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 MOFGA's 2010 Pest Reports - Compiled by Eric Sideman, PhD Minimize

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Pest Report - May 20, 2010

COLD-THEN HOT-THEN VERY COLD: This season has been a challenge for growers of fruits and other perennial crops. The very warm weather early in the spring pushed all these crops ahead by at least two weeks. That could be good, but more likely is bad; and it was bad.

Crops that normally can tolerate the typically cold weather of early May were not able to handle it at this advanced stage of development. And it was very cold. The week after Mothers day, I saw 24 degrees F here in the south, and heard 21 degrees F in central Maine. That killed much of the apple crop and most of the strawberries that were not protected with covers or irrigation.

That problem is a bad memory now and most of us want to move on. Of course, the best approach would be to learn from it before moving on. Learn how frost kills, what temperatures are killers, and methods of protection. Learn for next year, but also, in reality, we are not out of the woods yet. Frosts in Maine are not uncommon even now and into the first week of June.

Download a great article about frost and fruit with a table of killing temperatures for different fruits at different stages:


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. [It is a different species of flea beetles that feeds on the tomato family of crops. This is important information when planning rotations of potatoes, tomatoes with Brassica crops or cover 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 ‘shot-holes’. 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. Some crops simply out grow the beetle pressure and the damage can be tolerated. No damage can be tolerated in crops such as arugula.

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. Beetles overwinter in field borders near last year’s crop. Planting the same crop close by to where it was last year ensures a high population in the spring. The same could be true if you have fields full of mustard weeds.

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 or transplanting, 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.

Onion maggot damage. Photo by Eric Sideman.
(Reprinted and modified from Umass Veg Notes). Onion maggot (Delia antiqua) and cabbage maggot (Delia radicum) flies look nearly identical but are likely to be found only on or near their host crop. Cabbage root maggot attacks all types of Brassica crops, while onion maggots are highly specific for the onion family including onions, garlic, leeks, chives, and shallots. A good indicator of the start of cabbage root maggot flight is blooming of the common roadside weed, yellow rocket. This weed has been blossoming for a few weeks in southern Maine.

Life cycle: Onion and cabbage maggot flies spend the winter as small brown pupae in the soil. Adults emerge in spring and adults can travel considerable distances in search of host plants (1/2 to 1 mile). Cabbage root maggot flies are rather delicate, hump-backed gray-brown flies, about 5-7 mm long. Onion maggot flies are very similar. Female flies seek out their host crop to lay eggs at the base of the stem. Cool, moist soil conditions favor survival of the eggs, and soil temperatures over 95 F kill them. When the soil temperatures in the upper half to 1 inch are high (>100 degrees F) that soil temperature itself then provides control. It is possible we’ve reached these conditions in April this year, but this would only help if eggs were already present. This is the reason these pests are much more of a problem in the spring and summer generations of the pest are rarely noticed.

When eggs hatch, larvae feed on roots and can cause complete destruction of the root system. In crops such as broccoli or cauliflower the first sign of a problem is wilting of the plant on sunny days and yellowing of outer leaves. Later, plants collapse, wilt down, and die. If you pull one up you will see that the reason it is wilting is the roots are gone. You may find the legless white maggots feeding, or the small brown, oblong pupae. In Brassica root crops such as turnips, radishes and daikon, feeding tunnels make the root unmarketable.

In onions, newly hatched larvae crawl behind the leaf sheath and enter the bulb, and feed on the roots, stem, and developing bulb. Feeding damage also encourages entry of soft rot pathogens.

Avoiding damage by later planting. The first flight and egg-laying period is generally most intense in the first half of May, depending on accumulated growing degree days – thus, it will vary with the season and location. After the first flight is over, and as soils heat up, fewer eggs are laid and those that are laid are less likely to survive. I have observed that in most years that Brassica transplants set out after the flowers fall from forsythia did not suffer damaging infestations of cabbage maggots. In cooler areas of the state, however, scouting has sometimes found damaging levels into June. Each season will be different – and this year everything is early. It is impossible to name a consistent and reliable date after which it is safe to plant onions or cole crops, but late May into June will likely be safer than the first half of May. So, if you are seeing damage, there is still time to do another planting of onions and be less at risk. And, there is time for many more plantings of brassica crops.

Monitoring. Flies are attracted to bright yellow colors. Yellow sticky cards (3X5 inches) are inexpensive and easy to use; attach them with small wire stakes and place near the soil. Check and change traps twice weekly to record changes in fly activity. (sources: Great Lakes IPM, Gemplers)

Using Growing Degree Days. The beginning and peak of each fly generation can be forecasted using degree day accumulations. Most growing degree day information for
plants and insects is based on a base temperature of 50 F, but maggot flies are active at a lower base temperature of 40 F. For more information on using growing degree days, go to http://www.umassvegetable.org/ and look at the May 6, 2010 issue of the Umass Newsletter.

Floating row covers provide an effective barrier against these pests. Place the cover on as soon as the transplants are set. Use in a rotated field, as flies overwinter in soil after late season crucifers and could emerge under the cover if the same field has spring brassicas. Replace cover after weeding operations. As soil temperatures rise, the first flight ends and crops grow large, covers can be safely removed.

Crop rotation contributes to keeping populations low; greater distances are more effective. Fall tillage to bury crop residues and to expose over-wintering pupae is also important. For onions, bury or haul away onion cull piles. Rotting onion smell attracts the onion maggot fly. In an vigorous Brassica crop, cultivation that brings soil up around the stem may help encourage formation of adventitious roots from the stem, which can help compensate for root loss even if maggots are present.
Naturally-occurring fungal diseases occasionally will reduce onion maggot numbers significantly, particularly when flies are abundant and relative humidity is high. During a fungal epidemic dead, diseased flies, can be seen clinging to the highest parts of plants along field edges. Predaceous ground beetles, which eat onion maggot eggs, larvae and pupae, can also be important in reducing maggot numbers.

Nematodes for biological control. One alternative method that has shown promise but has not been widely field-tested is soil application of entomopathogenic nematodes, especially Steinernema spp. Steinernema feltiae has been found to be more effective compared to other Steinernema or Heterorhabditis species in attaching to and penetrating cabbage root
maggot larvae at low temperatures (10C) which is an important trait for use in spring when soils are cold. Common application methods include suspension of nematodes (infective juveniles) in water and application of water to transplants prior to setting in the field (as a spray or soaking drench), in transplant water used in the water wheel transplanter, as a drench after transplanting, or a combination of pre-plant and post-plant applications. Rates of 100,000 to 125,000 infective
juveniles per transplant have been shown to be needed to achieve reduction in damage.

Some Beneficial Nematode Suppliers:
The Green Spot: 603-942-8925 or www.shopgreenmethods.com
IPM Labs: http://www.ipmlabs.com 315 497 2093
Arbico Organics: http://www.arbico-organics.com +-800 827 2847
Griffin Greenhouse Supplies: 978-851-4346 or www.griffins.com
Integrated Biological Control Systems: 888-793-4227 or www.goodbug-shop.com
Koppert Biologicals: 800-928-8827 or www.koppert.com

--R Hazzard. References: Network for Environment and Weather Applications (NEWA) of NYS IPM Program; Univ of Wisconsin-
Minnesota Degree Day Calcuator (http://www.soils.wisc.edu/asigServlets/asos/SelectDailyGridDD.jsp); Ontario
Ministry of Agriculture, Food and Rural Affairs online fact sheet ; Schroeder et al 1996, Journal of Economic Entomology
89:1109-1115; Chen et al, 2003, BioControl 48: 713–724

WIREWORMS: (Reprinted from Becky Sideman's UNH Vegetable 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, ≤ -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.

BACTERIAL CANKER OF TOMATOES: I have heard of a few instances of bacterial canker in hoop house tomatoes this year. Please get in touch with me if you see this or think you may have it in your early tomato plantings.

Bacterial canker is a sporadic disease but devastating when it occurs. It could be spotty even within a planting. The symptoms of canker is a systemic wilt first appearing as downward turning of lower leaves with upward curling of the leaflet edges. Leaflets wilt often only on one side of a leaf. Marginal necrotic leaf lesions are often the first outward expression of bacterial canker.

It is primarily carried from year to year on contaminated seed, soil with infected tomato debris, stakes with debris, etc. and is spread during the season by splashing water, contaminated equipment and workers hands.

Clean seed is the first defense. This is the job of the seed company. If you have had the problem before, then be sure to use and/or sterilize soil mixes, stakes, pots, etc. Pruning tools should be sterilized between plantings. If you have bacterial canker in a field, then the debris should be incorporated into the soil to encourage decomposition. Crop rotation is recommended.

May 25 | Page 13 of 15 | May 3


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