Striped and spotted beetles
Description: Striped cucumber beetles (Acalymma vittatum) and spotted cucumber beetles (Cerotoma trifurcata) are serious pests of watermelon crops. Both insects are yellow-green in colour and 0.20 cm long. The striped cucumber beetle has three black stripes running the length of its back while the spotted has 12 black spots on its back. The adult striped cucumber beetle resembles the corn rootworm beetle, which can often be found feeding on the pollen of cucurbit blossoms later in the summer. The cucumber beetle has a black abdomen while the corn root worm has a yellow-green abdomen. Life cycle: Striped cucumber beetles overwinter as adults in protected areas. They become active in mid to late May. After mating, the female lays eggs in the soil at the base of cucurbits. The beetles are attracted to the cucurbitacin produced by the plants. Once the eggs hatch, the larvae feed on plant roots for two to three weeks. They pupate in the soil. Spotted cucumber beetles do not overwinter. Adult beetles migrate into the state in early to mid-July. Because they arrive later in the season, they pose less of a problem than their striped counterparts. Damage: Feeding damage by the striped cucumber beetle larvae can stunt or kill off seedlings or transplants. Adult beetles feed on the above-ground plant parts and fruit. More importantly, these beetles spread the bacteria that cause bacterial wilt. This disease plugs the water-conducting vessels of the plant, eventually resulting in plant die off. The adult cucumber beetles are such efficient carriers (vectors) of the bacteria that serious crop damage can occur even if only 10% of the beetles are inffective. Management: Beetles are usually localized in the field and spot treatments are effective. Some growers plant a row of vine crops near the upwind edge of the field on yellow or gold plastic mulch. This coloured mulch attracts cucumber beetles to the crops in that row where they can be killed or removed. Watermelon is not susceptible to bacterial wilt, so treatment should only be done to prevent heavy feeding damage. Insecticides kill bees, so spraying late in the day or at night protect these important pollinators. Discontinue treatments after the initial peak declines as the insecticides may reduce fruit set or cause flowers to abort. |
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