Vegetable Crop Insect Management

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu

Seed Corn Maggot in Spring Planted Vegetables
The warmer daytime temperatures in late February and March followed by cooler spring temperatures has resulted in very favorable conditions for seed corn maggot (SCM) infestations. Spring planted vegetables most susceptible to maggot damage include cole crops, melons, peas, snap beans, spinach, and sweet corn. SCM overwinter as pupae in the soil and adult flies start emerging in our area in late February and early March. SCM larvae (maggots) can cause damage by burrowing into seeds, cotyledons and the below ground hypocotyl tissue of seedlings. Maggots can also burrow into the main stems of plants.

SCMseed

Seed corn maggot damage to the pea seed.

SCMstem

Seed corn maggot damage to pea seed and tunneling in stem (arrow).

There are other maggots that can attack spring planted crops; however, the SCM generally occurs earlier in the season and has the widest host range. In Emmalea Ernest’s early season pea variety trial planted in mid-March at the Research and Education Center in Georgetown, DE , damage was significant, especially in untreated plots ( i.e. no insecticide seed treatment was applied)

PeaSeedTrt

Severe stand reduction from seed corn maggot in plot where no insecticide seed treatment was applied.

As most are aware, there are no rescue treatments for maggots, once damage is found it is too late to control them. Cultural control options to consider include: (a) avoid planting into fields where animal manure was recently applied and/or where a green manure was recently incorporated, (b) early disking or plowing under of crop residues to ensure that they are completely decomposed before planting; (c) good weed management; and (d) planting seeds as shallow as possible to encourage quick emergence. Chemical control options can include commercial applied seed treatments, or soil insecticides; however, not all options are available for all crops. In addition, if conditions remain favorable for seed corn maggot, a combination of a seed treatment and soil insecticide (where labeled) may be needed to provide effective control. Please refer to the Commercial Vegetable Recommendations for available control options. (http://extension.udel.edu/ag/vegetable-fruit-resources/commercial-vegetable-production-recommendations/)

Cabbage
Be sure to watch for imported cabbage worm (ICW) and diamondback moth larvae (DBM) within a week of transplanting. As a general guideline, treatment is recommended if you find 5% of the plants infested with larvae. If DBM is the predominant species, be sure to select an insecticide that is effective for this insect pest since it can be difficult to control. The pyrethroids have not provided effective control of DBM in many cases, especially where resistance has been documented. Please refer to the Commercial Vegetable Recommendations for suggested chemical controls

https://cdn.extension.udel.edu/wp-content/uploads/2012/03/20132545/ColeCrops.pdf

Peas
As soon as plants emerge, be sure to sample on a weekly basis for pea aphids. On small plants, you should sample for aphids by counting the number of aphids on 10 plants in 10 locations throughout a field. On larger plants, take 10 sweeps in 10 locations. As a general guideline, a treatment is recommended if you find 5-10 aphids per plant or 50 or more aphids per sweep. Although beneficial insects can help to reduce aphid populations, cool temperatures will favor an increase in aphid populations but will slow beneficial insect activity. As a general rule, you need one beneficial insect per every 50-100 aphids to help crash populations. Please refer to the Commercial Vegetable Recommendations for suggested chemical controls:
https://cdn.extension.udel.edu/wp-content/uploads/2012/03/20132545/Peas.pdf

Potatoes
As soon as plants emerge, be sure to sample fields for Colorado potato beetle adults, especially if an at-planting insecticide was not used. A treatment should not be needed for adults until you find 25 beetles per 50 plants and defoliation has reached the 10% level. Please refer to the Commercial Vegetable Recommendations for suggested chemical controls:
https://cdn.extension.udel.edu/wp-content/uploads/2012/03/20132545/Potatoes.pdf

Sweet Corn
The earliest planted fields should be scouted for cutworms. A number of cutworm species may be present at planting, including the black cutworm, dingy cutworm and clay backed cutworm. Regardless of the species, treatments should be applied for cutworms if you find 3% cut plants or 10% leaf feeding. Please refer to the Commercial Vegetable Recommendations for suggested chemical controls:
https://cdn.extension.udel.edu/wp-content/uploads/2012/03/20132545/SweetCorn.pdf

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