Fungal Pathogens for Biocontrol of Western Flower Thrips and Green Peach Aphids in Greenhouses

Fungal Pathogens for Biocontrol of Western Flower Thrips and Green Peach Aphids in Greenhouses

Summary

Western flower thrips (WFT) and the green peach aphid (GPA) are major economic threats to the greenhouse industry. Control with conventional techniques is difficult, costly and often ineffective. Furthermore, it is complicated by the development of insecticide resistance. Innovative approaches to management are needed.

Our objectives are to investigate the insect pathogenic fungi Verticillium lecanii, Beauveria bassiana, Metarhizium anisopliae and Paecilomyces farinosus for ultimate incorporation into greenhouse IPM strategies.

We have already shown that a number of our fungal isolates are highly pathogenic to WFT. Bioassay tests will allow us to select the most lethal and versatile ones for evaluation in greenhouse pilot tests. We will study the performance of select isolates against WFT on plants and in the soil. This will enable us to identify strains that are efficacious and the insect stage that can be most effectively targeted for WFT management. We will determine the persistence of fungal inoculum in soils to ascertain if stable fungal populations develop to provide a long term reservoir of infective material and ultimately how often treatments should be applied

Preliminary pathogenicity studies indicate that a number of the WFT-active fungal strains are also active against GPA. Additional fungal isolates need to be screened and quantitative bioassays performed. With this broad base of strain types, we will be able to identify the best and most suitable strains for management. By initially selecting WFT-pathogenic strains, we will also be able to show which strains have potential for use against other pests, making them more cost-effective for greenhouse use.

Objectives

(1) Determine the on-plant efficacy of select fungal isolates against western flower thrips (WFT).

In our research on the biological control of pear thrips, Taeniothrips inconsequens, a new pest of sugar maple, we discovered strains of the fungus Verticillium lecanii causing significant mortality. We have found that these strains are highly pathogenic to WFT. In fact, they are significantly more pathogenic than other V. lecanii strains, including those presently used commercially in European greenhouses.
WFT has become one of the most serious pests in the greenhouse industry. It attacks virtually all floricultural crops, and in the East is the primary vector of the tomato spotted wilt virus in ornamental and vegetable plants.
Damage thresholds for floricultural crops are low or nonexistent, particularly for pests like WFT that attack the flower - the most important part of these aesthetic crops. Flower damage results in reduced yields, lower market values and increased costs for repeated pesticide applications.
Chemical control of WFT is hard to ensure because the insect hides deeply in plant crevices making contact with pesticides difficult. Additionally, WFT develops resistance quickly. Reinfestation through migration into greenhouses is an additional problem.
Entomopathogenic fungi offer a viable biocontrol option as they are particularly well-suited to humid greenhouse conditions, and some species are relatively easy to mass produce. Pathogenicity varies among V. lecanii strains and among other entomogenous fungi so screening and evaluation is essential for development.
The pathogenicity to WFT of 11 V. lecanii isolates, and three strains of Beauveria bassiana (two from thrips) and Metarhizium anisopliae has been demonstrated. Each has specific biocontrol merits and we need to further select and develop them for greenhouse use.
WFT is found on plants and in soil, so we will evaluate the performance of select pathogens against stages found in these two environments. This will provide information that will enable us to identify which strains should be used, and the insect stage and greenhouse habitat to target.

(2) Determine the persistence of fungal inoculum in greenhouse soils.

Infection in soil of WFT and other greenhouse pests is dependent on contact with sufficient infective units. Persistence of infective propagules in soil increases the probability of contact with a susceptible host, thereby enhancing the possibility of infection.
Soil provides favorable moisture and temperature for persistence of fungal conidia and is the natural reservoir of fungi such as B. bassiana, M. anisopliae and V. lecanii. Conidia of B. bassiana persist in soil for 2 years, and M. anisopliae can survive and multiply in the soil over time. The survival of V. lecanii is uncertain, though we know it is strongly influenced by soil factors. Information on the survival of fungal ínocula is essential to ascertain how often treatments should be applied and whether a stable population is present to provide a long term reservoir of infective material.
We recognize that the situation in a greenhouse will differ according to spray practices and the compatibility of our fungi with other treatments must be assessed in future research. On-plant persistence and the transmission of the disease within a pest population must also be addressed. It is essential first, that the reaction of the fungi in a controlled environment be determined. Ultimately,, it will be possible to make recommendations to alter management practices to enhance their persistence.

(3)Determine pathogenicity of select fungal strains against the green peach aphid (GPA).

GPA is one of the most destructive pests of greenhouse vegetables and flowers in the Northeast. In this protected environment, its reproductive potential is such that destructive populations can build up within a matter of days. Economic damage results from direct removal of plant nutrients causing premature plant senescence, by excretion of honeydew on which unsightly sooty mold grows, and through transmission of plant viruses. Over 100 viruses are vectored by GPA to a variety of covered crops. Damage resulting from disease transmission is often more dramatic than direct feeding injury.

GPA preferentially feeds on the underside of leaves or in flowers, avoiding contact and limiting the efficacy of insecticides; and resistance develops quickly. Production and distribution of plants by large-scale propagators using intensive spray regimes contributes to the rapid development and spread of resistant strains.

These problems could be minimized by using entomogenous fungi, especially if they could become established in the pest population. V. lecanii is pathogenic to GPA, and control of aphids has been achieved in greenhouses in Europe. However, variation in the pathogenicity and disease-spreading potential of different V. lecanii isolates has been documented.

Some of our V. lecanii isolates from pear thrips are pathogenic to GPA. In addition, a number of B. bassiana, M. anisopliae and Paecilomyces farinosus strains, isolated from Vermont forest soils and forest insects, have recently been shown to be pathogenic to GPA in preliminary screening assays. B. bassiana and M. anisopliae also show variance in pathogenicity to aphids, so further screening assays against GPA, followed by bioassays of promising isolates, are required. With this broad base of strain types, we will be able to identify the best and most suitable strains for management. By initially selecting strains that exhibit toxicity to WFT, we will also be able to show which strains have potential for use against other pests, making them more cost-effective for greenhouse use.