Biological control of the eucalypt borers, Phoracantha semipunctata (Fabricius) and P. recurva Newman (Coleoptera: Cerambycidae) in South Africa

[摘要] ENGLISH ABSTRACT: The losses incurred to by the South African hardwood industry because of damagecaused by the larvae of the Australian eucalyptus borers Phoracantha semipunctata(Fabricius) and P. recurva Newman (Coleoptera: Cerambycidae) were countered by theintroduction of various biological control agents. Megalyra fasciipennis Westwood(Hymenoptera: Ichneumonidae), restricted to the southwestern Cape for nearly 91 yearsafter its probable establishment in 1910, is a specialist pupal parasitoid achieving aparasitism level of up to 52.5 %. It has an activity peak in early spring, which coincideswith the pupation of a large percentage of its hosts that had overwintered as larvae. Theaverage length of the ovipositor of M. fasciipennis (42.71 ± 5.33 mm S.D.) was longerthan the average tunnel length (31.34 ± 11.85 mm S.D.) to the pupal chamber ofPhoracantha spp, within the log despite variations in the thicknesses of the eucalyptstems. Stem thickness therefore did not adversely affect the level of parasitism.Megalyra fasciipennis adults are diurnal, with activity largely determined by temperature.Over 70 % were active between 25°C and 34 °C, the minimum threshold for activitybeing 16°C. Optimum temperature for oviposition was 30 °C. Males became activebefore the females and maximum oviposition occurred between 10hOOand 15hOO.In 1993, a host specific egg parasitoid, Avetianella longoi Siscaro (Hymenoptera:Encyrtidae), was introduced for the control of Phoracantha spp. A total of 7791 A.longoi adults and 80 parasitised eggs were released around Cape Town between 1993 and1995 before establishment was confirmed. Dispersal was monitored annually and wasdetermined to occur at a rate of 50 km/year. By 1998 A. longoi had dispersed 300 kmnorth of Cape Town to Lutzville and 270 km east to Riversdal. Subsequent to a satelliterelease in Knysna during 1994, it has been established 40 km from this release site, atPlettenberg Bay. The parasitoid has also bridged a LO km expanse of ocean to establishon Robben Island, immediately off the west coast of Cape Town. Avetianella longoi hasa preference for the eggs of P. semipunctata, which is the most likely cause for thedecline in the population of P. semipunctata. However, P. recurva remains relativelyunaffected. Average parasitism of Phoracantha spp. eggs by A. longoi was 59.4 %.An undescribed Cleonymus sp. (Hymenoptera: Pteromalidae) of unknown origin (thegenus being widely distributed on several continents), was discovered in the CapePeninsula parasitising late instar larvae of P. semipunctata and P. recurva. Thisectoparasitoid lays its eggs (mean number per larva = 20.3 ± 15.2 S.D.) through the barkinto the host chamber after the host has been paralysed. The host is entirely consumedand pupation takes place in the chamber with wasps emerging in the ratio of 1 male: 3females. Although uncommon in the field, mass rearing of these wasps in culture waseasily accomplished and a consignment was released in the Tzaneen district in 1993,where it was confirmed to have become established in 1996. Bark thickness constraintson the effectiveness of this parasitoid as a biological control agent because it's shortovipositor restricts the wasp to certain eucalypt species or trees with thin bark.The introduction into South Africa in 1995 and attempted establishment of the larvalparasitoids, Syngaster lepidus Brullé (Hymenoptera: Braconidae), Jarra phoracanthaMarsh & Austin (Hymenoptera: Braconidae) and J maculipennis Marsh & Austin provedunsuccessful in the Western Cape. However, the former two species were established inthe Tzaneen district and their recruitment for release in the Western Cape should beconsidered.The present guild of biological control agents has been insufficient to give the requiredcontrol. In the absence of biological control agents, intraspecific competition amongsthost larvae is the major mortality factor. Although high levels of mortality are achievedas a result of parasitism despite the biological constraints of the parasitoids (e.g. thenarrow activity peak of A. longoi and the restriction of Cleonymus sp. to thin barkedeucalypts), their combined parasitism has succeeded in reducing the competition betweenhost larvae, resulting in fewer yet larger host beetles emerging. The introduction ofadditional viable agents to assist in the biocontrol of Phoracantha spp. is required.

[发布机构] Stellenbosch University