E. C. Burkness and W. D. Hutchison
Department of Entomology, University of Minnesota
Striped cucumber beetle (SCB), Acalymma vittatum, is an economic pest of cucurbits in the Midwest. Besides causing direct feeding damage on foliage and fruit, SCB also vectors the bacterial wilt pathogen, Erwinia tracheiphila. The adult SCB is about 1/5 inch long, 1/10 inch wide, and has yellow wings with 3 longitudinal black stripes. Proper identification of SCB is important because they are easily confused with western corn rootworm adults, which are not economic pests in cucurbits. An easy way to distinguish between the two is to check the legs of the beetles. SCB adults have faint yellowish bands on the legs whereas western corn rootworm adults have solid black legs.
Biology and Life cycle
SCB overwinters as an adult under debris in wooded areas and along fence rows adjacent to cucurbit fields. In Minnesota, beetles usually do not become active until the first week of June in the southern portion of the state. We do not yet know whether SCB can overwinter in Minnesota. SCB will not take flight until mean daily temperatures reach 65°F. However, it is difficult to predict the emergence of SCB, because of differences in the micro-climates of overwintering sites. After emergence, the overwintering adults feed on the blossoms of many alternate hosts such as hawthorn, dandelions, and many other indigenous plant species. Once cucurbit plants emerge in the field, the beetles use cucurbitacin, a plant kairamone, to locate the cucurbit field. Feeding and mating begin shortly after colonization of a cucurbit field. After mating occurs females begin laying eggs in the soil at the base of cucurbit plants. Eggs hatch in about a week and larvae feed on the roots and underground portions of stems of the cucurbit plants. Pupation occurs in the soil and emergence of adults is 40 to 60 days after egg lay. Adult SCB emerge and begin to feed on foliage, flowers and fruit, if present. In Minnesota, there is usually only 1 generation per year. Occasionally a partial second generation may occur; however, this partial second generation usually will not have enough time to develop fat reserves to survive the winter.
Larvae develop as they feed on the roots and underground portions of the stems. 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. SCB hide in the soil around the plant, under clods of soil, or in cracks in the soil to escape predators or the heat of the day. Therefore, feeding on the stem will often occur right at the base of the plant where the stem meets the soil. Adult feeding is of particular concern when plants are in the cotyledon and 1-3 true-leaf stages. At this size, the plants are small enough that high populations of SCB can either defoliate the plants completely or girdle the stem. As plants grow beyond the 3 true-leaf stage, several cucurbits (cucumbers and pumpkins) can tolerate very high levels of defoliation. Once flowering occurs, SCB will usually move off the foliage and begin feeding on blossoms and pollen. This situation is more noticeable on the large flowering cucurbits like squash and pumpkin. Fruit or flower feeding does not usually become a yield concern. However, feeding on fruit may cause some cosmetic damage when high SCB populations are present in a field.
A major concern with SCB on cucumbers is the ability of the adult to vector bacterial wilt. The bacterium causes plants to wilt, and once the infection is established control is not possible. The bacterium was thought to exist and overwinter in the gut of overwintering SCB. However, research from Penn State University suggests that the bacterium may survive in symptomless weed hosts where SCB can pick up the bacterium after emerging, and before they move into cucurbit fields. The bacterium is spread through wounds created by SCB or other pests. Bacteria enter the plant when contaminated mouth parts and fecal matter come in contact with the wound sites. The bacterium enters the xylem (water conducting tissue) of the plant and starts to reproduce. In 1-3 weeks the pathogen will begin to block the flow of water to the infected area causing leaves and vines to wilt. A portion of the plant (a few leaves) may start showing symptoms and recover overnight. However, wilting will spread rapidly and within a few days the 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 and pumpkins are generally very tolerant or resistant to bacterial wilt.
One method used for field diagnosis of bacterial wilt is to take a stem that exhibits symptoms from a plant and cut it in half. Hold the 2 cut ends together for 10-15 seconds and slowly pull them apart. If bacterium is present, there should be a whitish bacterial ooze that forms a string between the 2 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. Squash bug feeding can cause vines to wilt; producing a similar symptomology to bacterial wilt. Therefore, the possibility of squash bug feeding should be considered when determining if wilt is present in a field.
A major concern with SCB is the potential impact of bacterial wilt particularly for the most wilt-susceptible cucurbits such as cucumbers and muskmelon. The best way to stop the occurrence and spread of bacterial wilt is to control adult SCB. Control tactics that result in fast knockdown are the most effective for preventing bacterial wilt. If plants do show signs of wilt, the best option (if acreage is small enough) is to remove the infested plant before more SCB can feed on the plant and spread the bacterium. Bacterial wilt can occur in any size plant. The majority of the control should be focused on the cotyledon and 1-3 true-leaf stage plants, when the plants are the most susceptible to defoliation and bacterial wilt. However, control may be necessary all the way up to fruiting as larger plants are susceptible to the wilt as well.
Despite the potential for bacterial wilt, we generally see wilt problems only about 1 out of 5 years in Minnesota. Therefore, the most common, initial concern occurs during stand establishment with direct feeding or defoliation damage on cotyledon and 1-3 true-leaf stages. Based on defoliation only, action thresholds were recently revised for Minnesota. The action threshold for first true-leaf plants is when SCB populations exceed 2 or more beetles/plant on 25% of the plants. Monitoring should occur weekly until initial infestations are detected. Once beetles are present, monitoring should occur more frequently to allow for a comparison of populations to the action threshold, or to detect the presence of any bacterial wilt.
Once plants are at the 2nd or 3rd 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. A defoliation level of 25% can be used as an action threshold based on recent Minnesota research. In southern Minnesota, infestations have occurred during the first week of June in '94 and '95; infestations were usually found 2-3 weeks later in central Minnesota.
Once infestations have been detected and action threshold levels have been reached, there are several different insecticides that provide effective SCB control. To ensure proper use of insecticides refer to the most recent edition of the Midwest Vegetable Production Guide (BU-7094-S). Excessive use of some materials that are toxic to pollinators may cause yield reductions. When flowers are present, every effort should be made to apply insecticides in the early morning (before 8 a.m.) or late evening (after 6 p.m.) to reduce contact with honeybees.
Although few alternatives to conventional insecticides are presently available, trap crops can be useful for managing early-season beetle infestations. A trap crop can be any cucurbit variety attractive to SCB, that is planted about 2 weeks earlier than the primary cucurbit acreage. The trap crop is planted early since SCB will usually migrate to the earliest emerging, or most mature cucurbits, on a given farm. A trap crop can be just a few rows to 2-5% of the total cucurbit acreage, and can be planted on the edges of fields. Once SCB build-up in these "traps", they should be treated with an effective insecticide to minimize further movement. This approach can be effective in reducing early-season SCB in a given area, and the need for treatment in the adjacent later-emerging cucurbits. Monitoring for SCB, however, should continue in the primary production fields.
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