Managing A Challenging Subterranean Clover Pest: Sustainable Control Using Insect Pathogens

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Commodities

• Agronomic: general hay and forage crops

Practices

• Pest Management: biological control, integrated pest management

Insect pathogens provide an effective means of suppressing pests but have received less attention compared with other biological control agents. For pests that develop below ground, insect pathogens may offer the best management option. My goal was to investigate the virulence of insect pathogens against the clover root borer, an economically important and unique bark beetle pest inadvertently introduced into the United States over 100 years ago. While bark beetles typically develop in the trunks of trees in forests, the clover root borer develops belowground within the roots of red clover. In my study, red clover seed fields were surveyed for the presence of naturally occurring entomopathogens and their virulence was evaluated against clover root borers. Based on morphology of spore bearing stuctures and sequences of ITS regions of fungal isolate genes, Beauveria bassiana, B. pseudobassiana, Isaria fumosorosea, Lecanicillium muscarium, Metarhizium anisopliae var anisopliae, M. robertsii, M. quizhouense and M. brunneum were detected in red clover seed fields. This is the first report of these entomopathogenic fungi in red clover fields in western Oregon. A laboratory study conducted with B. bassiana (isolate FD) documented that at high spore concentrations fungal spores are transmitted horizontally which facilitate spread of the disease pathogen. A second laboratory bioassay documented that field-isolated Beauveria bassiana and Isaria fumosorosea had similar levels of virulence compared to commercial products, Metarhizium anisopliae and Isaria fumosorosea that were tested. For determining the impact when clover root borers are exposed to entomopathogens in field collected soil, Beauveria bassiana (isolate FD) and Metarhizium anisopliae var anisopliae (isolate A4-MA) were evaluated. The study showed that Metarhizium anisopliae successfully established a colony, sporulated in the soil, and infected clover root borer adults while growth, development, and infection by Beauveria bassiana was minimal. Based on these studies, entomopathogenic fungi have potential for use as a biological control agent for clover root borer pests in red clover seed fields in western oregon.

Introduction

The clover root borer, native to Europe, was accidently introduced into the U.S. over 100 years ago. Early studies were focused on gathering information related to its biology, development, and damage to red clover (Westgate and Hillman 1911; Rockwood, 1926). Subsequent studies evaluated control strategies using organochlorine insecticides which killed the pest (Gyrisco and Marshall 1950; Cyrisco et al. 1954; Preuss and Weaver 1958; Koehler et al. 1961). These insecticides had long residual action and were thus effective in killing the pest during the short period when adults emerged from the roots below ground and dispersed to infest new hosts. After the ban on use of organochlorines, the newer insecticides that were developed had shorter residual action for mitigating negative impacts on the environment, but none were effective for controlling the pest. Since the ban on use of this group of insecticides, the pest has received little attention due to the challenges of developing a management strategy given its subterranean development. Insecticides currently registered for red clover are not effective against the pest (Rao et al. 2012). In Chile, new cultivars are being evaluated against the pest (Alarcón et al. 2010), but in the U.S., no efforts are being directed towards examination of host plant resistance. Meanwhile, entomopathogenic fungi and nematodes have been found to be effective against diverse beetle pests when applied to the soil or when incorporated into an autoinoculation strategy (Vega et al. 1995; Lacey and Shapiro-Ilan 2008).

1. Determine the presence and abundance of naturally occurring soil dwelling insect pathogens of the clover root borer.

2. Compare the virulence of entomopathogens of the clover root borer:

• Assess virulence of entomopathogenic fungi against the clover root borer
• Assess virulence of entomopathogenic nematodes against the clover root borer
• Assess horizontal transfer of entomopathogens in the clover root borer