Research Report
Can Combinations of Insecticides and Entomopathogenic Fungi Do the Job on Hard-To-Kill Large White Grubs? Virginia Tech Researchers: Sudan Gyawaly, Graduate Student, Curt Laub, Research Associate, and Tom Kuhar, Ph.D., Professor, Department of Entomology Research Sponsors: Virginia Turfgrass Foundation (partial funding)
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nnual white grubs remain the most important insect pests of turfgrass in Virginia. Among the handful of species of white grubs that may be encountered in Virginia turf, the most common are Japanese beetle (Popillia japonica Newman) and masked chafers (Cyclocephala spp). The life cycles of these species are similar in that adult beetles emerge about midJune and lay their eggs in the soil usually in July. The grubs that result from these hatched eggs feed on grass roots and can kill large patches of grass under heavy infestations (more than 10 per square foot) (Photo 1). Additional damage can occur when vertebrate animals dig up the turf in search of the grubs to eat. Insecticides such as the neonicotinoid Merit (imidacloprid) and the newer diamide insecticide Acelepryn (chlorantraniliprole) provide excellent control of young white grubs when these chemicals are applied during the summer months. However, these products, as well as most other registered turf insecticides, do not adequately control the larger, more mature white grubs that can be found feeding on turf during the fall and spring months.
Thus, summertime application timing is very important for insecticide efficacy. However, this is not always practical. Mixtures of insecticides with different modes of action may have a synergistic effect and provide a much higher level of efficacy than either product used alone. Here, we evaluated if large (late instar) white grubs could be controlled effectively with reduced rates of Merit or Acelepryn if each was mixed with an entomopathogenic fungus (EPF) product such as Botanigard (Beauveria bassiana) or Met F52 (Metarhizium anisopliae). Three experiments were conducted at Virginia Tech from 2012 to 2013 to investigate this question.
Our research: Experiment 1
Can EPF products enhance the activity of low application rates of Acelepryn on mature white grubs in the lab? Third-instar masked chafer grubs were collected from untreated turf at the Virginia Tech Turfgrass Research Center in fall 2012. Experiments were carried out at 25°C (77°F) and 90% relative humidity in a temperature chamber in 30-ml plastic cups filled with sandyloam soil and perennial ryegrass as food. One grub was placed in each cup.
Photo 1. White grub damage to turf in southwest Virginia (left) and masked chafer grub.
16 | Virginia Turfgrass Journal May/June 2015 www.vaturf.org
Treatments are listed in Table 1 and included reduced rates of Acelepryn, full recommended rates of Botanigard and Met F52, and each combination of Acelepryn plus fungi. For each treatment, a total of 2 ml of insecticide solution was applied to each container. For the control treatment, 2 ml water was applied. The cups were sampled each week for number of live grubs, and percentage mortality was ultimately assessed after four weeks.
Research results to date
There was a significant effect of treatments on grub mortality (Table 1). The combined application of half the recommended rate of Acelepryn plus Botanigard resulted in the highest grub mortality among the treatments. However, even this treatment resulted in only about 50% mortality of the grubs. None of the combined treatments resulted in a synergistic interaction, only additive effects on efficacy. Nonetheless, with the exception of a half rate of Acelepryn mixed with Met F52, combinations of EPFs with Acelepryn increased the overall grub mortality compared with either used alone.
Our research: Experiment 2 Do EPF products or combinations of EPFs with insecticides affect egg-laying by Japanese beetles? Research has shown that adult female scarab beetles may be able to detect whether soil is infested with entomopathogenic fungi or treated with insecticides, and choose to not deposit their eggs in that soil. In summer 2013, more than 1,000 adult Japanese beetles were collected from Kentland Research