rates between coniferous and deciduous host trees. The amount and type of forest cover in the landscape had important, but seasonally dependent, effects on survival that likely reflect changes in the habitat requirements of a shifting community of generalist predators. Based on the results, this study concludes that landscapes with intermediate forest cover are the least resistant to invasion by early feeding species such as the rusty tussock moth.