Among the more common and destructive pests that plague my garden every year is the Cucumber Beetle. Cucumber beetles are a vector for Bacterial Wilt Disease, Erwinia tracheiphila, which affect most members of the Cucurbit family of plants which include, squash, melons and cucumbers.
There are two species of this beetle which are common in the midwestern regions of North America: the Striped Cucumber Beetle, Chysomelidae Diabrotica and the Spotted Cucumber Beetle, Chrysomelidae Acalalymma. The adults of both species have yellow bodies with black markings and are about 1/4" long. Adults have chewing mouth parts and feed on the outer tissues of leaves, vines, fruit and, especially, flowers of the host plants. They also can be found feeding on plants other than in the Cucurbit family such as the ear silks of corn.
Cucumber beetles overwinter as adults amongst plant debris, compost piles and in lawns. In the spring, they emerge and locate host plants where they lay eggs in the soil at the roots of these plants. The larva hatch and feed on the roots which, when highly infested, may result in stunted plant growth. In mid-summer the adults hatch and feed on the mature plants. These beetles can reproduce two to three generations a year before the onset of winter.
The bacterium Erwinia tracheiphila resides in the digestive tracts of overwintering beetles and are transmitted to other beetles through feces and contaminated mouthparts. The bacterium affects the vascular structure of the host plant, inhibiting the transmission of fluids which turn sticky and sappy. The first sign of infection is droopiness of the leaves which can be mistaken as symptoms of drought. To identify the disease, cut a vine that has droopy leaves. If the sap is sticky, sappy or astringent, infection is likely. Once infected, the host plant cannot be saved and shall wilt until the entire plant is affected and dies. Remove all infected plants from the garden to reduce the possibility of spreading the disease to other plants.
The symptoms of Bacterial Wilt Disease are similar to another bacterium, Cucurbit Yellow Vine Disease, which is transmitted by squash bugs. Cucurbits, especially squashes, may suffer from co-infestation from both of these pests. Squash bugs and cucumber beetles have similar life cycles and similar methods of control.
"Early treatment is essential for beetle management in large commercial muskmelon or cucumber operations. A single post-transplant soil drench with Admire or Platinum can provide near season-long control. Repeated applications of contact insecticides are necessary to protect muskmelon plants from beetle feeding and transmission of bacterial wilt. Applications of foliar insecticides may be required twice per week during peak beetle activity."{1}
In the home garden, removing all debris from the garden including plant waste and mulch at the end of the growing season shall deprive cover for inscect pests to overwinter in. Rotate crops so that no cucurbits are growing in the same location in subsequent years. In the spring, cucurbit crops may be protected from egg-laying adults with floating row cover when seedlings emerge or are transplanted. This may limit the severity of local infestations. However when the plants are mature in the summer, employing row covers may not be practical.
For organic gardeners, controlling cucumber beetles on mature plants may be achieved by sprays containing pyrethrins which are natural compounds derived from chrysanthemums.
Caution should be used when using pyrethrins as they are powerful compounds which kill indiscriminately. ALL insecticides can harm beneficial predators and pollinators such as spiders ladybugs and bees. Always spray in the evening after bees and other pollinators have retired.
Cucumber beetles are avid flyers and difficult to kill by hand when found and disturbed. The best chance of finding and killing cucumber beetles by hand is to find them in groups inside flowers feeding on nectar and pollen.
For some crops, disease resistant cultivars have been bred, such as Slicemaster and Marketmore cucumbers. Presently there are no squash, cucumber or muskmelon cultivars known to have been bred with immunity to bacterial wilt. All watermelons, however, are immune. Check frequently with your favorite seed provider or your local garden stand for the latest disease resistant cultivars and methods of proper garden management.
{1}http://www.ca.uky.edu/entomology/entfacts/ef311.asp
{2}"The Organic Gardener's Handbook of Natural Insect and Disease Control", Rodale Press.