ABSTRACT

The entomogenous fungus, neozygites floridana Weiser and Muma, has been identified as causing periodic disease epizootics in orchard populations of the citrus red mite, Panonychus citri (McGregor) in the Lower Rio Grande Valley of Texas.

Epizootics occur most frequently in fall and winter months following periods of mild, wet weather with 100% humidity for several days. Foliar applications of copper fungicides made during spring time wet periods retard N. floridana development.

How the infection level of N. floridana affects red mite populations is shown in the above graphs of two orchards (A&B). N. floridana was first detected in mite populations in these orchards in November 1986, and monitored until early March 1987. Orchard owners sprayed with an acaricide and fungicide tank mix in early March 1987.

The citrus red mite, Panonychus citri (McGregor) (Acari: Tetranychidae) was first discovered in citrus orchards in the Texas Lower Rio Grande Valley (LRGV) in 1980 (French and Hutchinson, 1980). By 1983, it was already a major pest and infesting more than half the 70,000 acres of commercial citrus (French, 1984). The red mite was one of the first pest species to reappear in orchards following a major freeze in December, 1983.

Red mites are most prevalent in orchards during the fall and winter (Oct-Mar.) When cool-dry weather prevails in the LRGV. Populations of 100-200 mites/leaf are not uncommon; their feeding causes chlorotic silvering of leaves and fruit, leaf mesophyll collapse and severe tree defoliation. Red mite infestations are found on both grapefruit and orange cultivars, with slight preference toward the former.

In November, 1986 while making routine pest surveys we observe black cadavers of citrus red mite on foliage of grapefruit trees in orchards east of Harlingen, TX. Microscopic examinationof collected specimens revealed mite bodies crowded with dark brown fungal spores. These were subsequently identified as resting spores of the fungus Neozygites floridana, D.R. Smitley (Department of Entomology Michigan State University).

An endoparasitic fungus, Entomophthora spp., infecting citrus red mite and Texas citrus mite, Eutetranychus banksi (McG), was previously reported by Fisher (1951, 1954). Weiser and Muma (1966) reported that resting spores of Entomophthora (= Neozygites) floridana were seldom found in field collected Texas citrus mites.

Citrus red mites infected with resting spores of N. floridana. For comparison, a non-infected red mite is shown in the photo at 160X.

Subspherical in shape with a smooth three-layered wall, the resting spore (ca. 20 micron diameter) was the most common stage of N. floridana in diseased mites from LRGV orchards. Spores not only filled the body cavity but occurred in the legs and head capsule as well. Spores were observed in all red mite stages, except eggs and larvae. In one of the first orchards monitored in 1986, 90% of the red mites checked were infected with N. floridana.

Two methods were used to prepare mites for microscopic examination:

• to screen for resting spores, mites were placed in Hoyer's mounting media on glass slides and cleared at 70° C for 1 hr.

to detect other developmental stages of N. floridana, mites were stained with 0.1% cotton blue in lactic acid and heated at 80°C for 2 hr.

N. floridana Infection Cycle

The primary infective stage of the fungus is an anadhesive spore. Brown, striated and club-shaped (15-20 microns long), the anadhesive spore usually attaches to setae on the body or legs of the mite. The spores germinate and grow through the integument, expanding into hyphae that spread throughout the body. The hyphae eventually break into hyphal bodies that fill the mite's internal body cavity. Death of the mite usually occurs at this stage. The hyphal bodies then produce conidiophores that penetrate the mite's body wall and on whose ends are borne primary conidia. Upon discharge, and under high humidity, these conidia can infect other mites. However, a conidum landing on a dry surface produce an erect tendril with a terminal anadhesive spore. From initial infection of the mite to the production of primary conidia requires an average of six days in the laboratory at 26°C and 100% humidity (Selhime and Muma, 1966).

If environmental conditions are unfavorable the hyphal bodies (instead of producing conidia) will develop thick walls and form the resting spores.

Anadhesive spores attached to the legs and body of the red mite. Note: the long hyphal strand in the leg of the mite shown in photo at upper left. The two lower photos were taken using Nomarski Interference Microscopy.

Hyphal bodies of N. floridana crowding the inside of the citrus red mite, dorsal view. Note: the hyphal strand in the leg of the mite in the left photo.

Anadhesive spore attached to the long tendril arising from a conidium.

Conidia of N. floridana developing on the condiphores outside of the body of the citrus red mite.

We have had some success in culturing N. floridana under laboratory conditions. However, contaminate saprophytic fungi like the Alternaria spp (pictured right) had made culturing difficult.

LITERATURE CITED

Fisher, F. E. 1951. An Entomophthora attacking citrus red mite. Florida Entomologist 34:83-88.

Fisher, F. E. 1954. Diseases of citrus insects. Fla. Agr. Exp. Sta. Ann. Rep. For 1953:191.

French, J. V. and E.M. Hutchinson. 1980. Citrus red mite found in the Lower Rio Grande Valley. Citrograph 65(7): 197-199.

French, J. V. 1984. Establishment of citrus red mite, Panonychus citri (McGregor), as a major pest of Texas citrus. Proc. Int. Soc. Citriculture. Vol.2:472-475.

Selhime, A.G. and M. H. Muma. 1966. Biological studies on an Entomophthora attacking Eutetranychus banksi. Florida Entomologist 49:161-168.

Weiser, J. and M. H. Muma. 1966. Entomophthora floridana N. S. P. (Phycomycetes: Entomophthoraceae), a parasite of the Texas citrus mite, Eutetranychus banksi. Florida Entomologist 49:155-159.

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