The Effect of Stand Size and Vegetational Background on the Colonization of Cruciferous Plants by Herbivorous Insects

Abstract

The spatial dispersion of resources is a major factor in the population dynamics of many insects. This is especially true of herbivorous species which utilize ephemeral host plants, or which must leave the host during the life cycle and subsequently rediscover it. In such situations the pattern of colonization by insects will be determined by the interaction between the plants' spatial dispersion and the insects' behaviour. This investigation attempts to answer two questions. (1) What is the effect of host plant plot size on the numbers and kinds of herbivores that occupy the stand, and (2) how does the background of a stand (i.e. the presence or absence of other vegetation) influence its colonization ? Size and background of stands are not the only aspects of host plant spatial dispersion that affect insects. Others are plant density (Dethier 1959a, b; Pimentel 1961a; Davis 1966; Way & Heathcote 1966; Clark et al. 1967; A'Brook 1968, 1973; Heathcote 1969), distance from other stands (Janzen 1968, 1970) and proximity to other environmental features including overwintering sites and alternate hosts (Wolfenbarger 1940; Moreton 1945; Wright & Ashby 1946; van Emden 1965; Lewis 1969; Johnson 1969). The former two were chosen for investigation because these characteristics in natural vegetation and agricultural crops are the most consistently different. Crops are usually grown in patches which are larger and contain fewer other kinds of plants than natural patches of their wild relatives. The prevalence of insect outbreaks on cultivated plants is related in part to these differences (Pimentel 1961b; van Emden 1964; Smith 1972; Tahvanainen & Root 1972; Root 1973). Furthermore, the same factors are important in the biological control of weeds by insects. An insect's response to large concentrations of its host plant determines its ability to control the host population, and the colonization behaviour of the species is one of the determinants of the equilibrium densities of insect and host (Huffaker 1962; Janzen 1970). Despite recognition of the importance of plant spatial dispersion for insects (Huffaker 1962; Johnson 1969), there have been few attempts to document either responses of individual insect species or the size and composition of the entire herbivore fauna in stands of different sizes. Douwes (1968) found that the female geometrid moth, Cidaria albulata L., alighted more frequently on hosts (Rhinanthus sp.) in dense clumps than on scattered hosts. In the vicinity of larger patches of Rhinanthus the flight path was shorter and the moth changed direction more frequently, apparently in response to higher concentration of the host's odour. Away from these patches, the female made longer, faster flights. Thus the behavioural pattern of Cidaria albulata caused it to remain in or near larger patches of Rhinanthus. * Present address: ENVL, Stockton State College, Pomona, N.J. 08240, U.S.A.