The biology and economic impact of the weta, Hemiandrus sp. (Orthoptera : Stenopelmatidae) in an apricot orchard, Horotane Valley

Abstract

The basic aspects of the biology of the weta, Hemiandrus sp. (Orthoptera : Stenopelmatidae) were studied in an orchard, Horotane Valley, and in the laboratory during 1976 and 1977. The differences between preferred and least preferred ovi-position sites were investigated. Egg collections from the orchard showed that the mean fecundity was 50 ± 8 eggs. Fertility was in excess of 90 percent. The eggs have a winter quiescence and the presence of contact water was essential for egg development. A negative linear relationship between mean time for egg incubation with the time eggs were collected from the orchard was demonstrated as a potential tool for predicting field hatching.

Hemiandrus sp. was reared from newly hatched nymphs to adulthood on various diets. Various aspects of nymphal biology were investigated using such reared colonies. The effects of the types of diets and the mould inhibitor on instar weight, duration and size were discussed. The effectiveness of the mould inhibitor in controlling mould growth was also discussed and the types of mortality factors recorded.

Laboratory experiments using a starving technique showed that the most preferred food plant for Hemiandrus sp. was Italian ryegrass (Lolium multiflorum Lam.). Fathen (Chenopodium album L.) was also eaten as shown by field and laboratory investigations but was a far less preferred species than prairie grass. Hemiandrus sp. did not feed on spottedbur medick (Medicago arabica L.) and Oxalis sp., the other major orchard flora constituents.

Using pitfall traps, it was shown that hatching commenced in December during the 1976/1977 season. There were two overlapping populations present during the summer months. There was a preponderance of males at all stages where sex could be identified. The dispersion of the first five instars was negative binomial. The weekly dispersion of the male adults was generally negative binomial but that of the female was adequately described by the Poisson series. There was no difference in catch whether a trap was placed on the northern or the southern side of a tree. The soil moisture content and atmospheric temperature accounted for 80 percent of the variance in pitfall catch in the [fraction here] and 81 percent in the [fraction here] population. There was a difference in behaviour between adult males and females.

Damage patterns and economic impact were examined during the 1976/1977 season. Fruit damage was caused by mature females. The percentage of apricots damaged varied with time. Each apricot cluster was equally susceptible to weta attack. There appeared to be no significant difference in fruit damage between and within trees and within stems. Potential control measures and their cost benefit analysis were discussed.