BIOLOGICAL CONTROL OF WALNUT HUSK FLY (1983)

K. S. Hagen, R. L. Tassan and Mabel Fong

ABSTRACT

Four species of walnut husk fly (WHF) parasites were released in San Luis

Obispo Co. Three species are pupal parasites: Coptera evansi (1439released), Coptera occidentalis (435), Nasonia vitripennis (210) and one

species is a larval-pupal parasite Biosteres sublaevis (219). A few~.sublaevis, originally from Texas, were recovered from our experimentalrelease orchard near Fairfield, Solano Co. where it had been released

over the past several years. About 250,000 commercially obtained house

fly puparia parasitized by the pteromalid Nasonia vitripennis werereleased in 9 acres of an 18 acre experimental walnut orchard at harvest

time. This parasite has been reared from field collected WHF puparia. An

average of 4 parasites emerge from one house fly puparium. In laboratory

tests, the addition of 10 mg cupric acetate to adult WHF protein dietneither prevented egg deposition nor development of WHF larvae in walnuts.Four successive generations of WHF have now been cultured on an artificialdiet in the lab.

OBJECTIVES

To control walnut husk fly (WHF) biologically by use of natural enemies.

To culture introduced parasites of WHF and other fruit flies and release

them against the WHF. To improve techniques of culturing walnut husk

flies and its parasites in the laboratory. To eliminate bacterial

symbiotes from the WHF adult in order to hinder or retard development of

eggs or development of resulting husk fly larvae.PROCEDURES

The procedures of rearing flies and parasites were the same as that de-

scribed in 1981 research report. Table 1 shows the number of parasitescultured and released at Jardine's ranch near Paso Robles, San Luis

Obispo County during 1983. Nasonia vitripennis, a pteromalid wasp that

parasitizes various filth fly puparia, emerged from WHF puparia collectedin 1982 from our experimental plots (Wright's orchard) near Fairfield,

Solano County. Several years ago Mr. Wright had purchased house fly

puparia parasitized by B. vitripennis and released them around his barnto control filth flies. Therefore, we are conducting a test of the mass

release of B. vitripennis against the WHF. This fall, 1983, the growerpurchased 250 thousand house fly puparia parasitized by B. vitripennis.His cost was about one dollar per thousand puparia. Approximately 1000

parasitized puparia, each expected to yield 4 adult parasites, werescattered beneath each walnut tree in 9 of the 18 acre walnut orchard.

This test will be evaluated next year by placing unparasitized WHFpuparia in both the release and control halves of the orchard. 'fHFinfestation levels will also be determined.

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RESULTS

I. Biological Control

The larval-pupal parasite Biosteres sublaevis, originally collected from

native walnuts infested with WHF in the Davis Mountains in Texas, were

reared in the lab and released in Wright's orchard in Solano County for

3 consecutive years, 1979-1981. This year a few B. sublaevis emerged from

WHF puparia collected from Wright's orchard; thus~ it appears that this

parasite may finally be established in Solano County. No Biosteres tryoni

were recovered from Wright's orchard this year. Nasonia vitripennis wasreared successfully on WHF puparia in the laboratory. It is still too

early to determine if we have establishment of the 4 parasites released

at Jardine's "organic" farm near Paso Robles (Table 1). Next year pupariawill be collected from Jardine's release site and held for parasiteemergence.II. Larval Nutrition

Four successive generations of WHF have now been obtained by rearing

larvae on an artificial diet containing freeze-dried walnut husk,

dehydrated carrot powder and enzymatic protein hydrolysates of yeastand casein.

III. Elimination of Bacterial Symbiotes from WHF

In an attempt to eliminate bacterial symbiotes from the WHF adults and

larvae, the adults of WHF were fed the standard sugar-protein diet pluscupric acetate (10 mg cupric acetate in 50 ml of standard diet). There

were no differences in feeding rates on the standard diet compared to thesame diet plus the copper; furthermore, there were about as many puparia

produced from eggs deposited by flies fed both diets. Thus, it appears

that either bacterial symbiotes are not important for larval development

or that the copper dosage did not eliminate the bacterial symbiotes.CONCLUSIONS

It appears that Biosteres sublaevis is established against the WHF in

Solano County. A mass release of a commercially available house flyparasite Nasonia vitripennis which also attacks WHF was made in a 9 acre

experimental walnut plot. The results will be determined next year. Four

species of WHF parasites were released near Paso Robles, San Luis ObispoCounty. Successive generations of WHF have been cultured on an artificial

diet. Ten mg of cupric acetate in 50 ml of adult WHF diet apparently didnot eliminate bacterial symbiotes carried by the WHF.

rable I. The number of parasites cultured andat Jardine's farm near Paso Robles.

Family and SpeciesBraconidae

Biosteres sublaevis (from Texas)Diapriidae

Coptera occidentalis (from Solano Co.)Coptera evansi (from New Mexico)Pteromalidae

Nasonia vitripennis (from Solano Co.)

released on walnut husk fly

No. released

219

4351439

210

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