Weird Beetle on Watermelon Plants

Striped Cucumber Beetle

• Striped cucumber beetles are a quarter-inch long and yellow-green. Their main species are Acalymma vittatum (striped cucumber beetle) and the smaller A. trivittata (Western striped cucumber beetle). Both have long, dark antennae and three evenly spaced black stripes down their backs. Although similar in appearance to the the beetle that produces the western corn rootworm (Diabrotica virgifera 'LeConte'), striped species prefer cucurbits and are a greater problem for watermelons.

Spotted Cucumber Beetle

• Both striped and spotted cucumber beetles are members of the leaf-eating Chrysomelidae family. Spotted cucumber beetles (Diabrotica undecimpunctata) eat a broader range of plants, so they spread out their damage. Part of the western corn beetle's clan, the spotted beetle is also commonly called the southern corn rootworm and has 12 black spots on its back.

Growth Stages

• Striped and spotted cucumber beetles are found throughout the United States but are particularly troublesome in the South, where they can hatch up to four times a year, according to the Yardener website. During the winter, adult cucumber beetles hide under plant debris and in dense grass, invading gardens when temperatures reach 60 degrees Fahrenheit. As the garden grows, they lay their orange-yellow eggs in cracks in the soil near plant roots or on the undersides of cucurbit leaves. Hatching in five to eight days, the worm-like, light yellow larvae are three-quarters of an inch long when they hatch and bore into cucurbit roots and stems. After eating for 15 days, the larvae burrow into the soil and become pupae (a cocoon stage), then metamorphose into beetles in a week's time. Yardener notes that they only hatch once a year in cooler Northern areas.

Bacterial Wilt

• The bacterium (Erwinia tracheiphila) that causes cucurbit wilt overwinters in the intestinal tract of both kinds of cucumber beetles. But it is striped cucumber beetles that cause the biggest wilt problem for watermelon, because they dine only on cucurbits. The beetles spread the bacteria as they chew on plant parts or defecate. As the bacteria multiply inside the plant, they block the entry of water. When watermelon or other plant leaves begin wilting from the disease, there is no cure, according to the University of Kentucky.

Organic Controls

• Clear plastic row covers help keep cucumber beetles from alighting on plants. The covers need to be removed when blossoms emerge and it's time for insect pollination. Organic controls include interplanting watermelons and other cucurbits with companion plants that repel cucumber beetles such as radish, nasturtium and tansy or ones that attract beneficial insects that prey on the beetles. These include buckwheat, cowpeas and sweet clover.