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Cucumber beetles

  • March 28, 2020
  • Martha Proctor
  • Gardeners beware!  With the onset of warm days, there is a ravenous pest than can move rapidly onto its favored plants, curcurbits or melons (especially honeydew, crenshaw and casaba), and begin making large holes in the foliage and scarring the melon’s outer skin and crown.  In fact, this colorful pest is known to feed on more than 200 alternate host plants.  The adults consume the tender young shoots, flowers and stems on beans, squash, corn, potatoes, tomatoes, peas, beets, asparagus, eggplant, cabbage, and other vegetables as well as the shoots and blossoms of just about all the stone fruits (including peaches, apricots, plums).  This major pest of curcubits and culprit in question is the western-spotted cucumber beetle (Diabrotica undecimpunctata) and western-striped cucumber beetle. (Acalymma vittatum)

    The western-spotted cucumber beetle is the most abundant species in California.  Adult western-spotted cucumber beetles are shiny, 1/4" long, with long antennae and black heads.  Their greenish-yellow coloring and the twelve black spots on their back make them easy to identify.    Larvae are whitish and slender with three pairs of short legs; the head and tip of the abdomen are darker.  Don’t confuse our welcome visitor, lady beetles (commonly known as ladybugs), who have short and stubby antennae, with cucumber beetles.

    Beetles overwinter as adults in weedy areas and move into our gardens as soon as plants start to come up.  They lay yellow-orange eggs at the base of plants or in soil cracks.  Hatching larvae burrow into the ground seeking out roots, feed there for 2 to 6 weeks, pupate and emerge as adult beetles and then attack the aboveground portions of the plants they favor.  Fortunately, the feeding of the larval stage beetle causes little harm to plants.  There are three generations a year.    

    Cucumber beetles are difficult to control. Unfortunately, there are no effective cultural controls or effective natural enemies for these pests. Start checking your garden for cucumber beetles soon after transplanting or when seedlings emerge. Inspect chewed leaves, petals, and fruits for adults (especially leaf and fruit undersides touching soil), in flowers and at stem bases. The best strategy for most vegetable gardens is to place protective cloth, individual cups or cones over emerging plants and remove it when plants are old enough to tolerate damage.  On stone fruit trees, early harvest may be the only option.    

    Prevention and early control are essential. The least toxic method of extermination is hand picking and dropping the pests into a bucket of soapy water to drown. This can prove difficult as the beetles are most active between dusk and dawn, fly readily, and move quickly. Because the larvae are underground, the only life stage that’s treatable is the adult beetle stage.  Spray with pyrethrum, Neem, or spinosad products. In fall, remove garden debris as these are likely to become overwintering sites. In fall or spring, it can help to lightly till soil to kill eggs and larvae.

    Both species of cucumber beetle are vectors of squash mosaic virus and the pathogen that causes bacteria wilt of cucurbits, Erwinia tracheiphia.  Bacterial wilt causes susceptible plants to wilt and die.  Also, other cucumber beetles can pick up the bacterium from infected plants and transfer it to healthy plants.  To prevent bacterial wilt in susceptible crops, scout out beetles twice weekly at the seedling stage.  Remove and destroy any wilted plants.  Bacterial wilt is most severe in cantaloupe and cucumber, less so on squash and pumpkin and rare in established watermelon plants.

    The recommendation is to consider treatment on adult beetles with pesticides when numbers reach an average of 1 beetle/plant during the seedling to 4" tall stage.  If infestations are severe and an insecticide is used, it must be used with caution to avoid injury to bees. 

    Once plants get large enough, they usually survive.  To minimize the chance of having a cucumber beetle infestation, rotate plants to avoid overwintering and shield young seedlings.