UBC Theses and Dissertations
Food plant spacing and the dispersal tendency of the cinnabar moth larva Campbell, Barbara Jane
The tendency of larvae of the cinnabar moth, Tyria jacobaeae (L.), to disperse from their food plant, before it is defoliated and before they are ready to pupate, was investigated in 13 subpopulations of ragwort, Senecio jacobaea L. Dispersal of larvae from their original food plant to a new food plant was assumed to be acted upon by selection. The purpose of this thesis was to examine the likelihood that larvae in any given subpopulation could locate a new plant, and to examine the dispersal behaviour those larvae displayed. The research hypothesis, relating dispersal tendency to plant spacing and dispersal success, was that larval dispersal should be favoured in subpopulations with closer plant spacing, where it is more likely to be successful. Field measures of ragwort density and spacing, and larval distribution, were made at each of the 13 subpopulations. The behaviour of larvae from each subpopulation was studied in the field, or on dispersal plots of variable plant spacing, built at south campus, U.B.C. The basic method was to start from a known number of larvae of a given age on a centre plant, record the number that had disappeared after a given number of days, and, of those missing, the number that had arrived on new plants in the vicinity. The first measure was called disappearance, and the second, dispersal success. Of the 13 subpopulations, 8 were classified as being closely spaced on the basis of a median rank of 6 separate measures of plant density and spacing. Fifth instar larvae were found to disperse with a probability of success that varied from .74 when the probability was .33 that plant aggregations were separated by > 1 m (the more closely spaced populations) to .44 when p > 1 m was .75 (the more widely spaced populations). Thus the differential survival, necessary to any selection for different behavioural types, was demonstrated. And, in 2 out of 3 measures of larval distribution from the field, there was significant indication that larvae from widely spaced populations disperse less. However, disappearance, as measured on the south campus plots for larvae from areas of different plant spacing, offered no support to the hypothesis. These data were thought to be unreliable due to the influence of other factors on disappearance. Given the marked differential survival of dispersing larvae in closely spaced and widely spaced plant populations, it was concluded that selection could produce different behavioural types in these populations. The field evidence supports the hypothesis that dispersal tendency is influenced by the probability of dispersal success. If the higher dispersal mortality associated with wider plant spacing selects against dispersal behaviour, then dispersal mortality may be constant in populations with different plant spacing. Dispersal mortality will not act differentially as a population regulating mechanism in populations with different plant spacing. Lack of a relationship between larval density and disappearance suggests that dispersal tendency is influenced less by larval numbers on the original food plant than by the probability of dispersal success. There was no indication, from mean egg batch size, that selection favoured smaller egg batches and thus fewer overcrowded plants in those areas with wider plant spacing.
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