Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2003
Publication Date: September 1, 2003
Citation: Wraight, S.P., Galaini-Wraight, S., Carruthers, R.I., Roberts, D.W. 2003. Zoophthora radicans (zygomycetes: entomophthorales) spore production from naturally-infected empoasca kraemeri cadavers and dry-formulated mycelium under laboratory and field conditions. Biological Control. 28(1):60-77.
Interpretive Summary: Empoasca leafhoppers are widespread and important pests of agriculture. In the USA, several species cause serious damage to a broad range of food, forage and fiber crops, including potatoes, beans, alfalfa, and cotton. Zoophthora radicans, a common insect-pathogenic fungus, is an important natural enemy of leafhoppers, and its development for biological control has been pursued for many years. This fungus, however, produces fragile, thin-walled spores that are difficult to mass produce, harvest and formulate on an industrial scale. An alternative approach, which circumvents this problem, involves production of vegetative mycelium. After being applied in the crop, the mycelium produces spores that infect the target pests. USDA scientists have developed and patented a process for formulation of Z. radicans mycelium into dry granules that can be stored for many months under refrigeration. This paper reports studies which determined the effects of temperature on sporulation of this dried mycelium formulation and compared sporulation of the formulated fungus to wild fungus growing on naturally infected leafhopper cadavers in the field. Results indicated that the sporulation behavior of the formulated fungus was similar to that of the wild fungus and that the formulated product was therefore an excellent source of inoculum for infecting leafhoppers in the field. These findings will contribute to the continuing scientific and commercial development of this and other species of fungal pathogens for pest control.
Studies were conducted to assess the effects of temperature on sporulation of a dried-mycelium formulation of the entomophthoralean fungus Zoophthora radicans and to compare sporulation of formulated versus wild fungus on naturally infected cadavers of the leafhopper Empoasca kraemeri. Spore production by the formulation increased from 3.1 to 13.7 x 104 conidia/mg (dry weight) over the temperature range 5-20øC and then decreased to 10.7 104 conidia/mg at 25øC and to nearly zero at 31øC. The optimum sporulation temperature was estimated to be 23.6øC. Pieces of formulated mycelium (2 x 2 x 0.5 mm) placed on bean and cowpea foliage in the field generated conidia in a manner similar (both temporally and quantitatively) to wild Z. radicans on leafhopper cadavers. Sporulation commenced within a few h after evening dewset and ceased with the return of dry conditions after 08:00 h. Both the formulated fungus and wild fungus on adult leafhopper underwent multiple daytime desiccation/nighttime rehydration cycles, sporulating on up to eight consecutive nights. Second, third, fourth, and fifth-instar cadavers supported sporulation of the wild fungus for only 5-6 nights. On a dry weight basis, the wild fungus produced substantially more conidia than the formulated fungus; however, differences were less pronounced based on hymenium surface area. These findings indicate that the dried-mycelium formulation is well suited as a source of inoculum for initiating or augmenting epizootics in leafhopper populations.