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

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Pest Report - June 18, 2010

Illustration from NC State University.
ONION THRIPS (Thrips tabaci).
Onion thrips are active now and if they have been a problem for you in years past it is time to start scouting for them. They are very tiny and easily overlooked until the onion plants start showing leaves that look as if they have been rasped. In fact they have. The thrips feed by using their moth parts to rasp and pierce the onion leaves, releasing juices for them to feed upon. If the population of thrips is large you will see silvery patches on all the young leaves and when really bad the whole field will look white and silvery and the leaves wither. Obviously onion yields can be greatly reduced because the onion plants lose food and water through the damaged tissue.

Onion thrips hide between the leaves at the base of the onion plant. I find the best way to find them is to pull up a plant and hold it upside down over a white sheet of paper and pull apart the leaves as you shake gently. Even when they are walking about on the paper you will still need to look hard to see the thrips. My eyes are not that good and I need a hand lens to even spot them. The immature thrips are white to a pale yellow, elongated with very short antennas and dark eyes. Remember, tiny. The adult is tiny too, but it has wings. Thrips' wings are unique. They are fringed with hairs. Thrips are very poor fliers, but they do fly when disturbed and get blown in the wind easily. Keep in mind that this means thrips will be blown to new fields downwind. Adults are pale yellow to dark brown.

There are many generations per season and they can be very quick in warm weather. Also, parthenogenesis is common, meaning females that cannot find males produce progeny all by themselves. Each female can produce up to 80 eggs, which tells you that the population can explode quite quickly under good conditions. Good conditions are warm and without heavy rains. Heavy rains wash the weak insect off the plant.

Growers can simulate heavy rains with heavy overhead irrigation. As well as disturbing the insect the extra water will help the rasped onions. Extra fertilization will help too.

Thrips survive in onion debris so clean up after harvest. I have heard that inter-cropping with carrots confuses thrips.

If the natural enemies or the cultural practices fail to keep thrips populations down then you may need to turn to a pesticide. Remember, thrips are often around in small numbers that can be tolerated. IPM practices recommend an economic threshold of an average of 3 thrips per green leaf. When scouting, sample about 50 plants around the field from at least 10 different locations in the field and then figure the average per leaf. Entrust is the most common recommended material. Follow the label instructions and be sure to spray into the leaf axis.

Image from University of Minnesota.
FOUR-LINED PLANT BUG (Poecilocapsus lineatus).
The four-lined plant bug is a general feeder that attacks many kinds of crops and weeds. Basil and mint are favorites but it will also feed on hundreds of other species. The damage is sometimes confused with disease symptoms. Both the nymphs and adults suck plant sap. Their feeding leaves tiny spots about the size and shape of flea beetle feeding holes but the plant bug does not completely puncture the leaf and so there is a sort of window look to the spot. There is a toxin in the saliva of the plant bug that is meant to digest the leaf tissue. This leads to, in addition to the spots, distorted leaves that may turn dark brown and curl up and eventually fall off, hence the confusion with disease. Also, since the plant bug is very flighty and drops off and falls to the ground quickly when disturbed it is common that the pest is not seen, i.e., just the damage is seen.

The four-lined plant bug adult is greenish yellow with four black lines running longitudinally down its back. The nymphs are bright red with tiny black beginnings of wings. For some good pictures of adults, nymphs and damage see:


The four-lined plant bug lays banana shaped eggs in the fall in slits in the plant stem. The eggs overwinter and hatch in mid spring and the nymphs begin feeding. It takes about a month and a half for the nymphs to develop into adults. The adults feed for a month or so and then lay eggs. The adults do not overwinter and there is just one generation per year.

For most vegetable crops the damage is tolerable. Herbs may be ruined. Insecticidal soap and summer oils have proven useful. Some fact sheets say you can scout for and destroy the eggs in the fall, but that seems unreasonable. I've never even have seen them.

Image from University of Minnesota.
Cutworms are still a real challenge for some growers, especially for small transplants and carrots and onions. Cutworms are the caterpillars of a few different species of night flying moths. Some of the species fly in very early in the spring and others arrive in the fall. They lay eggs at the base of plants (weeds and cover crops as well as your cash crop). The eggs hatch into tiny, dark gray, greasy caterpillars that feed at night. Some species just simply cut off your plants just about at ground level. Other species climb up and cut off leaves or eat notches out of them. The caterpillars can be found in the soil by digging around near the damaged plants. They curl up into a "c" shape when you handle them.

The big problem is you never know whether it will be a problem or not. On a small scale, Dixie cups with the bottoms cut out placed around a transplant make a good barrier.

One method that works on a large scale but is usually not practical is to starve them out. If you can keep a field completely free of ANY growing plants (weeds or crops) for a few weeks after they hatch then they will starve and die.

I have received good reports about making bait from wheat bran, a Bt solution and molasses and then sprinkling it or making patties and putting the patties along the row of effected crops. This year I tried using alfalfa meal instead of bran and I really think it works. No, I don't have a specific recipe. This idea is a copy of baits that were recommended fifty years ago, but then with toxins way to frightening to mention. I suggest you make the most concentrated solution of the Bt allowed on the label. A solution of Entrust would work too. Make the solution of Bt and molasses and then add the bran or alfalfa meal to make it damp. Put it out in the evening because the cutworms feed at night and the Bt breaks down quickly in sunlight.

I have also heard very good reports from folks who have used parasitic nematodes. The best results for cutworms is achieved when a combination of two types of nematodes are applied in a mixture because the different species work different levels in the soil and attack the cutworms both while they hide deeper in the soil during the day and when they move up at night. A mixture of Heterorhabditis bacteriophorea (Hb) and Steinernema carpocapsae (Sc) has essentially eliminated cutworm problems for some of our growers. The nematodes are usually shipped on a sponge ready to mix with water and apply to the soil. It is important not to let the soil surface dry out after application. 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). It may be too late this year for nematodes to help, but remember for next year.

June 19 - Late Blight Update | Page 10 of 15 | June 8


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