Immigration, Frequency Distributions and Dispersion Patterns of the Psyllid Trioza erytreae (Del Guercio) in a Citrus Orchard

Abstract

SUMMARY A citrus orchard at Nelspruit, South Africa was pruned back to the main branches to stimulate the growth of new shoots which are highly attractive to the psyllid Trioza erytreae (Del Guercio). Adults invaded the orchard in exponentially increasing numbers, which led to an exponential increase in number of shoots supporting eggs. In terms of area, there was an increase of 4% of new trees infested per day. The frequency distribution of the first immigrants was random, leading later to extreme contagion. A Poisson model fitted the first sampling date, and the negative binomial the second. After that, the frequency distribution curve was too skew for the negative binomial to fit. The log-normal was the only model to fit all sampling dates. Its standard deviation, which represents the increase in contagion, increased linearly with time. The value of b in a variance/mean plot (Taylor's power law) was 1.69 for this species. Dispersion patterns showed that adults tended to accumulate in the row adjacent to the source of infestation and along the edge bordered by large silver oaks, Grevillea robusta A. Cunn., commonly used as windbreaks. Short shoots supported no or only a few psylla, while long shoots supported zero to many tens of psylla. Valencia seems to be the most attractive variety to this species, while grapefruit was the least attractive and navel and Ortanique intermediate. These results suggest that trap trees in the form of heavily-pruned Valencias treated with an insecticide may be one way to manage T. erytreae populations.