Striped cucumber beetles
Published in Yard & Garden Line News, June 1, 2005
Striped cucumber beetle
Plant Disease Clinic
Keep your eyes open for the striped cucumber beetle, Acalymma vittatum. This insect is a periodic pest in home gardens of cucurbits, i.e. cucumbers, squash, melons, and pumpkins. The striped cucumber beetle generally occurs in low numbers from year to year but occasionally is very abundant and damaging to plants.
You can recognize a striped cucumber beetle by its 1/5 inch long oblong body, its yellowish green wing covers, and its three longitudinal black stripes. It also has a black head and antennae as well as an orangish prothorax (the first area behind the head).
There are a couple of similar looking insects you might also see in your garden. The spotted cucumber beetle is similar in size, shape, and color and also likes to eat cucurbits but is much less commonly encountered. Instead of stripes, this beetle has 12 black spots on its wing covers. The western corn rootworm is also yellow green with three black stripes but has a darker colored prothorax and a yellow-green abdomen underneath (a striped cucumber beetle has a black abdomen underneath). Although you may find this beetle feeding on cucurbits blossoms, it is not a pest on squash family plants.
A striped cucumber beetle overwinters in protected sites on the ground in wooded areas and fence lines adjacent to cucurbit fields. It is unclear whether it actually survives winters around the Twin Cities and areas to the north. Limited studies show that they are not tolerant of the cold and usually die. It is possible that in cases of mild winters some may survive. These areas are repopulated each spring by striped cucumber beetles flying in from the south. It does appear that striped cucumber beetles do overwinter successfully in southern Minnesota.
When the temperature warms to an average of 65ºF, generally late May or early June, these beetles emerge and feed on the blossoms of early flowering plants, such as dandelions, apples, and hawthorn, until their host crops are available. Once cucurbit crops are growing, adult striped cucumber beetles fly to gardens, feed on the leaves, and lay eggs at the base of plants. This feeding is the most damaging to cucurbit plants.
Eggs hatch in several weeks and whitish larvae feed on plant roots and underground parts of stems. Despite this feeding, the larvae are not usually considered to be pests. The 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. When populations are high, they can also chew on stems and fruit. There is typically one generation per year.
Striped cucumber beetles are also injurious because they can infect cucurbits with bacterial wilt. These beetles harbor the bacteria in their bodies and infect feeding wounds with contaminated feces. All cucurbits can become infected, however, cucumbers and melons are most susceptible and likely to die from bacterial wilt while squash and pumpkins are generally tolerant to it. Infected plants wilt and leaves can discolor before eventually dying. Once a plant is infected with bacterial wilt it can not be saved.
You can test a plant for bacterial wilt by cutting it with a knife and then squeezing the ends. Infected plants should ooze a milky white bacterial substance. You can also hold the two cut ends together and then slowly pull them apart. If it has bacterial wilt, you should observe a string of bacterial ooze.
Don't confuse bacterial wilt with other cucurbit problems. Two insect pests, squash bugs and squash vine borers can inflict damage that can cause leaves to wilt. Watch for heavy numbers of squash bugs or orangish frass exuding from stems (a symptom of squash borer damage). Squash family plants can also exhibit wilting leaves when suffering from drought. More information on bacterial wilt.
When managing striped cucumber beetles, plants are most susceptible to damage when plants are in their first to the third true (or normal) leaf stages, i.e. the leaves that emerge after the first seed leaves. Watch your plants regularly for striped cucumber beetles during this time.
If you find at least 25% defoliation during this time, you should treat your plants to protect them from these beetles. Use a garden insecticide labeled for cucurbit crops. Common examples include permethrin, esfenvalerate, or carbaryl. It isn't important to treat striped cucumber beetles you see later in the summer as they do little or no damage to plants.
It is very difficult to protect cucurbit plants from bacterial wilt. If you have a history of this disease in your garden, you can try to protect your cucurbits by erecting 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 your cucurbits start to flower.