Vegetable Crop Insects – May 22, 2009

Joanne Whalen, Extension IPM Specialist;

Economic levels of diamond back and imported cabbage worm larvae continue to be found. A treatment should be applied when 5% of the plants are infested and before larvae move to the hearts of the plants.

Continue to scout all melons for aphids, cucumber beetles, and spider mites. Economic levels of aphids and spider mites continue to be found. Also, we are starting to find cucumber beetles, especially in cantaloupe fields. As soon as we get a day of warm, sunny weather we could see a significant increase in activity. So be sure to scout carefully since damage can occur quickly. Since beetles can continue to re-infest fields as well as hide under the plastic, multiple applications are often needed to achieve control. Foliar products labeled for cucumber beetle control on melons include Assail, a number of pyrethroids, Lannate, Sevin and Thionex. The Actara label only states cucumber beetle suppression. Be sure to check all labels for rates, precautions and restrictions, especially as they apply to pollinators.

Continue to sample for thrips and corn borers. On young plants, corn borer larvae can bore into the stems and petioles. In areas where peppers are isolated or corn is growing slowly, moths are often attracted to young pepper plants. Therefore, you should watch for corn borer moths laying eggs in all fields. As a general guideline, treatment may be needed if there is no corn in the area or you are using rye strips as windbreaks. You should also look for egg masses on the leaves. For the most recent trap catches, you can check our website at ( or call the Crop Pest Hotline (in state: 1-800-345-7544; out of state: 302-831-8851).

We are starting to see an increase in Colorado potato beetle egg laying and the first small larvae have been detected. A treatment should be considered for adults when you find 25 beetles per 50 plants and defoliation has reached the 10% level. Once larvae are detected, the threshold is 4 small larvae per plant or 1.5 large larvae per plant. If adults are the predominant stage, the following neonicotinoids are labeled but should not be used if an at-planting neonicotinoid was applied: Actara, Assail, Endigo, Leverage, Provado (imidacloprid), or Venom. These materials should provide control as long as beetles are not resistant to this class of chemistry. Once eggs hatch and larvae are present, the previous materials as well as Avaunt + PBO, Agri-mek (abamectin), Coragen, cryolite, Radiant, Rimon, or Spintor have provided control. Be sure to read all labels to select the correct rate, maximum number of applications and observe resistance management statements on the labels.

A corn borer spray may be needed 3-5 days after an increase in trap catches or when we reach 700-degree days (base 50). If you are counting infested terminals, the first treatment should be applied when 10% (fresh market) or 20-25% (processing) of the terminals are infested with small larvae. A number of insecticides are labeled for corn borer control. Please refer to the Delaware Commercial Vegetable Production Recommendations for labeled materials.

Snap Beans
Continue to sample all seedling stage fields for leafhopper and thrips activity. The thrips threshold is 5-6 per leaflet and the leafhopper threshold is 5 per sweep. If both insects are present, the threshold for each should be reduced by 1/3. If both insects are present, Lannate, Brigade (bifenthrin), Mustang MAX, Proaxis and Warrior (lambda-cyhalothrin) are labeled for both insect pests on snap beans. In addition, be sure to watch for bean leaf beetle. Damage appears as circular holes in leaves and significant defoliation can quickly occur. As a general guideline, a treatment should be considered if defoliation exceeds 20% prebloom. A pyrethroid, dimethoate or Sevin are labeled for control.

Sweet Corn
Continue to sample for cutworms and flea beetles. As a general guideline, treatments should be applied if you find 3% cut plants or 10% leaf feeding. In order to get an accurate estimate of flea beetle populations, fields should be scouted mid-day when beetles are active. A treatment will be needed if 5% of the plants are infested with beetles. Small corn borer larvae can be found in the whorls of the earliest planted fields. A treatment should be applied if 15% of the plants are infested. The first earworms have been detected in light traps and pheromone traps. In sweet corn planted under plastic, silk sprays will be needed for corn borer and corn earworm as soon as ear shanks are visible. You can call the Crop Pest Hotline for the most recent trap catches (in state: 1-800-345-7544; out of state: 302-831-8851) or check our website at

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