Scouting and managing cucumber beetles
Know the strategies to help track and manage cucumber beetles on diverse vegetable farms. Learn about this pest and others at the 2014 Integrated Pest Management Academy, Feb. 18-19.
Cucumber beetles are defoliating herbivores. In Michigan, growers are likely to see striped cucumber beetles emerge just after Memorial Day and remain active all season long. Spotted cucumber beetle adults arrive later in the season from southern states in late July or early August. Large populations of either beetle can cause heavy defoliation and damage the surfaces of fruits later in the season, but primarily cause crop damage by transferring a disease called bacterial wilt during feeding (Photo 1). Plants suffering from this disease wilt in a patchy fashion as the disease spreads with some leaves looking healthy while others wilt.
Photo 1. The beginnings of bacterial wilt causing some leaves to wilt, while others remain healthy on the same cucumber plant. Photo credit: C. Welty
Infected plants attract more beetles that continue to spread the disease until whole plants eventually die due to a total bacterial blockage of their water-carrying xylem (Photo 2). Adult beetles overwinter with the bacteria in their gut all winter and emerge the following year ready to infect more plants. There is no cure for this crop disease. Management relies on control of the beetle vectors and prevention through scouting and integrated pest management (IPM).
Photo 2. The bacteria cause wilting by blocking the xylem of cucurbits. Photo credit: M.P. Hoffman
When cucumber beetles emerge from their winter slumber, they are hungry! They will munch on wild cucumber, early corn, tomatoes or peppers to stay alive until cucurbits are planted and available. Uninfected mature cucurbits can handle 20-50 percent defoliation without a change in yield. However, the seedling stage is very vulnerable to cucumber beetles and can be eaten all the way back to the soil line. Delaying planting, using mature transplants and protecting succulent young plants with row covers are all valid methods of reducing feeding damage from cucumber beetle.
Cucumber beetles are very mobile and scouting should occur at least once per week to ensure early detection once they arrive. Once economic thresholds are reached, control actions should be promptly used to knock back their populations and reduce bacterial wilt. For cantaloupe and cucumber, the economic threshold warranting an insecticide application is one beetle per plant; for squash it is two beetles per plant, and for pumpkins and watermelon it is five beetles per plant.
Since these beetles first move into the crop through edges before moving to the center of fields, a trap cropping system can be planned to border the main crop and catch these beetles. Effective trap crops on the edges should be a more attractive variety of the main crop and planted one to two weeks earlier than the center of the field. With careful monitoring, beetles are often detected in trap crop borders first and can be selectively sprayed to minimize chemical costs and adverse effects on pollinators and natural enemies (Photo 3).
Photo 3. A parasitoid fly maggot developed inside this striped cucumber, and has killed it in the process. Photo credit: Ben Phillips, MSU Extension
If you want to learn more about IPM strategies for insect pest management, then consider joining Michigan State University Extension at the third annual IPM Academy, Feb. 18-19, 2014, in Okemos, Mich., just outside of East Lansing, Mich. The Academy includes an afternoon session devoted to Managing Pests in Diverse Vegetable Rotations. We will cover insect pest management strategies based on crop rotations for Michigan vegetable growers and discuss how to build a program tailored to your operation. This session is meant to prepare you to address the weed, disease and insect problems you already have, and prepare you to manage those that emerge in the future.