Is switchgrass a host for Japanese beetle and spotted wing Drosophila?

Research is being conducted to determine if switchgrass is a vector and host for common insect pests and crop diseases in cropping systems.

Switchgrass is a native perennial plant species that grows across much of the United States. It is used to control soil erosion, as a wildlife cover and a feedstock for biofuel production. Due to its widespread nature, switchgrass grows in or adjacent to many different types of cropping systems, including commercial fruit crops. Research is being conducted around the United States, including Michigan, to determine if switchgrass is a vector and host for common insect pests and crop diseases in cropping systems. Blueberry growers in Michigan have asked if switchgrass is a host for Japanese beetle and spotted wing Drosophila. The answer to their question lies in understanding the life cycle of the two insects.

Japanese beetle adults emerge in the summer and feed on the foliage of many plants. Japanese beetles lay their eggs in grass, since the beetle larvae feed on grass roots. They prefer to lay their eggs in irrigated and mown grass over grass that is non-irrigated and tall. The mowed grassy row middles in irrigated blueberry fields are attractive egg laying sites. Some blueberry growers have removed their grassy row middles to reduce the number of larvae in their fields.

The spotted wing Drosophila (SWD) is an invasive pest of soft thin-skinned fruit such as raspberries and blueberries. The insect has the ability to attack sound fruit as it ripens. The SWD lays its eggs under the skin of the fruit, in the soft flesh of fruit. When the larvae hatch they consume the fruit from the inside out. The fly reproduces very quickly. It has a short life span and female flies lay hundreds of eggs. The whole life cycle of the insect revolves around soft-skinned fruit and the SWD population moves from one species of fruit to another through the growing season as different fruit ripen and then disappear.

So can switchgrass serve as a host and vector for these two fruit pests? Since switchgrass does not produce a soft thin-skinned fruit, it is not attractive to SWD and is therefore not a host or even an attractive site. By virtue of the fact that switchgrass is a grass, it is possible that switchgrass could serve as a vector for Japanese beetle. However, it is commonly grown without irrigation and allowed to grow until it is harvested in the fall after a killing frost. It is important to note that switchgrass is not listed on Japanese beetle preferred grass feeding lists, nor is it listed under any Japanese beetle host specie lists. Rufus Isaacs, fruit specialist in the Entomology Department at Michigan State University, points out that switchgrass plantings are more likely to support predators that eat Japanese beetles. Japanese beetle populations have steadily declined over the past decade due to the effectiveness of its natural enemies.

Scouting fields is the most effective way to monitor Japanese beetle and spotted wing Drosophila activity. Scouting information can be found at the Integrated Pest Management website.

As new uses for crops such as switchgrass develop, it is important to consider the unintended impacts of the introduction or expansion of new or minor crops. The examples of the insects above are a reminder that the diversity of Michigan agriculture requires diligence in protecting our important ecosystem.

Michigan State University Extension continues to offer an unbiased perspective of the biofuel industry to inform citizens on this topic. For more information on switchgrass, biofuels, and Japanese beetle and spotted wing Drosophila control, refer to archived articles on the MSU Extension News website. 

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