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MOFGA's Pest Alert - July 27, 2012
Compiled by Eric Sideman, PhD - MOFGA's Organic Crop Specialist

I would still call this a difficult growing season. Many of us across the Northeast have gone from very wet, muddy fields to dry-as-a-bone fields in a short bunch of weeks. Now it is cloudy and rainy and we should all be glad, except late blight has been reported here and there in Massachusetts, New Hampshire, and Maine. With cloudy, damp weather the spores released from this infected fields are able to move in the winds more than 50 miles and survive. In the sunny weather of the past few weeks any spores that got into the wind were likely to die quickly. So, scout your fields carefully and be prepared.


Damage from Onion Thrips. Photo by Eric Sideman.

Onion Thrips - (Thrips tabaci) - Onion thrips are active and taking down onions. 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 loses 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, but this late in the season it is questionable for onion.

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

Damage done by thrips


Purple blotch in onions. Photo by Eric Sideman.

Purple Blotch (Alternaria porri) - Purple is a very common and sometime destructive disease that effects onions, garlic, shallots and leeks. Lesions begin as whitish sunken areas that elongate and develop purplish centers. Under favorable conditions (warm with wet leaves) the purple blotch lesions grow large and oval with concentric rings. The lesions may merge and take down whole leaves, and may become covered with brownish spores. The older leaves are more susceptible than the younger leaves.

Onion residue is the source of the disease in the spring. The fungal mycelium and conidia spores persist as long as the onion debris in the field or in cull piles. New conidia are produce on infected tissue in the spring and wind blown or carried in water to the new crop. The leaves have to have liquid water on them for the spores to germinate, but germination is very quick, less than an hour. Symptoms may appear in less than a week after germination and new conidia spores are quickly produced.

Cultural Control - Sanitation is very important to limiting spread. Infected crop debris should be destroyed or buried after harvest. Cull piles should not be kept near the new onion field. Onions should be grown in rotation with non host crops.

Materials Approved for Organic Production - Serenade has been shown to be effective against purple blotch.


Tomato hornworms. Photo by Eric Sideman.

Hornworms (Manduca quinquemaculata and Manduca sexta) - Hornworms are probably the most destructive insect attacking tomatoes and they are showing up now. They are giant caterpillars that can do a vast amount of eating in a very short time.

Right now we are finding young, tiny ones.

Sometimes it seems that overnight healthy looking tomato or pepper plants are striped of their leaves leaving bare stems. The hornworms will also attack the fruit eating gouges out so large that they look more like bites of a furry animal than an insect.

Look now for the damage and the frass, which is black pellets laying all around plants hosting hornworms. The frass may be your first sign there is a problem. The frass of young hornworms is small.

The adults are large, fast flying hawk moths, which, in flight, may look like a hummingbird. At dusk they hover over flowers sucking nectar. Eggs are laid on tomato leaves and hatch in 5 days.

Hand picking is a bit frightening but does work and chickens enjoy fighting with the challenging pest. The problem with hand picking is that they blend in very well and it is easy to overlook one or two caterpillars, which can do significant damage in a day or two. Bt works very well on this caterpillar.

Tomato hornworm with Braconid wasp cocoon. Photo by Eric Sideman.

What are those white things on tomato hornworms? Tomato hornworm larvae are parasitized by a number of insects.

One of the most common is a small braconid wasp, Cotesia congregatus.

Larvae that hatch from wasp eggs laid on the hornworm feed on the inside of the hornworm until the wasp is ready to pupate.

The cocoons appear as many small white projections protruding from the hornworm's body. Parasitized hornworms should be left in the field to conserve the beneficial parasitoids.

The wasps will kill the hornworms when they emerge from the cocoons and will seek out other hornworms to parasitize. (Reprinted from 2005 Vermont Veg and Berry News by Vern Grubinger.)


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