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 MOFGA's 2009 Pest Reports - Compiled by Eric Sideman, PhD Minimize

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Pest Report - May 19, 2009

This is the first Pest Report for the 2009 growing season. The Pest Report is sent out every week or two during the heart of the season and highlights problems happening now or likely to show up soon. I hear about problems from growers all around the region so please help out and let me know if something big or unusual is happening.

I write much, and much of what is in the Pest Report is reprinted from newsletters by crop advisors around the region. The ones I use the most are from the University of Vermont (Vern Grubinger), University of Massachusetts (Ruth Hazzard), and the University of New Hampshire (Becky Grube). As you can guess, using material from there often gives us a bit of time to prepare because many pests move into Maine from the south and west during the season, i.e., do not spend the winter here. Also, this year I will be picking out some of my own pieces from previous Pest Reports because usually the warning is the same every year.

The Pest Report pieces include a description of the pest, the biology of the pest, a description of the damage, and some suggested management practices allowed under organic standards.

Seedcorn Maggot

The frost last night reminded me of a recommendation that I often make and that is to plant your garden based on the calendar for sure, but on top of that pay attention to the immediate weather. Even if the calendar says the soil should be warm do not put seeds out that require warm soil to germinate if the weatherman is predicting days of cold weather.

Peas, beans, corn, potato sprouts and even cucurbits in the greenhouse are attacked by the larvae of this fly named the seedcorn maggot. They are yellow-white maggots about a quarter of an inch long and sharply pointed at the head end. The symptom is usually that you see no germination, and when you dig around you may find nothing left or may find the maggots burrowing into the seed. Sometimes the seed germinates but only a weak or partially eaten plant is seen. The injury is most likely to occur in cold wet seasons where the germination is slow, and also in soil high in organic matter.

The attack is early in the spring because the critter spends the winter as pupae in the soil, or maybe free maggots in manure or unfinished compost. The adult is a grayish brown fly only about a third of an inch long. It emerges in early spring and deposits eggs in rich soil and even compost piles usually near seeds and seedlings. Exposed peat or potting soil mix of transplants can also serve as attractive sites for females looking for a place to lay eggs. There are a few generations each season.

The best method of dealing with this critter is to do everything you can to encourage quick germination and rapid growth. In the cold, wet soils the seeds are just sitting ducks. Shallow planting helps when conditions are poor. Best yet, wait for things to warm up and dry out

Wireworms (Reprinted from Becky Grube's Newsletter)

I have gotten quite a few questions about problems with wireworms recently. Wireworms damage root and tuber crops such as 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 Life of the Wireworm. Wireworms are the larvae of click beetles (family Elateridae). There are many species of wireworm. The larvae have slender, hard, ¾ -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 sytem 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.

Flea Beetles in Brassicas

Flea beetles are busy feeding in spring plantings of brassica crops in Massachusetts. Numbers are likely to rise in coming weeks here in Maine as beetles move out of field borders where they spent the winter. Crucifer and striped flea beetles feed on Brassica crops as well as weeds that are in the same family, such as yellow rocket or wild mustard. [Different species of flea beetles feed on the tomato family of crops.] The crucifer flea beetle (Phyllotreta cruciferae) is uniformly black and shiny, about 2 mm in length, while the striped flea beetle (Phyllotreta striolata) has two yellow stripes on its back. Flea beetle adults feed on leaves and stems, resulting in numerous small holes, or Œshot-holes¹. Eggs are laid in the soil starting in late May, and beetle larvae feed on roots. The non-waxy greens (arugula, bok choi, tatsoi, mustard, Chinese cabbage, komatsuna) are preferred to the waxy cabbage, kale and collard types of brassicas. In brassica greens, beetles feed on the whole surface of the leaf, and will continue feeding from the seedling stage until harvest. Waxy crops are most susceptible at the cotyledon and seedling stage and feeding is more limited to leaf margins on older plants. Occasionally in tender greens such as arugula, tarnished plant bug feeding may be confused with flea beetle feeding. In addition to the shot holes from flea beetles, there may also distorted leaves that are typical of TPB feeding, which injures leaf tissue when leaves first emerge.

To reduce and delay flea beetle invasion of spring crops, move them as far away from the fields that were used for fall Brassica crops as possible. Remember when you choose rotation fields that the species of flea beetle on Brassicas are different from those on the tomato family. Beetles overwinter in field borders near last year¹s crop. Planting the same crop close by to where it was last year ensures a high population in the spring.

One of the best ways to protect Brassica crops from flea beetles is to place a floating row cover over the bed or row. It is critical to seal the edges immediately after seeding, because Brassica seeds germinate quickly and beetles rapidly find the cotyledons. Flea beetles can fit through extremely tiny cracks. Edges of the cover must be sealed on all sides using soil, plastic bags filled with soil, or some other method. Spinosad (Entrust is organic formulation) is proving to be effective in suppressing flea beetles and reducing damage. Pyrethrin (Pyganic EC 5) showed poor to moderate efficacy in trials, and has a short residual period. Yet some growers have reported a good knockdown with this product. You can spray the Pyganic right through the floating row covers and knock down any flea beetles that may have gotten inside.

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