MOFGA's Pest Report - May 9, 2011
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist
LATE BLIGHT: Late blight has been found in Maine in a greenhouse where tomatoes were alive through the winter. At this time the plants have been destroyed. Late blight only overwinters in our region in living tissue; that is in the potato tuber and host crops kept alive in greenhouses. These overwintering hosts are the potential source of any outbreaks of late blight. If you have kept late blight hosts alive through the winter be sure to carefully monitor them for any symptoms of the disease. If you suspect that tomato plants in your greenhouse may be infected, immediately contact someone who can positively identify the problem. DO NOT TRANSPLANT, OR SELL FOR TRANSPLANT ANY TOMATO PLANTS FROM A GREENHOUSE WITH INFECTED PLANTS.
As the season progresses make sure you scout your fields for any sprouting volunteer potato plants and destroy them. Plant only clean potato seed.
We are always at risk from poor practices of others, and the blowing wind, but for sure let’s all do our part to avoid filling the air with late blight spores.
WIREWORMS: (Modified a bit from an article in Becky Sideman's UNH Vegetable Newsletter). This is the time to decide where you are putting the different crops. Now is your chance to avoid wireworm problems because once you see the damage it is too late. Wireworms damage root and tuber crops such as potatoes, sweet potatoes and carrots by tunneling, causing unsightly holes as well as providing an entry point for pathogens. Young seedlings with small root systems can be weakened or killed. Favorite young seedlings of wireworms include peas and artichoke.
This is the time to decide not to put these host crops where there are large populations of wireworm, which includes land recently in sod, or near the edges of fields where wireworms can migrate from the sod headlands.
Life of the Wireworm. Wireworms are the larvae of click beetles (family Elateridae). There are many species of wireworm. The larvae have slender, hard, ≤ -1 inch long bodies that range from tan to orange to brown in color. The larvae pupate in the spring, and the adult beetles emerge and are active in the summer. The adult beetles are not typically pests. Female click beetles lay eggs during May and June. They lay eggs in the soil, primarily in weedy or grassy fields. The eggs hatch in 3-4 weeks, and the larvae then look for food. The larvae can live for several years, depending on the species, availability of food, temperature, and soil moisture. Because eggs are typically laid in grassy fields, wireworm problems are usually most severe in fields that were recently sod. However, because the larvae can live for many years, problems can persist in fields that have not been sod for some time. And grassy cover crops can attract the adults for egg laying. Wireworms are often more prevalent in moist areas of fields, and in areas with high organic matter.
What they Eat. Wireworms are omnivores. They feed on the roots of grasses or weeds and on other soil insects. It is hard to starve wireworms out of a particular area, because of their diverse diet. Crops that tend to increase wireworm populations include potato, carrot, sweetpotato, small grains (wheat, barley), onion, beet, and clovers. A clean (non-weedy) alfalfa crop can reduce populations. This may be in part because of alfalfa's deep root system reduces soil moisture, making the environment less favorable for the larvae.
Minimizing Damage. Rotation into alfalfa or crops that are not preferred can reduce wireworm populations. Avoid planting highly susceptible crops such as potato and carrots into sites with a high potential for damage, such as fields previously planted to grass sod, pasture or small grains, or fields with a prior history of wireworms. The edges of fields (near sod) can also be a problem, because the larvae can move through the soil in search of food. Baits can be used to determine wireworm pressure prior to planting a susceptible crop. This can be done by placing carrots or potatoes in a softball-sized hole about 4-6‰ deep, covering with loose soil, and then covering the area with a piece of black plastic to warm the soil. Wait 4-7 days, and dig up the bait to check for the presence of wireworms before planting. On small scale plantings this baiting system can be used to reduce numbers in gardens. Another version of this method involves burying a fist-sized clump of corn, wheat, or rolled oats. It may also help to harvest crops as soon as possible. Some reports indicate that the wireworms seek out the moisture in potato tubers if soil conditions become dry, and wireworm damage increases over time in potato crops left in the ground.
CUTWORMS: (many species). Cutworms are occasional pests of many crops early in the season, including carrots, peas, onions, spinach, broccoli and the list goes on and on. Some years they result in major losses, other years result in no loss at all. They are larvae of a dozen or so different species of night flying moths. They are greasy looking caterpillars that have a habit of curling into a “C” shape when disturbed. Most species that attack vegetables overwinter as partially grown larvae and so can become active very early in the spring when the plants first germinate and are very susceptible. Cutworms hide in the soil during the day and crawl on the surface at night. They feed and cut off young seedlings at the soil surface.
Cultural Control. Since most species lay their eggs in the late summer/fall on vegetation, keeping fields clean of weeds and crops in the fall helps. Of course, this goes counter to the recommendations to keep the soil covered cropped, so it is a management decision based on how bad the problem has been. Fall plowing exposes larvae to birds. Cultivating fields in the spring just after vegetation has appeared and grown a few inches, and keeping it clean cultivated can starve the cutworms out before the peas go in. But, in must areas this is not practical because crops need to be planted.
Materials Approved for Organic Production. Entomopathogenic nematodes show good efficacy when environmental conditions are favorable. Steinernema carpocapsae has been shown to be very effective against cutworms, although reports are not specifically in pea production. Success with nematodes depends on proper application methods. Be sure to follow the instructions from the supplier carefully. A few suppliers of these insect-attacking nematodes are The Green Spot (www.greenmethods.com), IPM Laboratories (www.ipmlabs.com), and ARBICO (www.arbico.com).
Baits. Spinosad or Bt will kill the caterpillars, but getting the pest to consume the insecticide as a sprayed on material before significant damage is done is not likely. However, farmers have reported good results using these materials in baits. The bait is spread on the ground near the plants, or prior to planting to clean out an area.
Spinosad - Seduce (OMRI Listed) is a new commercial bait. I have no experience yet with this, so if any of you try it, please let me know what you think.
Bt - A bait made from Bt is often recommended and has received good reports from farmers. This is a method of use of Bt that is not described on the label. This off-label use is permitted by EPA under FIFRA 2ee, but growers should check with their state pesticide regulators about their state regulations. Make the bait by mixing the highest concentration solution of Bt allowed on the label and then mixing in a bit of molasses and alfalfa meal or bran. Then dampen this mix if necessary. Spread the bait along the planted or planned rows in the evening.