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BIOLOGICAL CONTROL OF THE NAVEL ORANGEWORM

Amyelois transitella (Walker) Phycitidae

       Two external larval parasitoids, Goniozus legneri Gordh and Goniozus emigratus (Rohwer) and one internal egg-larval parasitoid, Copidosomopsis plethorica Caltagirone, which are dominant on navel orangeworm in south Texas, Mexico, Uruguay and Argentina, were successfully established in irrigated and nonirrigated almond orchard in California\'s Central Valley in 1979-86 (Caltagirone 1966, 1964; Legner & Silveira-Guido 1983). Separate k-value analyses indicated significant regulation of their navel orangeworm host during the warm summer season. Variable percentages of field-collected larvae of the navel orangeworm and the imported parasitoids have required significantly longer developmental periods to the adult stage than those in laboratory control cultures. These differences indicate diapause in the host triggered by several seasonally varying factors, and a diapause in the parasitoids triggered by hormonal changes in the host. Possible latitudinal effects on diapause are also observed. The ability of the imported parasitoids to diapause with their host enables their permanent establishment and ability to reduce host population densities to economic levels (Legner 1983).

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        The relationship between residual almond mummies on densities of the navel orangeworm and parasitism has been demonstrated (Legner 1983a). The two imported Goniozus parasitoids have also been shown capable of regulating navel orangeworm at low densities (Legner & Silveira-Guido 1983, Legner & Gordh 1992). Superimposed upon the whole system is a diapausing mechanism in both the navel orangeworm and the parasitoids (Legner 1983b). Efforts are now required that would tie together all these forces into a sound, reliable integrated management, which would allow growers to make reasonable decisions on whether or not to remove mummied almonds, or to use within season sprays.

       Populations of navel orangeworm have been followed since 1979 in six almond orchards near Paso Robles, Hilmar, Chowchilla, Selma, Westley and Atwater, to determine the impact of the parasitoids. A clear drop in the average density of navel orangeworm in all orchards is coincident with the establishment of the three parasitoids (Legner & Gordh 1992). However, the almond reject levels are not always below the economic threshold of in all orchards. Careful investigations show that invariably such rejects are due to other causes, such as ant damage and fungus infections. In certain years the peach twig borer has been found to be the principal cause, which subsequently stimulates oviposition by navel orangeworm moths. Packing plant appraisals frequently attribute damage incited by twig borer to the navel orangeworm.

Copidosomopsis plethoricus and Goniozus legneri, and to a lesser extent Goniozus emigratus overwinter in release orchards year after year. However, only Copidosomopsis can consistently be recovered at all times of the year. The Goniozus species are not recovered in significant numbers until early summer. However, adults of the latter are frequently observed in large numbers during autumn and early spring months.

Goniozus legneri has been reared from codling moth and oriental fruit moth in peaches in the Paso Robles area in addition to navel orangeworm from almonds. Field data suggest that a certain number of old mummied nuts is necessary to maintain a desirable synchrony of these parasitoids with navel orangeworm to produce the lowest average densities (below 4% damage at harvest). In fact, at Paso Robles mummies often exceed 1,000 per tree through the winter months, and produce navel orangeworm densities at harvest at below 1% on soft-shelled varieties.

       Pest management in almond orchards frequently involves periodic releases of Goniozus legneri and/or Copidosomopsis plethoricus to reestablish balances that were disrupted by insecticidal drift or by the absence of overwintering mummied fruit refuges.

Goniozus legneri (Discovery & Habits) --The discovery of Goniozus legneri in South America involved making initial contact with Dr. José Pastrana of the University of Buenos Aires. Arrangements were made for Dr. Legner to meet with Dr. Pastrana in Punta del Este, Uruguay in 1977. The navel orangeworm was not a common insect at higher latitudes in South America, and Dr. Pastrana only recalled having studied it in his collections from central Argentina. He advised Dr. Legner to travel to Concordia, Argentina to inquire there.

       In Concordia, Legner was accompanied by Dr. Aquiles Silveira-Guido, where both of them searched through collections in the experiment station there. A dusty room, filled to the ceiling with wooden insect collection boxes, was searched intensively. Several specimens of the navel orangeworm were found from collections made in 1938, and from the host coral tree, Erythrinia crista-galli. This knowledge enabled a further search in the wild on this host tree.

       Subsequently, collections were continued in Argentina and Uruguay with the aid of Dr. Silveira-Guido. Goniozus legneri turned out to be the most frequently collected parasitoid from navel orangeworm and the imported carob moth, Ectomyelois ceratoniae (Zeller), with other parasitoid species of only ca. 5% occurrence being Temelucha sp, Coccygonimus sp. and Venturia canescens (Graven-Stein), Bracon sp. & Copidosoma sp. Cultures sent to Riverside, California were attempted, but only G. legneri succeeded.

       Collections were also made from Texas where it was found that Pentalitomastix (Copidosoma) plethorica parasitized navel orangeworm on Nonpareil almonds as far north as Brownwood (33 deg. N. lat.) and on Texas ebony and western soapberry seeds along the Gulf of Mexico coast and throughout south Texas. At the latitude of Corpus Christi, another parasitoid, a biparental strain of Goniozus emigratus was found attacking this host at low densities in all seasons on western soapberry and Texas ebony. Although identified as G. emigratus by Gordh & Hawkins (1981), its biparental behavior and fecundity differed significantly from the uniparental Hawaiian form to indicate its possible sibling status.

REFERENCES:

Caltagirone, L. E. 1966. A new Pentalitomastix from Mexico. The Pan Pacific Entomol. 42: 145-151.

Caltagirone, L. E., K. P. Shea and G. L. Finney. 1964. Parasites to aid control of navel orangeworm. Calif. Agric. 19(1): 10-12.

Gordh, G. 1982. A new species of Goniozus imported into California for the biological control of navel orangeworm[Hymenoptera: Bethylidae; Lepidoptera: Pyralidae]. Entomol. News 93: 136-138.

Gordh, G. & B. Hawkins. 1981. Goniozus emigratus (Rohwer), a primary external parasite of Paramyelois transitella (Walker), and comments on bethylids attacking Lepidoptera [Hymenoptera: Bethylidae; Lepidoptera: Pyralidae]. J. Kansas Entomol. Soc. 54: 787-803.

Gothilf, S. 1978. Establishment of the imported parasite Pentalitomastix plethoricus [Hym: Encyrtidae] on Ectomyelois ceratoniae [Lep: Phycitidae] in Israel. Entomophaga 23: 299-302.

Legner, E. F. 1983a. Influence of residual Nonpareil almond mummies on densities of the navel orangeworm and parasitization. J. Econ. Entomol. 76: 473-475.

Legner, E. F. 1983b. Patterns of field diapause in the navel orangeworm (Lepidoptera: Phycitidae) and three imported parasites. Ann. Entomol. Soc. Amer. 76: 503-506.

Legner, E. F. & G. Gordh. 1992. Lower navel orangeworm (Lepidoptera: Phycitidae) population densities following establishment of Goniozus legneri (Hymenoptera: Bethylidae) in California. J. Econ. Ent. 85(6): 2153-60.

Legner, E. F. & A. Silveira-Guido. 1983. Establishment of Goniozus emigratus and Goniozus legneri [Hym: Bethylidae] on navel orangeworm, Amyelois transitella [Lep: Phycitidae] in    California and biological control potential. Entomophaga 28: 97-106.

Legner, E. F., G. Gordh, A. Silveira-Guido & M. E. Badgley. 1982. New larvicidal wasp to attempt control of navel orangeworm. Almond Facts 47(3): 56-58.

Legner, E. F., G. Gordh, A. Silveira-Guido & M. E. Badgley. 1982. New wasp may help control navel orangeworm. Calif. Agric. 38(5-6): 1, 3-5.