Cucumber beetles in vegetable gardens
There are two cucumber beetle species to be aware of in Minnesota; the striped cucumber beetle, Acalymma vittatum, and the spotted cucumber beetle, Diabrotica undecimpunctata howardi. Both species can be found on cucurbits (squash, cucumbers, melons, etc.), however, spotted cucumber beetles are found much less frequently in Minnesota. The striped cucumber beetle generally occurs in low numbers from year to year but occasionally is very abundant and damaging to plants. Besides causing direct feeding damage on foliage and fruit, striped cucumber beetle also vectors the bacterial wilt pathogen, Erwinia tracheiphila. Cantaloupe and muskmelons are especially vulnerable to bacterial wilt spread by these beetles.
Adult striped cucumber beetles are about 1/5 inch long, 1/10 inch wide, and have yellow wings with three longitudinal black stripes. The spotted cucumber beetle is similar in size, shape, and color, but instead of stripes, this beetle has 12 black spots on its wing covers. Larvae of both species are small (3/8 in) and creamy white colored. Eggs of both species are pale orange-yellow and are laid in groups.
Striped cucumber beetles overwinter as adults in leaf litter. They emerge in late May to early June. Adults feed on blossoms of flowering plants, mate, and lay eggs in the soil at the base of host plants (cucumber, squash, etc.). Eggs hatch in several weeks and larvae feed on plant roots and underground parts of stems. Larvae pupate in the soil and emerge later in the summer as adults. It takes about 40 to 60 days for this insect to go from an egg to an adult. Adults return to cucurbit plants and feed on the foliage later in the summer. There is typically one generation per year.
Unlike striped cucumber beetles, spotted cucumber beetles do not overwinter in Minnesota. Adults migrate from the Southern U.S. and usually arrive in our area in late June or early July. Adults lay eggs on non-cucurbit plants such as corn and other grasses. Larvae feed on the roots of grasses and pupate in the soil. Like the striped cucumber beetle, it takes approximately 40-60 days from egg to adult and there is only one generation per year.
Larvae feed on cucurbit roots and underground portions of the stems. However, this feeding generally has little impact on plant health. Once adults have emerged, feeding occurs on the foliage. If populations are high, the beetles will feed on the stems of the plants as well.
Adult feeding is of particular concern when plants are in the cotyledon and first through third true-leaf stages. At this size, the plants are small enough that high populations of striped cucumber beetles can either defoliate the plants completely or girdle the stem. As plants grow beyond the third true-leaf stage, several cucurbits (cucumbers and pumpkins) can tolerate very high levels of defoliation. Once flowering occurs, striped cucumber beetles will usually move off the foliage and begin feeding on blossoms and pollen. Fruit or flower feeding is not usually important although feeding on fruit may cause some cosmetic damage when populations of striped cucumber beetles are high.
A major concern with striped cucumber beetle is the ability of the adult to transmit the bacteria that causes bacterial wilt. The bacterium infects the plants' vascular system and causes plants to wilt. Once the infection is established, control is not possible. The bacterium is acquired by striped cucumber beetles feeding on infected symptomless weed hosts or on neighboring infected cucurbits.
Symptoms of bacterial wilt on cucurbits include leaf and vine wilting. A portion of the plant (a few leaves) may start showing symptoms and recover overnight. However, wilting can spread rapidly and within a few days the entire plant will be dead. A key point to remember about bacterial wilt is that not all cucurbits are equally susceptible; cucumbers and muskmelon are most susceptible, while squash, pumpkin, and watermelon are generally very tolerant or resistant to bacterial wilt.
To determine if your plant is infected with bacterial wilt take a stem that exhibits symptoms from a plant and cut it in half. Hold the two cut ends together for 10-15 seconds and slowly pull them apart. If bacterium is present, there should be whitish bacterial ooze that forms a mucus-like string between the two cut ends. However, this technique is sometimes difficult to use for diagnosis and should not be relied on as the sole indication that the bacterium is either present or absent.
Monitoring your cucurbit plants early in the season for striped cucumber beetles is important to determine if management is needed. Focus monitoring efforts on the cotyledon and first to third true-leaf stage plants, when the plants are the most susceptible to defoliation and bacterial wilt. However, management may be necessary until fruiting as larger plants are susceptible to bacterial wilt as well. Be prepared to manage striped cucumber beetles, particularly if you have had problems with them in the past. It is rare to see spotted cucumber beetles early in the season and, in general, spotted cucumber beetle numbers are too low to require treatment.
Remove weeds in and around your garden because they may be potential hosts for adults. During the planting season apply a heavy layer of mulch around established cucurbit plants to reduce the attractiveness of the plants for egg laying adults. If you have determined that you have a plant that is showing signs of bacterial wilt, the best option is to remove the infested plant before more striped cucumber beetles can feed on the plant and spread the bacterium. After the summer ends, remove garden debris and leaf litter to reduce the overwintering area of striped cucumber beetle adults.
A trap crop is a plant that attracts pests away from your main garden plants. Planting a few highly attractive cucurbits prior to planting your garden cucurbits can be useful for managing early-season beetle infestations. Once striped cucumber beetle numbers build up in these "traps", treat them with an effective insecticide to minimize further movement.
Before you apply an insecticide, be certain that it is necessary. This can be done by monitoring your plants for striped cucumber beetle activity. Start monitoring plants at cotyledon stage. If you find two or more beetles/plant on 25% of your plants, an insecticide should be applied.
Once beetles are present, monitor more frequently (every couple of days) to determine whether cucumber beetle numbers have reached the action threshold as well as to detect the presence of any bacterial wilt. Once plants are at the second or third true-leaf stage, monitoring efforts should shift from monitoring beetles to checking the defoliation level. Monitoring the defoliation level is much faster and still allows for the detection of bacterial wilt. If you find 25% of a given plant defoliated, an insecticide application may be warranted.
There are numerous insecticides labeled for use on cucurbits for the management of cucumber beetles. When possible choose an insecticide that has a low impact on natural enemies, such as lady beetles and pollinators, such as bees. There are several options when considering a low impact insecticide. Neem is a plant based insecticide and acts primarily as an anti-feedant. Although insects are not killed quickly, it causes them to stop feeding and they eventually die. There are also products that contain pyrethrins. It is necessary to hit the beetles directly with pyrethrins as there is no residual activity.
Conventional, or broad-spectrum insecticides, are longer lasting but can kill a variety of insects, so use them carefully and judiciously. Examples of broad spectrum insecticides available include permethrin, bifenthrin, lambda-cyhalothrin, and carbaryl.
Please be cautious with timing when spraying ANY insecticide. Every effort should be made to apply insecticides in the early morning or late evening to reduce contact with honeybees. If possible, do not apply any insecticide while plants are flowering.
CAUTION: Read all insecticide label directions very carefully before buying and again before using to ensure proper application. Be sure that the label specifies it can be used on the specific plant you wish to treat. The label is the final authority on how you may legally use any pesticide. Whenever using any pesticide, including low impact, natural or organic insecticides, follow all label directions and use standard pesticide safety practices for transporting, storing, mixing, applying and disposing of pesticides to protect yourself, your neighbors and the environment.