Ecology of the oriental fruit moth, Grapholita molesta (Busck) in New Zealand

Abstract

The current distribution of the recently arrived stone and pip fruit pest, the Oriental fruit moth (Grapholita molesta (Busck), Lepidoptera: Tortricidae) (O.F.M.) is mapped and the sharp decline in numbers in the Auckland region, as monitored by pheromone trapping over the years 1980-84 described. As a background to future integrated control strategies, this thesis concentrates on problems in the field biology of O.F.M., investigation of the timing of events in the field and of the proximal causes of these timings, and makes a close study of parasitism of O.F.M.. O.F.M. is established as having 4 larval feeding instars on peach in the field in Auckland but 5 feeding on apple at 25°C under laboratory conditions. Methods of separating the instars are provided. Diapausing larvae feed for longer and have a tendency to develop through 4 rather than 5 instars at 25°C. A re-analysis of data in the literature revealed no agreement on the temperature thresholds of development for the different life stages - this has very real implications for modeling work. The use of 7.2°C lower temperature threshold of development for pupae is supported by the summations of developmental day degrees in two thermally distinct environments. There is a temperature impediment to egg hatch early in the spring in Auckland. Oviposition schedules in the field typically span c.15 days with 65% of oviposition over by day 9. Under Auckland conditions the oviposition period is effectively temperature independent. Ist instar entry into shoots and fruits is highly competitive with most shoots and fruits supporting only one feeding larvae, regardless of egg densities. Fruits ae utilised as feeding sites from their first availability in November. Roughly 70% of larvae in all generations feed only on shoots with a mean of c.2.5 shoots/larvae. Crop damage levels at harvest on an unsprayed block of peach variety 'Golden Queen' was c.5-17% as opposed to 1-1.6% on a comparable, insecticide sprayed block. Brown rot Sclerotinia fructicola damage was strongly associated with O.F.M. damage. Of all rotten fruit at harvest c-30% has associated O.F.M. damage, whether O.F.M. populations are high or low. A larval tagging study showed pupation sites to vary over the season, with 52-83% of larvae pupating off the tree, mainly in weed stems, peach stones and the upper soil layers. Larvae generally cocooned in the first available sites. Distances moved in the search for pupation sites were greater late in the year. No phototropic effect with relation to sun direction or tree silhouette could be determined. A laboratory rearing technique utilising apple thinnings is presented. The 'quality' of insects reared in this way, as measured by flight cage sexual responses and field cage overwintering success, was not impaired in comparison with field reared insects. Diurnal and seasonal timing of moth sexual activity was examined. The lower temperature threshold for male flight is c.16°C. The spring threshold for female calling is at least as low as the male flight threshold. The summer figure was below the lowest consistent dusk temperature of around 18°C. Within the restrictions of the lower flight temperature threshold, male moths flew from c.5hrs to c.1.5hrs before sunset in the spring and in the 3hrs before sunset in the summer. Mean flight time was earlier in relation to sunset on cooler days and this effect was not due to early curtailment of flight. This shift may be sufficient to ensure fertilisation in cool spring weather. However, female calling showed no tendency to occur earlier on cooler days. No differences were determined between moths having experienced diapause as larvae and those which had not: either in temperature threshold, male flight or female calling times, or in the timing of responses to pheromone as opposed to virgin female traps. Suction trap captures of wild moths over 3 seasons at Kumeu Research Orchard confirmed the flight cage timing. There is a very small morning flight. Flight timing over the season was examined using a pheromone trap network in Auckland and Hawkes Bay. The 50% points in the cumulative spring moth emergence is around 27 September (flight from late August to early November) and 25 October in Hawkes Bay (flight from mid-September to mid-November). Three summer flights follow, the third being rather truncated by diapause in Hawkes Bay. There is a small partial fourth flight in Auckland; larger in warmer seasons. The overall timing of pheromone trap catch in the overwintering generation is constant throughout the Auckland region but sites show considerable variation in the duration and timing of summer peaks within years. The peak moth catch in any generation varies considerably at any one site between years. Neither examination of regional mean flight dates nor the plotting of results on a physiological time scale, improve the predictive value of the results. Sampling of sub-adult life stages in 'Golden Queen' peaches at Kumeu R.O. 1981-4 revealed 3 clear summer feeding generations with a fourth commencing development just before harvest in March. Intergenerational physiological distances were constant over the two years examined, with the time between the Ist and 2nd summer generations being shorter by c.14% than that between the 2nd and 3rd generations. It is shown that the timing of the presence of the Ist larval peak on 'Golden Queen' relates as much to the time of host shoot availability as to the time of early oviposition and subsequent egg hatch. Moth emergences in the spring are a function of the time of diapause breakage. Although there is a strong bimodality shown in pheromone trap catches (especially in Hawkes Bay), it is demonstrated experimentally, using a field cage at Kumeu R.O., that the date of diapause breakage is not a function of the date of entry into diapause, and that the emergence peaks do not therefore represent cohorts from the two final summer generations. However, neither is diapause breakage controlled by the accumulation of a certain number of chilling day degrees, nor is it terminated at a specific daylength. The specific cause of diapause breakage remains unknown. Diapause induction, is shown to occur progressively at photoperiods from 14 to 13hrs (c. 25 February to 20 March in Auckland). Predictions are made for the dates of diapause entry elsewhere in New Zealand; these are confirmed for the Hawkes Bay district. A small survey of parasitism of cocooned larvae and pupae was carried out in Auckland and Hawkes Bay in 1983-4 and all life stages were examined for parasitism over 3 years at Kumeu R.O.. No egg parasitism was recorded. Only one larval parasite, a braconid, is reported on one occasion. The major parasitism is of the cocooned larva, prepupa and pupa. One tachinid, one braconid, one pteromalid, and 4 ichneumonid species are recorded. Of these, only 1 ichneumonid is of significance. Two Hemiteles spp. are primary parasites of cocooned O.F.M. at very low levels and hyperparasitic on the major primary parasite Glabridorsum stokesii at levels of around 5-17% during the summer generations. G.stokesii was present at all Auckland sites surveyed, attacking O.F.M. from October to May. A method of calculating percentage parasititism from field data is presented. This takes into account the relative developmental times of the parasite and unparasitised host. Pupal parasitism by G.stokesii was consistently high in late summer (up to 100% in individual weeks). Circumstantial evidence links the decline in O.F.M. numbers in the Auckland region with the rise of G.stokesii numbers over the same period. In addition to O.F.M., G.stokesii also parasitizes other tortricid species in the orchard situation. A review of the world literature identified a number of species within New Zealand which my have future significance as O.F.M. parasites. The potential for future bio-control introductions is discussed.