Immigration, frequency distributions and dispersion patterns of the psyllid Trioza erytreae (Del Guercio) in a citrus orchard.
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 Trioza erytreae (Del G.). Adults invaded the orchard in exponentially increasing numbers, leading to an exponential increase in the 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 the sampling dates. Its standard deviation, which represented the increase in contagion, increased linearly with time. The value b in a variance/mean plot (Taylor's power law) was 1.69 for this species. Dispersal 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 species commonly used as windbreaks. Short shoots supported no or only a few psyllids, while long shoots supported zero to many tens of them. Valencia seemed to be the most attractive variety to this species, while grapefruit was the least attractive and navel and Ortanique were intermediate. These results suggest that trap trees consisting of heavily-pruned Valencias treated with an insecticide may be one way to manage populations of T. erytreae.