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

Moore, Judy A. (2003-12)

Thesis (MSc)--Stellenbosch University, 2003.

Thesis

ENGLISH ABSTRACT: The losses incurred to by the South African hardwood industry because of damage caused by the larvae of the Australian eucalyptus borers Phoracantha semipunctata (Fabricius) and P. recurva Newman (Coleoptera: Cerambycidae) were countered by the introduction of various biological control agents. Megalyra fasciipennis Westwood (Hymenoptera: Ichneumonidae), restricted to the southwestern Cape for nearly 91 years after its probable establishment in 1910, is a specialist pupal parasitoid achieving a parasitism level of up to 52.5 %. It has an activity peak in early spring, which coincides with the pupation of a large percentage of its hosts that had overwintered as larvae. The average length of the ovipositor of M. fasciipennis (42.71 ± 5.33 mm S.D.) was longer than the average tunnel length (31.34 ± 11.85 mm S.D.) to the pupal chamber of Phoracantha spp, within the log despite variations in the thicknesses of the eucalypt stems. 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 activity being 16°C. Optimum temperature for oviposition was 30 °C. Males became active before 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 and 1995 before establishment was confirmed. Dispersal was monitored annually and was determined to occur at a rate of 50 km/year. By 1998 A. longoi had dispersed 300 km north of Cape Town to Lutzville and 270 km east to Riversdal. Subsequent to a satellite release in Knysna during 1994, it has been established 40 km from this release site, at Plettenberg Bay. The parasitoid has also bridged a LO km expanse of ocean to establish on Robben Island, immediately off the west coast of Cape Town. Avetianella longoi has a preference for the eggs of P. semipunctata, which is the most likely cause for the decline in the population of P. semipunctata. However, P. recurva remains relatively unaffected. Average parasitism of Phoracantha spp. eggs by A. longoi was 59.4 %. An undescribed Cleonymus sp. (Hymenoptera: Pteromalidae) of unknown origin (the genus being widely distributed on several continents), was discovered in the Cape Peninsula parasitising late instar larvae of P. semipunctata and P. recurva. This ectoparasitoid lays its eggs (mean number per larva = 20.3 ± 15.2 S.D.) through the bark into the host chamber after the host has been paralysed. The host is entirely consumed and pupation takes place in the chamber with wasps emerging in the ratio of 1 male: 3 females. Although uncommon in the field, mass rearing of these wasps in culture was easily accomplished and a consignment was released in the Tzaneen district in 1993, where it was confirmed to have become established in 1996. Bark thickness constraints on the effectiveness of this parasitoid as a biological control agent because it's short ovipositor restricts the wasp to certain eucalypt species or trees with thin bark. The introduction into South Africa in 1995 and attempted establishment of the larval parasitoids, Syngaster lepidus Brullé (Hymenoptera: Braconidae), Jarra phoracantha Marsh & Austin (Hymenoptera: Braconidae) and J maculipennis Marsh & Austin proved unsuccessful in the Western Cape. However, the former two species were established in the Tzaneen district and their recruitment for release in the Western Cape should be considered. The present guild of biological control agents has been insufficient to give the required control. In the absence of biological control agents, intraspecific competition amongst host larvae is the major mortality factor. Although high levels of mortality are achieved as a result of parasitism despite the biological constraints of the parasitoids (e.g. the narrow activity peak of A. longoi and the restriction of Cleonymus sp. to thin barked eucalypts), their combined parasitism has succeeded in reducing the competition between host larvae, resulting in fewer yet larger host beetles emerging. The introduction of additional viable agents to assist in the biocontrol of Phoracantha spp. is required.

AFRIKAANSE OPSOMMING: Die verliese wat die Suid-Afrikaanse hardehoutbedryf ly as gevolg van skade veroorsaak deur die bloekomboorders Phoracantha semipunctata en P. recurva (Coleoptera: Cerambycidae), is bekamp deur die invoer van verskeie biologiese beheeragente. Megalyra fasciipennis Westwood (Hymenoptera: Ichneumonidae), beperk tot die Suidwes-Kaap vir byna 91 jaar nadat dit waarskynlik in 1910 daar gevestig is, is In spesialis papieparasitoïd wat In parasitismevlak van tot 52.5% bereik. Dit het In aktiwiteitspiek in die vroeë lente wat saamval met die papievorming van baie gasheerlarwes wat oorwinter het. Die gemiddelde lengte van die eierboor van M. fasciipennis (42.71 ± 5.33 mm S.A.) was langer as die gemiddelde tonnellengte (31.34 ± 11.85 mm S.A.) na die papieholte van die gasheer binne in die hout, ten spyte van die variasie in die dikte van die bloekomstamme. Stamdikte het dus nie In nadelige uitwerking op die vlak van parasitisme nie. Volwassenes van M.fasciipennis is bedags aktief en aktiwiteit word hoofsaaklik deur temperatuur bepaal. Meer as 70% was tussen 25°C en 34 °C aktief, met 16°C as die minimum drumpel vir aktiwiteit. Mannetjies het voor die wyfies aktief geword en maksimum eierlegging het tussen 10hOOen 15hOO plaasgevind. In 1993 is die gasheerspesifieke eierparasitoïd Avetianella longoi Siscaro (Hymenoptera: Encyrtidae) vir die beheer van Phoracantha spp. ingevoer. Van 1993 tot 1995 is 7791 volwassenes van A. longoi en 80 geparasiteerde eiers rondom Kaapstad vrygelaat en dis vasgestel dat die spesies gevestig het. Die verspreiding daarvan is jaarliks gemonitor en dis vasgestel dat dit teen 50 km per jaar plaasvind. Teen 1998 het dit versprei tot 300km noord van Kaapstad na Lutzville en 270 km oos na Riversdal. Na 'n satelliet-loslating by Knysna in 1994 het dit 40 km verder by Plettenbergbaai gevestig. Die parasitoïd het ook 10 km van die oseaan oorgesteek om op Robbeneiland, wes van Kaapstad te vestig. Avetianella longoi gee voorkeur aan die eiers van P. semipunctata en dis waarskynlik die rede vir die afname in die getalle van hierdie spesies, maar P.recurva word relatief min beïnvloed. Die gemiddelde graad van parasitisme van Phoracantha spp. was 59.4%. Dit is gevind dat 'n onbeskryfde Cleonymus sp. (Hymenoptera: Pteromalidae) van onbekende oorsprong (die genus kom wyd verspreid in verskeie vastelande voor) die laat instar larwes van P. semipunctata en P. recurva parasiteer. Hierdie ektoparasitoïd lê sy eiers (gemiddeld 20.3 ± 15.2 S.A.) in die gasheerholte nadat die gasheer eers verlam is. Die gasheer word heeltemalopgevreet en pupering vind plaas in die holte plaas. Volwassenes kom uit in verhoudingvan drie mannetjies tot een wyfie. Alhoewel skaars in die natuur, kan hierdie wesp maklik in massa geteel word. 'n Besending is in die Tzaneen distrik vrygestel en in 1996 is vasgestel dat hulle gevestig het. Basdikte is 'n beperkende faktor in die gebruik van hierdie parasitoïd as effektiewe beheeragent vir biologiese beheer omdat die kort lengte van die eierboor die wesp sal beperk tot bloekomsoorte met dun bas. Die invoer na Suid-Afrika in 1995 en vestiging van die larwale parasitoïde Syngaster lepidus Brullé (Hymenoptera: Braconidae), J. phoracantha Marsh & Austin (Hymenoptera: Braconidae) en J. maculipennis Marsh & Austin was onsuksesvol in dieWes-Kaap. Die twee spesies is egter in die distrik Tzaneen gevestig en versameling met die oog op loslating in die Wes-Kaap behoort oorweeg te word. Die huidige gilde van biologiese beheer-agente is onvoldoende om die vereiste graad van beheer te verskaf. In die afwesigheid van biologiese beheer-agente is intraspesifieke kompetisie tussen gasheerlarwes die belangrikste mortaliteitfaktor. Alhoewel hoë vlakke van mortaliteit as gevolg van parasitisme bereik word, ten spyte van die biologiese beperkings van die parasitoïde (bv. die kort aktiwiteitspiek van A. longoi en die beperking van Cleonymus tot bloekoms met dun bas), het die gekombineerde parasitisme daarin geslaag om die kompetisie tussen gasheerlarwes te verlaag, met die gevolg dat minder maar groter gasheerkewers verskyn het. Dit is dus nodig dat addisionele organismes gevestig word om by te dra tot die biologiese beheer van Phoracantha spp.

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