A life system study of Cryptophlebia ombrodelta (Lower) (Lepidoptera: Tortricidae) in southeast Queensland

Abstract

The larval stages of Cryptophlebia ombrodelta (Lower) (Lepidoptera: Tortricidae), the macadamia nut borer, a native to Australia, has been recorded as a pest of Macadamia spp. in Australia. Macadamia spp. (commonly known as "macadamia") yielding edible nuts are Australian native trees grown commercially. The industry is presently undergoing expansion.

This study, based on macadamia orchards in Southeast Queensland, was undertaken to investigate the damage caused to the crop by the insect, establish factors contributing to the insect's abundance, and to provide a sound base from which future studies may be directed. To maintain direction, each part was envisaged as .contributing to a computer model which would summarize current knowledge of the life system. The study developed in three main sections - the host-plants, the subject insect, and its natural enemies. These were then investigated as interacting subsystems of the whole.

Host plants: 46 species of host plants (from six botanical families) are known. Most, although not natives, are now found in Australia. Macadamia trees were studied to establish the number of plant parts, their stability, and distribution within the tree for sampling purposes. Nut numbers decline from a maximum early in the season (October) with two well defined periods of fall, coinciding with natural crop thinning, and maturity. Other plant parts were considered stable for the purposes of this study. Distribution of nuts within trees was very variable. Nut crop development was studied to determine the effect of insect attack at different times. Processing size is achieved in October, November. Nut shells harden in December, and kernels mature in February; each event is well defined. Alternative Hosts occur in abundance only near housing settlements. Overlapping of fruiting periods ensures food supply throughout the year.

Cryptophlebia ombrodelta was reared satisfactorily on an artificial medium. The use of head capsule widths to identify larval instars was complicated by the variable number of instars (five or six). A computer programme assisted in determining instars of field collected larvae. The physiological time (hour degrees) of development of each immature stage was determined and appears to depend on daylength. The main field sampling unit for population studies was one fruit. Population trends were consistent from season to season but results were imprecise, due to the relatively low numbers of insects per fruit. Examination requirements limited the sample size. In macadamia, populations increased rapidly in late December, declining gradually after February. In the alternative hosts population intensity was usually much greater and infestation periods shorter. A pheromone lure proved effective in trapping males. Traps for females were ineffective. Studies on the biology and behaviour revealed a marked preference for oviposition on damaged nuts. Fruit entry by young larvae is easier on damaged nuts. A considerable proportion of mature larvae leave the fruit in search of pupation sites.

Natural enemies: Six parasites of the immature stages were recorded with an overall average species apparent percent parasitism of 0.05% to 12.20%. One hyperparasite was found. Observed predation was negligible; no disease factor was discovered.

The interactions: The alternative hosts were important sources of C. ombrodelta. In macadamia, partial budgets indicated that mortality was important in population regulation. Two of the most important factors are probably larval establishment mortality, and death of mature larvae leaving the nuts. Parasitism is probably more important than the average percent parasitism figures indicate. Crop damage is caused by direct kernel damage and husk damage before and after maximum kernel maturity. At one site it was estimated that C. ombrodelta reduced potential crop value by approximately 20%.

The computer model explained and summarized the population processes as they are understood. Although not suitable for predicting the outcome of commercial management practices, it is a useful teaching tool.

The study concluded that it is essential for the industry to define product quality more clearly, and to conduct population studies in specific commercial areas. If new orchards are to be planted, an area isolated from housing settlements would be preferable. Otherwise a natural enemy, reducing C. ombrodelta populations in alternative hosts, is likely to provide the most acceptable control.