Cucumber Beetle

Order:Coleoptera
Family: Chrysomelidae

Nomenclature:
Several species populate this family of pest. They include the Striped Cucumber Beetle, which are further divided into the Eastern Striped Cucumber beetle and the Western Striped Cucumber Beetle. Another group are the Spotted Cucumber Beetle, The Western Spotted Cucumber Beetle and the Banded Cucumber Beetle.

Appearance
Most members of this family attain a length of 1/4 to 1/3 inch. They have moveable mouth parts which enable them to cut and chew plant materials. Their names derive from their coloration which is on their characteristic hard wing covers. They have several different color schemes so there is no single description that covers all species in this family. Most have black heads with antennae.

Distribution
Cucumber beetles are native to North America and are found in all areas of the United States from Canada to Mexico. They are most abundant and destructive in their southern range. They are generally not a problem in sandy soils. Spotted Cucumber Beetles cannot overwinter in the harsh northern climate zones, but migrate in from their southern range each year. The Striped Cucumber Beetles do overwinter to emerge each spring. All species are strong fliers and when aided by air currents can travel as much as 500 miles in only 3 to 4 days.

Plants attacked:
Plants of economic impact to humans include the cucurbit species (including but not limited to cucumbers, cantaloupes, winter squash, pumpkin, gourd, summer squash, and watermelon) as well as beans, corn, peanuts, potatoes, and other crops. Overwintering adult cucumber beetles emerge when the temperature rises above 50 degrees F. They feed on up to 200 plant species such as hawthorn and dandelion until cucurbits become available.

Damage description: Damage by cucumber beetles fall roughly into four catagories:

  • seedling destruction
  • flower and foliage destruction
  • root feeding
  • transmission of disease

Destruction begins in the spring when feeding by overwintering adults on the seedling stage of plants occur. The beetles feed on newly emerged cotyledons and stems, and are known to go below ground level to feed on plants as they emerge. After feeding on seedlings for a few days mating occurs. Adult females lay from 200 to 1200 eggs in the soil near seedlings. Larvae hatch in 7-10 days and begin feeding on the roots of the plants. The larvae chew holes and tunnel into the roots. Damage by larvae, except under dry conditions, is usually considered minor. The first generation of new adults emerge to feed on the foliage and flowers. Feeding damage to foliage is usually minor but severe feeding on flowers can result in poor fruit set. A second generation emerges about 3 months after the first. The entire life cycle occurs in 6-9 weeks. In northern climes there is only one generation per year, but in the southern range there are 2-3 generations per year.

Usually the worst damage caused by cucumber beetles is from transmission of disease. Bacterial wilt (Erwinia tracheiphila) can kill many plants in a field, seriously reducing yield. Wilt usually starts with a single leaf and spreads to the entire plant, killing it. The growth of the bacteria blocks the channels for sap circulation causing wilt due to lack of nutrients/fluids. Imagine as a comparison a growth in a human arm which blocks blood flow to the limb. Within a short time the arm would die (wilt). Untreated, this condition would lead to the death of the entire organism. Cucumber beetles also transmit squash mosaic virus and various bean viruses. The impact of disease transmission is probably the most important aspect of the cucumber beetle's biology.

Control practices: There are four strategies for control of cucumber beetles.

Row crop coverage

The earliest method that provided good results were reported in 1841. Cucurbit plants were covered with light covers such a cheese cloth which allowed light to enter but which excluded the insects. This method is still employed. Modern row crop covers can provide protection from insects, provide late frost protection and help in moisture retention.

Trap Cropping

The second method is to employ trap cropping. An early planting of cucurbits can be made to attract the overwintering cucumber beetles where they can be destroyed by insecticides. This reduces the numbers of beetles left to breed and to feed on the main crop planted later. It is necessary to pull out and burn the remaining vines of the trap crop after destroying the beetles. Trap crops are usually not recommended as the sole method used to control cucumber beetles.

Pesticides

A third method is chemical control (pesticides), used particularly in commercial planting. A soil insecticide can be employed at planting time for control of beetles during the seedling stage, and foliar treatments can be applied later in the season as required.

Parasitic control/Predatory Organisms

The use of parasitic organisms such as nematodes has been employed to naturally curb cucumber beetle populations. Other natural predators which help control cucumber beetle populations are bats, soldier beetles, tachinid flies, and braconid wasps.

Summary
The identification and control of cucumber beetles is an important part of managing the productivity of a valuable food crop. Unchecked, they can do considerable damage causing economic loss of these important food crops.


Sources:
www.ext.vt.edu/departments/entomology/factsheets/cucbeet.html
www.uky.edu/Ag/Entomology/entfacts/veg/ef311.htm
http://attra.ncat.org/attra-pub/cucumberbeetle.html
http://www.ghorganics.com/CucumberBeetles.htm

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.

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