Two cucumber beetle species occur in South Dakota; the striped cucumber beetle (Figure 1) and the spotted cucumber beetle (also known as the southern corn rootworm) (Figure 2). The striped cucumber beetle is most common and is the main threat to cucurbits. The spotted cucumber beetle appears less frequently, especially in the western half of the state.
The adults of both cucumber beetle species are approximately ¼ of an inch long and have yellow elytra (hardened wing covers) with black markings. Striped cucumber beetles have three distinct black stripes that run lengthwise on their backs, whereas spotted cucumber beetles have twelve black spots. The larvae of both species are small and cream colored and live in the soil, where they feed on roots. The larvae are rarely observed unless the roots of the plants are dug up.
Figure 2. Spotted cucumber beetle. Note the twelve black spots on the back.
Credit: A. Varenhorst
Adult cucumber beetles overwinter in debris and become active in the spring and early summer. They search for cucurbits to feed on, mate, and lay eggs. Generally, these beetles escape unnoticed until the first summer generation of adult beetles emerges and populations dramatically increase. Up to this point, the larvae developing in the soil have little to no impact on plant health. Upon emergence, the adult beetles begin feeding on the stems and foliage of cucurbits. As the plants mature, the beetles will progress to feeding on the flowers, and eventually the fruit. Cucumber beetle feeding can result in girdled stems, widespread defoliation, and spoiling of cucurbit fruit (Figure 3).
Cucumber beetles are also the vectors of bacterial wilt, which is a disease that can quickly decimate many types of cucurbits. The bacteria that causes bacterial wilt plugs up the vascular tissue of cucurbits, causing the vines to wilt and die. There is no recovery for the plant once it is infected.
One of the main concerns with cucumber beetles is that they are a difficult pest to manage. Several cultural practices can be applied to prevent infestations from becoming severe. At planting time, place mulch around cucurbits to deter cucumber beetle adults from laying eggs near the plants. Removing mulch and debris after harvest can reduce the number of overwintering sites for striped cucumber beetles in the fall. If you detect signs of bacterial wilt, be sure to remove the infected plants quickly so that cucumber beetles are unable to feed on them and spread the disease. Destroy the infected plants – do not put them in your compost pile, as they can remain a source of inoculum for the following season.
Early detection is critical, so be sure to regularly scout cucurbits throughout the growing season. The threshold for managing cucumber beetles is when you detect two or more beetles per plant or when estimated defoliation of leaves reaches 25%. Once cucumber beetles reach threshold levels, management action should be taken to avoid extensive injury to the infested crop. Organic options include products such as neem oil, spinosad, and pyrethrin. These products can reduce populations or at least deter feeding; however, they do not have long-lasting residuals. Conventional products like carbaryl (Sevin), permethrin, and pyrethroids can be effective. When using any insecticide in the garden, be sure to time applications for when pollinators will be least active, and read and follow the label directions.
Reference: World-Burkness, S. and J. Hahn. 2017. Cucumber beetles in vegetable gardens. University of Minnesota Extension.