Description. Striped cucumber beetles (SCB) are about 1/4-inch long, have black undersides, with yellow (2) and black (3) stipes on top. The three black stripes on the wings have distinct straight edges. (fig. 1). Larvae are approximately 3/8-inches long, have white worm-like bodies with brown heads and three tiny pairs of legs.
Beetles sometimes confused with SCB. Striped cucumber beetles are sometimes confused with the adult three-lined potato beetle, generally found on solanaceous plants, and the western corn rootworm, sometimes found feeding in cucurbit flowers. The western corn rootworm has black and yellow lines on its back, but the edges of the lines are curved and less distinct (blurred), and the underside of the beetle is yellow. Spotted cucumber beetles (fig 2) are often present on cucurbit crops at lower populations than SCB. The spotted cucumber beetle has a greenish-yellow body with 12 distinct black spots on its back and is slightly larger than the SCB.
Life-cycle. Striped cucumber beetles overwinter as adults in protected sites along hedgerows and in wooded areas. Some may pass the winter within the field under old crop debris. They begin to emerge from overwintering sites in the spring when temperatures exceed 55 degrees F, and feed on the pollen of wild flowering plants. Once crops begin to emerge from the soil, or are transplanted into the field, the beetles migrate to cucurbit plants to feed and mate. Colonizing beetle populations are initially highly concentrated along the margin of the field, but become less aggregated as the season progresses. In Connecticut, beetle infestations generally tend to peak about the third week of June. Eggs are laid in the soil at the base of cucurbit plants 10 to 20 days after adults mate, and hatch a week later. Larvae take 2 to 4 weeks to mature, then pupate over the next 7 days. Summer adults emerge and feed in flowers, on foliage or on the surface of fruit. Adults eventually leave the field to feed on pollen until winter.
Feeding preference. Striped cucumber beetles show a distinct preference for certain types and varieties of plants in the cucurbit family. Preferred cucurbit crops include some gourds, certain winter squash varieties like 'Turk's Turbin' and 'Blue Hubbard,' zucchini, cucumbers, yellow summer squash and acorn squash. Cucurbit crops that are slightly less preferred by the beetles include pumpkins, muskmelons, butternut squash and watermelons. Beetles prefer to feed on cotyledons of seedlings, wilting plants and certain cucurbit types/varieties due to higher concentrations of cucurbitacins (feeding stimulants produced by cucurbit plants). Once feeding begins, beetles use an aggregation pheromone to call others to an acceptable food source. Spotted cucumber beetles have a much wider host range than SCB, which restrict their feeding to cucurbit species.
Damage and management. Adult cucumber beetles damage cucurbit crops primarily by feeding directly on the cotyledons, foliage and stems of newly emerged seedlings, thereby reducing plant stands, and also by transmitting (vectoring) the pathogen Erwinia tracheiphia, which causes bacterial wilt disease. The bacterial wilt pathogen survives through the winter in the hind gut of the cucumber beetle and spreads when the beetle defecates in feeding wounds. Over 10% of the beetles may harbor and vector the bacteria. Seedlings need to be protected until the plants reach the 3-true-leaf stage or beyond. Protecting older plants generally does not reduce plant death due to bacterial wilt or direct feeding, but blossom or fruit feeding and excessive defoliation on more mature plants may delay growth, reduce yields or render fruit unmarketable. Successfully controlling (killing) colonizing beetle populations though June will generally eliminate problems with this pest later in the season. Beetle feeding can also vector mosaic viruses, predispose plants to black rot and spread powdery mildew spores between plants. Larval feeding on roots increases the incidence of Fusarium wilt. Cucumbers and muskmelons are especially susceptible to bacterial wilt, summer squash are prone to virus problems and black rot is a common butternut squash and pumpkin ailment.
Cultural, mechanical and biological controls. Crop rotation to a distant field (> ½ mile away) can help minimize the size of the beetle population. Simply rotating to new ground within a field, or to an adjacent field, is not effective due to the mobility of the beetle.
Delaying cucurbit plantings until the last week of June and using heavy seeding rates can help assure that plant stands survive. Floating row covers can also be used to exclude the beetles from the plants, but must be removed by bloom to allow bees to pollinate the crop. Planting on black plastic mulch reduces the survival of SCB larvae by up to 50%.
Perimeter trap cropping can be used to concentrate beetle populations in the border areas by completely encircling many types of cucurbits with a more attractive trap crop, like 'Blue Hubbard.' Do not use a trap crop that will act as a reservoir for the bacterial wilt pathogen (i.e. Turk's Turban) or disease incidence may increase and yields may decline. Even with light-moderate beetle populations, it may be necessary to spray the trap crop to control the beetles, to prevent the population from redistributing throughout the field over time. Perimeter trap cropping may not work in unrotated fields with extreme beetle populations or for the most preferred cucurbit crops. See "Perimeter Trap Cropping for Yellow and Green Summer Squash."
The tachinid fly, Celatoria diabrioticae, parasitizes both spotted and striped cucumber beetles in New England. Nematodes (Steinernema riobravis) can provide partial control of SCB larvae.
Chemical controls. The following materials are registered for cucumber beetle control on summer squash: esfenvalerate (Asana), carbaryl (Sevin), permethrin (i.e. Ambush), imidacloprid (Admire), kaolin clay (Surround). All of the above materials, except imidacloprid, should be applied to the foliage in a timely fashion (5-10 day schedule) to prevent excessive crop damage. Kaolin clay is an insect repellent that suppresses light populations of cucumber beetles. Imidacloprid is a systemic insecticide which is picked up by the roots of the plant and translocated to the foliage. Imidacloprid should be applied as an in-furrow or banded application, or through drip irrigation at the proper rate to avoid phytotoxicity problems. In areas with 2 beetle generations per season (i.e. Pennsylvania), additional foliar applications of a second (non-systemic) material may be necessary to control mid-summer adults. Imidacloprid can be used at reduced rates (0.67 fl. oz./1000 plants) to treat transplants prior to field setting, but only provides effective control for 10 to 14 days. Higher rates will produce phytotoxicity and are not advised.
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Photos by Robert Durgy, University of Connecticut
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