Eric Sideman's Pest Report
August 31, 2002

Crop Condition

Conditions- There was some rain yesterday in much of southern Maine....too little too late is all I can think of. With many crops nearly dead and it being nearly September it is unlikely to make a big difference for many growers. But it may make it possible to plant fall cover crops and may extend the life of some fall harvests.

It is now time to start thinking about next year and irrigation. Also, it is a good time to look carefully around and assess the disease and pest situation in order to make fall cultural decisions and get ready for pests that you know will be back next year. Learn the biology of your pests because some big hitters this year may only have gotten a hold at you place because of the weather.

ONION THRIPS

I have heard reports and seen a lot of onions go down to thrips this summer. Thrips are tiny, elongate insects that have four narrow wings fringed with delicate hairs. They feed on onion leaves by piercing the plant tissue with their mouthparts and sucking up the plant juices. This feeding causes silver streaks that coalesce into white patches on the leaves. If the infestation is severe the leaves start dying from the tip. The whole plant can die, especially in hot dry conditions like we had in mid summer. If you had a severe thrip infestation you now may be seeing onions that ripened prematurely and produced only small bulbs.

Onion thrips over winter as adults in crop debris and weeds. Winter wheat and rye and alfalfa are good overwintering sites. Adults move into fields from the edges and reproduce. There may be a few generations per season here in Maine.

Here is the control methods reprinted from the ATTRA web site (http://www.attra.org/attra-pub/thrips.html)(Note, I have modified it so the materials mentioned are likely to be approved for use in organic fields by next season....BE SURE TO CHECK THE OMRI LIST TO BE CERTAIN):

Cultural Control Methods

The choice of cover crops can affect the number of overwintering thrips. Thrips-susceptible crops should not be planted following wheat or rye, which provide excellent over-wintering sites for thrips. Oats is a better choice, but requires later fall planting than rye or wheat-a factor that constrains its use as a cover crop in colder regions (6).

Weed destruction in the field and surrounding margins can help to reduce thrips populations, since these areas serve as overwintering and re-infestation sites. Growers should take care, however, that vegetation management does not conflict with strategies designed to reduce soil loss (through maintaining soil cover), to increase biodiversity, and to make the farm system more sustainable.

Drought stress increases the susceptibility of onions to thrips damage. Adequate irrigation throughout the growing season is a critical factor in minimizing damage (2).

The fact that thrips are color-sensitive suggests that colored mulches may be effective in their control. Louisiana researchers conducted a study to see if aluminum-coated mulch would repel the pest (7). Black plastic was spray-painted in the field with aluminum paint. The reflective mulch repelled 33-68% of the thrips. Ultaviolet-absorbing plastics-used to build walk-in field tunnels-have proved effective in protecting crops from western flower thrips, as well as sweet potato whitefly, cotton aphid, and the diseases they vector (8).

Soil fertility management may also affect thrips infestation and damage. According to one source (9), a lack of adequate soil calcium may invite higher populations of thrips. Another writer suggests that nutritional balance can reduce thrips attack. High nitrate levels will invite thrips, and the effects of excessive nitrate are compounded by shortages of potassium, sulfur, boron, and manganese. Foliar applications of soluble calcium and kelp will balance the excess nitrogen. These nutrient levels can be monitored, using plant tissue analysis on a weekly basis to make accurate adjustments (10).

In iris, gladiolus, daylilies and other vegetatively propagated flowers, severely infested plant parts should be cut away and destroyed. Dig corms early in the fall and cut off the tops before thrips move down into them. Destroy the remaining debris (3).

Genetic Resistance

Among onions, varietal resistance in some sweet Spanish types is noted in the literature. It is suggested that these may be older cultivars with more open canopy growth (5). There is also some genetic resistance among cabbage cultivars. 'Masada,' 'Brutus,' 'Galaxy,' and 'Amtrack' are reported to be highly resistant; 'Fresco,' 'SuperElite,' 'Satelite,' 'Protector,' 'Bartolo,' and 'Supergreen' demonstrate moderate resistance (11).

Biological Control

Numerous beneficial organisms work to suppress thrips. These include lady beetles, minute pirate bugs, ground beetles, big-eyed bugs, lacewings, hover flies, predatory mites, and spiders (5). Unfortunately, these predators and parasites may be hampered by the fact that thrips feed under close-fitting leaves and down in the leaf sheaths where they are difficult to access. Insecticides, even those cleared for use in organic production, also tend to work against beneficial predators and parasites. They should be used minimally and applied with caution.

On occasion, thrips populations may explode as large numbers migrate from nearby vegetation that is mowed, harvested, or drying up. In such circumstances, in-field populations of beneficial predators and parasites may not be adequate. The grower can increase the numbers of predatory and parasitic insects and arachnids on the farm by providing protective habitats for them. These habitats-often called refugia-can be integrated into farm rotation planning, and can supplant "weedy" field borders and waste areas. For more detail on refugia, ask for ATTRA's Farmscaping publication.

Naturally occurring fungal diseases can also devastate thrips populations (5). Limiting the use of foliar fungicides by using forecasting systems and cultural controls helps to protect beneficial fungi and maximize this natural control mechanism.

Alternative Pesticides

Several alternative pesticides might be suggested for controlling thrips. Sulfur, insecticidal soap, and diatomaceous earth have all demonstrated efficacy in suppressing thrips in several crops (3, 12, 13). Being contact pesticides, however, their effectiveness in onions would probably be limited because the thrips can hide between the leaves. In contrast, three applications of superfine sulfur are recommended at monthly intervals in fruit crops for spring thrips control. Lime sulfur has also been suggested as an alternative (13).

Spray formulations of the biological agent Beauvaria bassiana (e.g., Naturalis-O®) are also recommended for thrips control. The agent is a fungus that grows and reproduces in the host; therefore, effective control may not be observed until 7-10 days after application. B. bassiana is most effective when used early, before large populations have built up (14).

The botanical pesticides rotenone and pyrethrum, have been suggested for thrips control (9, 15, 16). Another botanical pesticide, sabadilla, also has a record of use in controlling thrips, and is suggested when other botanicals fail (3). Various formulations of neem such as Neemix® and Align®, which are registered for use on food crops, might also be investigated. However, not all neem products are equally effective against thrips-especially in the adult stage (17, 18). Several ATTRA clients have reported disappointment in neem's effectiveness as a "rescue treatment." Furthermore, neem has proved less friendly to beneficial insects than once believed. Washington State research has found neem to be toxic to ladybeetles, especially in their early larval stages (19).

Noted organic proponent J. Howard Garrett recommends use of a homemade botanical spray formulated from garlic and pepper (16). Garrett's garlic/pepper tea is made by liquefying two bulbs of garlic and two cayenne or habañero peppers in a blender 1/3 full of water. Solids are strained out and enough water is added to make one gallon of concentrate. Garrett mixes ? cup concentrate with two tablespoons vegetable oil and enough water to make 1 gallon of ready-to-use spray mix (20). A commercial product called Hot Pepper Wax®, containing capsaicin (the active ingredient of cayenne pepper), is also recommended for thrips control (21). A highly refined horticultural oil can be used during the growing season for control of thrips and several other insects (22).

Spinosad (the manufacture is working on an organic formulation that may be available next season. The present formulation is not permitted) may also be useful in controlling thrips in the field. Spinosad is a recently discovered insecticide, derived from the fermentation of actinomyces bacteria commonly found in the soil (23). Always follow label instructions when using any pesticide, and keep in mind that the decision to use a pesticide should be made only when other approaches to pest management fail to provide adequate crop protection. Be aware that some "natural" pesticides may be unacceptable or limited in certified organic production. Growers are advised to check with their certifying agent before purchasing or using any such products.

Surround® Crop Protectant is labeled for suppression of thrips and several other insects for apples, stone fruits, citrus, small fruits, and onions. Surround is a kaolin clay film sprayed onto the crop as a barrier to insect pests. Supplemental control measures may be needed in addition to Surround. ATTRA has more information on use of Surround in fruit crops, available on request.

Note: check the ATTRA site above for the citations.

CRUCIFERS

Flea beetles are still attacking new brassica seedlings and older plants - except in rotated fields. On one organic CSA farm this week, we observed a late-planted field, isolated from earlier brassica fields, where flea beetles were virtually absent. On the same farm, brassicas planted in the same field where early greens were grown had several flea beetles per plant. It is helpful to have isolated, separated fields - and to use this to advantage. Where early season brassicas are harvested (or bolted) be sure to till deeply, not only to speed decomposition but to disturb flea beetle larvae feeding on the roots. Adults we see now emerged in late summer.

Watch for cabbage root maggot fly eggs on sensitive crops like fall turnips, rutabagas, and Chinese cabbage. Moist, cooler soils after the rain, combined with the flight period that typically occurs in late August, could allow survival of eggs and some damage to roots or tubes.

SWEET CORN

European Corn Borer: Pheromone trap catches of moths remain high in most locations, so it is still very important to protect silking corn to prevent larvae from moving into the ears. Fields already being sprayed for corn earworm should also have adequate control of borer.

Corn Earworm: Corn earworm moth activity is somewhat lower this week, due to cooler temperatures, but levels are still high enough to warrant protection of all silking corn. Earworm is likely to remain a threat to any late silking corn for the remainder of the season. A three day spray interval for all silking corn was recommended in Cape Elizabeth and Monmouth this week. A four day spray interval was recommended in Auburn, Lewiston, Litchfield, Dayton, Dresden, Oxford, New Gloucester, Old Town, Poland Spring, Nobleboro, North Berwick and Wells. A five day interval was recommended in Corinth, Levant and Warren. We organic growers should not put the Zea-Later away yet. Fall Armyworm: Fall armyworm moth activity remained high in some areas, and larval feeding damage is evident in many late fields. Moth captures have exceeded the action threshold (5) for silking corn in Auburn, Cape Elizabeth, Dayton, Dresden, Lewiston, Litchfield, New Gloucester, Poland Spring and Wells. Sprays for corn earworm should also provide control of fall armyworm.

Start Getting Ready for Next Year 1. Plow down corn stalks and stubble to destroy overwintering larvae of European corn borer
2. Plant a cover crop, such as winter rye, to prevent soil erosion and to add organic matter to the soil.
3. Take a soil test to determine if lime or other nutrients should be applied.
4. Plan to rotate your crops to prevent pests from building up in any one location.
5. Evaluate your weed management results. What worked well and what didn't? Which weed species were the biggest problem? How can you improve control next year?

Sincerely,

(Modified from Sweet Corn Newsletter from David T. Handley Vegetable & Small Fruit Specialist)

(About the author: Eric is MOFGA’s Technical Services Director, essentially an organic "extension agent". He can be reached at the MOFGA office to answer your questions about farming and gardening. Link to MOFGA Contact Page, or email Eric directly.)