Effects of natural enemies and host condition on populations of insect pests of trees

The purpose of this dissertation is to evaluate the effects of natural enemies and host condition on insect pests of ornamental trees. The goal of my first chapter is to evaluate the potential for conservation biological control practices to manage armored scale insects, Chionaspis species (Hemiptera: Diaspididae), in Christmas tree plantations. I manipulated the application of insecticides and the mowing of grass (which removes floral resources that could otherwise be used by natural enemies). There was a drastic change in population density of scale insects over time, initially highest in the blocks that were neither sprayed nor mowed, then highest in blocks that were both sprayed and mowed at the end of the study. This pattern suggests that the greater abundance of natural enemies in areas where trees were not treated with insecticides, and vegetation was not mowed, and the corresponding greater levels of mortality of scale insects in those areas resulted in a clear reduction in population densities in just one year.;The goal of the second chapter is to test the hypothesis that sympatric species of cerambycid beetles share host species but differ in the condition of hosts that they attack. I manipulated the condition of pin oak, Quercus palustris Munchh and Norway maple, Acer platanoides L. trees by girdling the trunk or by felling, and I also felled white pine Pinus strobes L. trees. The majority of the cerambycid beetles emerged from felled trees, while few beetles developed in trees that were completely girdled. These findings suggest that these treatments were not effective in stressing tree to the extent that they were susceptible to attack. The number of beetles that completed development in logs of both oaks and pines was significantly correlated with the diameter of logs. More beetles emerged from the oak logs with greater diameters, while more emerged from the pine logs which had an intermediate diameter.;Three species of parasitic wasps emerged from felled oak trees in small numbers, with an overall parasitism rate averaged only 0.029 +/- 0.137 (SD) per beetle. No parasitoids emerged from oak trees in the other treatments, nor from maple logs. I reared from pine logs a tachinid fly in the genus Billaea (probably Billaea monohammi Townsend), with an average parasitism rate of beetles of 0.1 +/- 0.26. The most common cerambycid in the pine logs was Monochamus carolinensis (Olivier), and B. monohammi may have potential as a biological control agent of Monochamus species, many of which are economically important pests of forest trees and in lumber production. This is a new host record for B. monohammi.;In the third chapter, I reviewed the primary and secondary literature on parasitoids of the wood-boring larvae of cerambycid beetles to examine the relationship between the feeding niche of beetles and the number and types of parasitoids that attack them. Significantly fewer species of parasitoids attacked beetle larvae that feed on roots than those that feed within twigs and branchlets, with intermediate numbers of parasitoid species associated with beetles species whose larvae feed in herbaceous stems or the large branches or trunks of woody plants. Of the parasitoid species that attack multiple host species, about half are limited to hosts of a single feeding niche, suggesting that some species of parasitoids may be limited in their ability to parasitize hosts of some niches. The highest rates of parasitism are reported from cerambycid larvae feeding in herbaceous stems. Knowledge of any trends in parasitism might help guide efforts to find biocontrol agents.