Striped (Acalymma vitattum; StCB) and, to a lesser extent, spotted (Diabrotica undecimpunctata; SpCB) cucumber beetles are damaging pests on crops in the family Cucurbitaceae (e.g., cucumber, squash, pumpkin, watermelon). These pests not only inflict severe damage to various plant parts, including roots, leaves (Figure 1), flowers (Figure 2), and fruits (Figure 3), but transmit the bacterial wilt pathogen (Erwinia tracheiphila) that can kill cucurbits (Figure 4).
Figure 1. Feeding damage on zucchini by the adult beetles on leaves (Photo by John Obermeyer).
Figure 2. Adult StCB beetles are feeding from melon flowers (Photo by John Obermeyer).
Figure 3. Feeding damage on zucchini fruit caused by the adult beetles (Photo by John Obermeyer).
Figure 4. Wilted cucumber plants indicate the presence of bacterial wilt disease, caused by the pathogen Erwinia tracheiphila, transmitted by cucumber beetles (Photo by John Obermeyer).
How to identify cucumber beetles?
Adult striped cucumber beetles are about 1/5 inch long and 1/10 inch wide. They are yellow-green in color with three black stripes running the length of their bodies, and they have a black head and antenna (Figure 5). Striped cucumber beetles are frequently mistaken for western corn rootworm beetles (Diabrotica virgifera virgifera) (Figure 6), although the last do not typically pose a threat to cucurbits but can be found feeding on plant parts. A simple way to differentiate between the two is by inspecting their undersides: striped cucumber beetles have black abdomens on the underside and four distinct stripes on the top side that extend the full length of the abdomen, whereas western corn rootworms have yellow-green abdomens, and the stripes on the top side are often smudged and do not extend the entire length of the abdomen.
Figure 5. Striped cucumber beetle feeding squash leaf (Photo by John Obermeyer).
Figure 6. Adult female Western corn rootworm (Photo by John Obermeyer). Notice the stripes on the abdomen are smudged and don’t extend the entire length of the body.
Spotted cucumber beetle adults are similar in size to striped cucumber beetle adults. They are yellow-green in color, with 12 black spots on their backs (Figure 7).
Figure 7. Adult spotted cucumber beetle (Photo by John Obermeyer).
Life cycle
Beetles overwinter as unmated adults on the edges of agricultural fields. In Indiana, their life cycle includes two distinct generations. The first generation typically emerges from their overwintering sites in late April or early May. Subsequently, the second generation, originating from the offspring of the preceding one, emerges around late July to early August. After beetles emerge, they engage in mating and females deposit their eggs at the base of a host plant. StCB are cucurbit specialists, while SpCB can reproduce and live on other crop hosts. The larvae then develop in the soil, primarily subsisting on plant roots. While this feeding behavior generally does not lead to significant economic damage, there are occasions when the larvae can cause harm by infiltrating developing fruits that come into contact with the ground. Both types of cucumber beetle larvae exhibit a worm-like appearance, characterized by a white body with a dark head and three pairs of legs. After completing their larval stage, pupation occurs in the soil, culminating in the emergence of adult beetles approximately one week later.
Damage
These two species of beetles serve as vectors of bacterial wilt (mentioned earlier). Upon acquisition, the bacterium spreads through their feces or on their mouthparts. As they feed on the leaves, they create openings that facilitate the entry of the pathogen into the plant. Once inside, the bacterium proliferates within the xylem, leading to rapid wilting and eventual death of the plant. Unfortunately, there are no effective methods for saving an infected plant. Infected plants pose a risk as other cucumber beetles can pick up the bacterium from these plants and transmit it to healthy ones. Therefore, it is crucial to promptly remove and destroy any wilted plants to prevent further spread of the disease.
Susceptibility to bacterial wilt varies among cucurbit crops. Cantaloupe and cucumber are very susceptible and succumb to the disease. Squash, pumpkin, and watermelon are resistant to the disease but still vulnerable to damage from beetle feeding. In crops unaffected by bacterial disease, a higher beetle population per plant can be tolerated. For cantaloupe and cucumber, the economic threshold stands at 1 beetle per plant, while for watermelon, pumpkin, and squash, it rises to 5 beetles per plant before action is needed.
How to control it?
On small farms and for organic growers, physical exclusion of the beetle is the best management strategy. It can be very effective in high tunnels to install exclusion netting over the openings to eliminate the pest from entering and bringing with it bacteria. See this YouTube video for a demonstration. For large-scale commercial growers, insecticides are the best tool. They do not need to be applied via seed treatments if you are planting transplants in the field. Rather, if you are planting when beetles are present, a soil drench at planting will be most effective, using the low rate on the label. If you have delayed planting or don’t suffer from beetle damage until midsummer, scouting the fields until you reach an economic threshold and then applying a foliar spray of insecticide is the best option. Using the Midwest Vegetable Production Guide, choose a product suitable for your needs and based on the phenology of the plant; avoid systemic products and most neonicotinoids during bloom to preserve pollinators.
You can find more detailed information about alternative tools to manage beetles in organic production: https://vegcropshotline.org/article/organic-control-methods-for-striped-cucumber-beetles/