Categories
BLOG

yellow bug with black spots in garden

How to Control Pesky Cucumber Beetles in Your Garden

These garden pests can kill or stunt your plants if you don’t take care.

You know you might have a cucumber beetle problem if your plant leaves are yellowing, wilting, or have holes appearing on them, or if your seedling stems are being chomped on.

(Whether you’re starting your first garden or switching to organic, Rodale’s Basic Organic Gardening has all the answers and advice you need—get your copy today.)

The beetles swarm on seedlings, feeding on leaves and young shoots, often killing plants; they also attack stems and flowers of older plants and eat holes in fruit. Their feeding can transmit wilt and mosaic viruses to your plants. The beetles can be found across North America, mostly in the West.

There are two types of pesky cucumber beetles: striped and spotted. Striped cucumber beetles mostly feed on circubit vegetables vegetables like cucumbers, squash, pumpkins, and beans, and rarely on other plants, while the similar spotted cucumber beetles feed on over 200 different crop and non-crop plants. They are both yellow, and and you can differentiate them by the shape of the black marks on their wing covers. Striped cucumber beetle adults are yellow, elongate, 1/4-inch beetles with black heads and three wide black stripes on wing covers (as opposed to the spots on the spotted beetle’s wing covers). Striped cucumber beetles lay eggs at the base of cucurbit plants and their larvae then feed on the roots of these plants. The adults feed on squash family plants, beans, corn, peas, and blossoms of many garden plants. Larvae feed on roots of squash family plants only, killing or stunting plants.

Life cycle of the cucumber beetle

Adults overwinter in dense grass or under leaves (so get rid of these right away to prevent a cucumber beetle infestation!), emerging in early spring to early summer. They eat weed pollen for 2 weeks, then move to crop plants, laying eggs in soil at base of plants. Eggs hatch in 10 days; larvae burrow into soil, feed on roots for 2 to 6 weeks, pupate in mid- to late summer. The larvae are slender, white grubs.

Adults typically emerge in 2 weeks to feed on blossoms and maturing fruit, with one to two generations per year. The spotted cucumber beetle is a bitdifferent, primarily laying its eggs on corn and other grasses such that the larvae of spotted cucumber beetles are not damaging to cucurbit crops.

How to control cucumber beetles naturally

There are several strategies you can use if you spot cucumber beetles popping up in your garden. The first thing you can do to prevent them is to remove any dense grass, leaves, garden trash and other debris shortly after harvest to reduce their potential overwintering sites. The best course of action once you spot them should be to inspect plants frequently for these beetles and handpick or vaccum any that are discovered and remove them.

Another step can be to introduce a natural predator. Commercially available beneficial insects, such as ladybugs, green lacewing and the spined soldier bug, will feed on pest eggs. (Here are 10 beneficial insects you actually want to have in your garden.) Beneficial nematodes also work well to curtail immature stages developing in the soil.

Remove and destroy crop residues where adults overwinter; cover seedlings or plants with a floating row cover, and hand-pollinate covered squash family plants. You can also pile deep straw mulch around plants to discourage beetles from moving between plants. For uncovered plants, you can apply kaolin clay, especially to leaf undersides, and reapply after rain. The clay is a good natural mineral deterrent.

Almost anyone who has grown cucumbers or squash has probably dealt with the cucumber beetle. Find out what you can do to prevent or control these pests.

Cucumber beetles in home gardens

Quick facts

There are nonchemical and chemical options available for managing cucumber beetles.

Cucumber beetles become active in late May or early June and feed on the blossoms of early flowering plants, such as dandelions, apples and hawthorn, until their host crops are available.

How to identify cucumber beetles

Adult striped cucumber beetle (Acalymma vittatum)

Adult spotted cucumber beetle (Diabrotica undecimpunctata howardi)

  • Similar in size, shape and color.
  • 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.

Biology of cucumber beetles

Striped cucumber beetles

They live through the winter as adults in leaf litter and 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 transform to pupae in the soil and emerge later in the summer as adults.
  • Adults return to cucurbit plants and feed on the foliage later in the summer.
  • It takes about 40 to 60 days to go from an egg to an adult.
  • There is typically one generation per year.

Spotted cucumber beetles

  • Do not live through the winter in Minnesota.
  • Adults migrate from the Southern U.S. 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 transform to pupae in the soil.
  • It takes about 40-60 days from egg to adult and there is only one generation per year.

Damage caused by cucumber beetles

Feeding damage by striped cucumber beetles

  • Larvae feed on cucurbit roots and underground portions of the stems. This feeding does not affect plant health.
  • Adults feed on the foliage, and when populations are high, they feed on the stems as well.

Adult feeding is most damaging up to the third true-leaf stages of cucurbit plants.

Bacterial wilt

Striped cucumber beetles can carry the bacteria that causes bacterial wilt.

  • The bacterium infects the plants’ vascular system and causes plants to wilt.
  • Once the plant is infected, it cannot be saved.
  • Striped cucumber beetles pick the bacterium when they feed on infected weed hosts or infected cucurbits.

Infected plants wilt and leaves can discolor before eventually dying. Wilting can spread rapidly and within a few days the entire plant can die. Not all cucurbits suffer the same amount of damage:

  • Cucumbers and muskmelon are most affected.
  • Squash, pumpkin and watermelon are generally very tolerant or resistant to bacterial wilt.

To check 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.

This technique is sometimes difficult to use for diagnosis and should not be used as the only indication that the bacterium is either present or absent.

Do not confuse bacterial wilt with other cucurbit problems. Squash bugs and squash vine borers can cause leaves to wilt. Watch for heavy numbers of squash bugs or orangish frass leaking from stems (a symptom of squash borer damage). Squash family plants can also wilt when suffering from drought.

How to protect your garden from cucumber beetles

Check for cucumber beetles early in the season

  • Pay attention to plants in the cotyledon and first to third true-leaf stage, when the plants can suffer defoliation and bacterial wilt.
  • Be attentive until fruiting as larger plants can be affected by bacterial wilt as well.
  • Concentrate your efforts on striped cucumber beetles. It is rare to see spotted cucumber beetles early in the season and their numbers normally are too low to require treatment.

Keep your garden clean

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 discourage egg laying.
  • If a plant is showing signs of bacterial wilt, remove the infested plant before more beetles can feed on the plant and spread the bacterium.
  • After the summer ends, remove garden debris and leaf litter to reduce sites for adult beetles to spend the winter.

Use trap crops

A trap crop is a plant that attracts pests away from your main garden plants.

  • Plant a few highly attractive cucurbits prior to planting your garden cucurbits
  • Once striped cucumber beetle numbers build up in these traps, treat them with an effective pesticide to minimize further movement.

Use a physical barrier

You can try to protect your cucurbits by building a floating row cover or similar barrier during early to mid-June to keep the striped cucumber beetles away from your plants.

Be sure to remove the barrier when cucurbits start to flower.

Using pesticides

Use pesticides only when necessary.

Watch your plants regularly for striped cucumber beetles when new leaves are coming up from seed leaves. If you find two or more beetles/plant on 25% of your plants in the cotyledon stage, apply a pesticide.

  • Once beetles are present, monitor more frequently (every couple of days).
  • Once plants are at the second or third true-leaf stage, check 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, apply pesticides.

Striped cucumber beetles seen later in the summer can be ignored as they cause little or no damage.

Low impact pesticides

Choose a pesticide that has a low impact on natural enemies, such as lady beetles and pollinators.

  • Neem is a plant based pesticide that prevents insects from feeding, which eventually kills them.
  • Pyrethrins have no residual and treatments need to come in contact with the beetles to be effective.
Conventional or broad-spectrum pesticides

They are longer lasting but can kill a variety of insects, so use them carefully and judiciously. Examples of broad-spectrum pesticides available include: permethrin, bifenthrin, lambda-cyhalothrin and carbaryl.

CAUTION: Mention of a pesticide or use of a pesticide label is for educational purposes only. Always follow the pesticide label directions attached to the pesticide container you are using. Remember, the label is the law.

Be sure that the vegetable you wish to treat is listed on the label of the pesticide you intend to use. Also be sure to observe the number of days between pesticide application and when you can harvest your crop.

Jeffrey Hahn, Extension entomologist and Suzanne Wold-Burkness, College of Food, Agricultural and Natural Resource Sciences

Cucumber beetles become active in late May or early June and feed on the blossoms of early flowering plants, such as dandelions, apples and hawthorn, until their host crops are available.Identify and control cucumber beetles in Minnesota home gardens