Cucumber Beetles and Bacterial Wilt

Insects can cause two kinds of damage in plants. First of course, is their direct feeding on the plants. But some insects can also transmit diseases to those plants as well. A good example in muskmelons is the cucumber beetle. These insects can be extremely damaging to a crop, not only because of their feeding, but because they can also vector bacterial wilt, which can quickly destroy infected plants.

Cucumber beetles come in two varieties, the striped cucumber beetle and the spotted cucumber beetle, which is also known as the southern corn rootworm. These insects overwinter as adults in protected places. Eggs are then laid in the soil near host plants. The larvae feed on the stems and roots of the host plant, with pupation in the soil. Up to 4 generations per year may be seen in the southern United States.

Feeding damage can be destructive, especially on young plants. All parts of the plants are fed upon, including roots, stems, leaves, tender terminals, and flowers. Occasionally, damage may occur to the fruits as well, attacking the rinds. Many growers have told me that they don't usually try to control cucumber beetles after the plants are older, since they don't cause as much problems then. This may be true in some years, but it's risky to assume that. It's true that a larger plant has a better chance of withstanding attack, simply because there is more plant to regenerate new growth compared to a small plant. But damage can still occur, and in some years, the fruit may be attacked as well, rendering it less marketable. In addition, you are only providing more meals for adult beetles who will give you more offspring for the following season.

The other type of damage that cucumber beetles inflict is bacterial wilt disease. Cucumber beetles are the only known vector of this disease, which is spread around from plant to plant through their feeding. After the disease is transferred to the plant by the beetle's feeding, the bacteria reproduce and are transferred through the plant's vascular system. Eventually the bacterial material plugs the vascular system and the plant wilts. Without the ability to transport water to the leaves, the plant quickly dies.

This is another reason why control of the cucumber beetle is needed no matter what the age of the plant. Even mature plants are subject to bacterial wilt infection from cucumber beetle feeding. While every cucumber beetle may not carry the disease, it’s risky to assume that your crop will be safe. It only takes a little feeding from an infected beetle to transmit the disease.

Once bacterial wilt is established, cucumber beetles spread the disease around the field. They feed on infected plants, then move to uninfected plants and inoculate them. Even if your crop is symptom-free when you plant, the beetles can pick up the disease from other cucurbit crops and weeds.

The key to controlling this disease is to control the cucumber beetle. If they are not there, you shouldn't have a problem. If you already have some diseased plants, controlling the beetle activity should reduce the spread of the disease. If only a few plants are infected, you might have better results if you remove those plants along with spraying for beetles. This would be impractical on a large scale, and is only effective if it is done when the disease is first noticed. If the disease is widespread, the better approach is to eliminate the beetle.

(by Tim Baker, Horticulture Specialist, Grundy County)