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Early Season Soil Pests

A cold winter will should slow the start of insect population development. But when the insects do become active, will there be significantly lower densities caused by mortality to the overwintering life stage? Species that are adapted to overwinter in cold climates, as far north as Canada, or those that have the ability to burrow deep in the soil (assuming you have land with deep soil) may well have survived the winter well, especially if you had no-till or good snow cover conditions. Here's a review of some early season soil pests. Chemical control options have been incorporated into the 2003 Commercial Vegetable Production Recommendations - but please note one error: Furadan 4F is not labeled on peppers, and on cucurbits and sweet corn the target pests are not wireworms.

Wireworms

Wireworms are becoming an increasing problem in the mid-Atlantic area. Wireworms are long, slender, hard-bodied, wirelike larvae of "click beetles". They are about 1.25 inches long by 1/8 inches in diameter. The larvae are the damaging stage, not the adults. The adults are called click beetles because of their habit of snapping and flipping their bodies when turned upside down. Wireworms have variable life cycles, depending on the species. Most species take 2 to 5 years to complete their development, so there is considerable overlap of larval sizes; the larger larvae do more damage. One species that is troublesome in potato (Melanotus communis; there is no common name) takes 6 years to complete it's life cycle. Wireworms overwinter as eggs, larvae, or adults.

Wireworms do more damage during cool wet springs, especially in fields following sod or other grasses. They damage crops by devouring seeds in the soil, cutting underground stems and roots, and by boring into the larger stems and roots. Often the seed is hollowed out, leaving only the hull. All crops are susceptible to attack to one degree or another, and particularly susceptible are potatoes, carrots, peas, onions, corn, sweet potatoes, lettuce, melons, beans, cowpeas, and sugar beets.

Plowing or cultivating infested soils in the late summer or fall exposes wireworms to natural enemies and freezing temperatures. Crop rotation helps reduce wireworm populations; continuous planting of vegetables and field crops, especially potatoes and wheat, tend to increase wireworm abundance. No-till fields may allow wireworm populations to increase.

Insecticides can be applied either in the spring or fall when the soil temperature at 6 inches deep is at least 50 F. In general, seed treatments with only lindane or permethrin protect only the germinating seed from wireworms; commercially treated seed with imidacloprid provides longer control. Imidicloprid (Admire or Platinum) applied through drip-irrigation is not labeled for wireworms, but when applied to labeled crops for pests on the label, it can also help with wireworm suppression. When using diazinon, try to work it gently in to the soil immediately before transplanting so it is in the root zone.

A monitoring technique is to set up bait stations using 1 cup untreated wheat plus 1 cup untreated shelled corn, about 4 inches deep, covered with black plastic. Uncover the bait 10-14 days later. The germinating seed gives off volatiles (CO2 and others) that attract the wireworms. A threshold for insecticide application is 1 wireworm per bait station. One guess for why we are seeing increased wireworm problems, and on more crops, is that we are putting in transplants into plastic mulch on recently prepared ground earlier in the season. The ground preparation may remove other food resources, and the transplants then act similar to the bait station.

If you are having wireworm problems, collect some (about 20-30) in alcohol and send them in for identification. Management for a species with a short (1-year) life cycle will be different for species with a long (5-6 year) life cycle.

White Grubs

White grubs are the immature stages (larvae) of June beetles, May beetles, and Japanese beetles. There are over 100 species of white grubs. They have a C-shaped body, a brown head, three pairs of legs, and a slightly enlarged abdomen. Full grown grubs range from 0.75 to 1.75 inches long.

Adults feed on leaves of trees, whereas the larvae feed on roots, particularly bluegrass, other lawn grasses, timothy, corn, soybeans, tubers of potatoes, and other crops. Root-feeding causes wilting, stunting, and death of the plant if enough feeding occurs. Similar to wireworms, cool, wet springs and areas previously in sod may have heavier infestations.

The life cycles of the more abundant and injurious species may extend over three years. Eggs are laid 1 to 8 inches deep in the soil, especially near woodlands; after 3 weeks the larvae hatch and begin feeding on roots. During the winter the larvae migrate to deeper portions of the soil.

Crop rotation helps reduce populations. It is best to plant deep-rooted legumes (alfalfa, clover) in rotation with susceptible crops. In some regions a rotation of oats, barley or wheat with clover and corn has been satisfactory. Corn or potatoes may follow clovers but they should not follow grasses in the year of a heavy beetle flight. The most severe damage occurs on crops that follow grass sod. Late summer or early fall plowing destroys many larvae, pupae, and adults in the soil and exposes these stages to predators, which includes many vertebrates, as well as parasitic wasps. Soil insecticides applied for wireworm control may also effectively reduce grubs.

Seedcorn Maggot

The adult seedcorn maggot is a fly similar to a housefly, but you are unlikely to see it. The adult is only 5 mm (~ 1/4 inch) long, and is grayer in color than a housefly. The damaging larvae or "maggots" are the immature larval stage. They grow from a newly hatch larva up to 1/4 inch long, they are yellowish white, legless, cylindrical, and tapered at one end. This tapered end contains a single hook-like appendage that is part of the mouth. There are no other readily visible mouthparts. Pupae are inside a puparium (a hardened skin) which starts as an ivory color and hardens into a reddish brown color. Pupae are ~ 1/4 inch long.

These insects overwinter as a pupa in our soils (farther south all life stages can be found during the winter). Adults emerge in early spring and lay an average of 270 eggs per female in moist soil. Soil containing abundant decaying vegetation is also attractive to the ovipositing female. Exposed peat or potting soil mix of transplants can also serve as attractive sites for females looking for a place to lay eggs. Larvae hatch and crawl to germinating seeds or plants roots, and complete their development within 2-3 weeks. Several generations per year may occur. The maggots burrow into the seed, causing seed death or poor germination. Damage tends to be spread throughout the field. The larvae feed on peas, beans, corn, cabbage, turnip, radish, onion, beet, spinach and sprouting potato. Damage can sometimes be avoided by delaying planting until the first generation larvae have pupated. This date varies with locality, but is approximately June 10 for New York State. It takes about 450 degree-days to complete a generation, which is a bit fast for an insect species. In field corn, if you have passed 450 degree-days, you are typically past the 1st generation, and after that soil conditions make it unlikely that seedcorn maggot would be a serious problem. However, in vegetable crops the later plantings of multiple crops can be attacked. Cultural controls include:

  • thorough incorporation of organic matter into the soil
  • preparation of seedbeds for rapid germination
  • shallow planting (encourage rapid plant growth and minimize the time the germinating seed is sitting in the soil)
  • covering rootball of transplants when transplanting
  • planting when soil temperature are warm

This last recommendation is especially effective for transplants. Studies in Indiana with melon transplants have shown that root damage is directly related to soil temperature.

Seed treatments applied at planting should give effective chemical control with minimal amount of pesticide. For some crops, we have the option of transplant application of Admire. There are also several materials available for pre-plant incorporation that control can be applied. Post-applications, soil drenches after the damage is present, are not effective.

Black Cutworms

As opposed to early-season pests which may be found on many crops, black cutworms are primarily pests of corn, but they can also attack tomato, pepper and eggplant. The adult moths become active in April and May in Pennsylvania. Females lay eggs in dead vegetation on the soil surface and in weeds, where moisture is high. The larva is greasy gray to black with a light stripe down its back. Full grown larvae are about 1.75 inches in length. Young larvae feed on the leaves of emerging corn, whereas the older larvae cut the plant off at the base (hence the name "cutworm") or bore into the plant. After four or five weeks of feeding in May and June, the larvae pupate in the soil. Two more generations may occur, but damage does not tend to occur from later generations.

Cultural controls may help control cutworm populations: good weed control, fall plowing, spring cultivation after weeds have started some growth (height of 2"). Also avoid planting hill or row crops after grassy sod. No-tillage or reduced tillage may increase the amount of damage. Pre-planting or at-planting treatments for black cutworm can be used, but post-planting treatments based on scouting during the leaf stages are also effective.

Blacklight traps can be used to monitor moths, but it is as effective to monitor for feeding damage. In sweet corn, check each planting weekly during the spike through the 5-leaf stage. Check for small irregular holes in the leaves, as well as missing or cut plants. If cutworms are present, examine 10 sets of 20 plants throughout the field and record the percent of cut or damaged plants. Look under clods of dirt and vegetation and the bases of plants for the larvae; if you see the larvae, record the average size of the cutworms and the number per 100 plants. In sweet corn during the two-leaf stages, apply a treatment if more than 10% of the plants show fresh signs of feeding. At the three to four-leaf stages apply treatment at a 5% level. Also, use your judgment based on stand count: if you are at the minimum stand count, you may need immediate treatment, whereas more feeding can be tolerated if the stand is heavier than needed. During drier conditions, treatments may be less effective because cutworms may be feeding below the soil surface; in these cases, rotary hoeing or cultivation, as well as using higher spray volumes, may help increase the chances of contacting the insects with the pesticide.

Warning

Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.

Authored by: Shelby Fleischer, Professor

February 2000

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