Eric Sideman's Pest Report

July 6, 2001

Crop Condition

The big news that is even big enough to make front page of many newspapers is the armyworm. Growers often miss the problem until the damage is really bad because the caterpillar feeds at night. Most of the populations that I have seen are approaching the end of their feeding period and the damage is done. Here is information to help you decide if anything should be done.

Army worm

The critter is the army worm. Luckily there are many natural enemies of the army worm and it is rarely a major problem two years in a row in any location because as the pest populations build up so do its natural enemies. There are many natural enemies of the army worm including ground beetles, parasitic wasps, and birds. The most effective enemy is the red-tailed tachina-fly.

The army worm is related to the cut worm and somewhat has that same greasy appearance. They are gray to black with a narrow yellow stripe on their back and a wider yellow stripe on each side. There is some question as to whether the army worm can over winter in Maine. Where it does, it over winters as a hibernating, partially grown caterpillar. The young larvae feed in the spring, not usually doing much damage, pupate in May, and the moths of the first generation appear in June. The females lay about seven hundred eggs each that hatch in about ten days. The moths tend to cluster when egg laying and so you see patches of very dense populations of caterpillars. The caterpillars from these eggs are the destructive worms that you see now feeding. The worms feed mostly at night when their enemies are sleeping. Leaves of grass are their favorite food and many fields of hay and small grains have been destroyed already. As the worm finishes the food in an area they march along spreading out from the patch and devouring grass along the way, hence the name army worm.

It takes about three to four weeks for the worms to reach their full grown size of about 1.5 to 2 inches, at which time they burrow into the soil and pupate. New moths emerge in about two weeks. These lay eggs for another generation of army worms but this will not be nearly as destructive as what is happening now.

Controls may be late now. In hay crops the best bet is to harvest the crop if close to ready. The best pesticide available to organic growers will be a Bt. Dipel Df (2 lb/A) or Xenteri (1.5 lb/A) are labeled for army worm.

I tried Condor yesterday and saw good results with it. However, the larger stages of the caterpillar are much harder to kill. The Condor killed caterpillars that were an inch or so. I am not sure whether it is cost effective to spray hay ground, but if the army worm is in your sweet corn you better spray.

In Dwight Sanderson's 1912 copy of Insect Pests of Farm, Garden and Orchard he talks about building a dust furrow as a barrier to their progress. He says to make a dust furrow plow a deep furrow around the field to be protected and thoroughly pulverize the soil by dragging a heavy log back and forth in the furrow making the side towards the protected crop as steep as possible. In attempting to climb this barrier, the dust will slide from under the insect and large numbers will collect in the bottom of the furrow and die.

Leaf hopper

The adult potato leaf hopper is pale green (the species I have seen this year so far are dark, olive green). It is wedge-shaped, about 3 mm long. It is very active and like the nymph, it can run sideways or backwards as rapidly as they move forward. Like all leaf hoppers, it is a sucking insect and as it feeds it injects its saliva into the phloem (food conducting cells) which disturbs transport in the plant. This causes what is referred to as "hopper burn", which is a yellowing and browning of the leaves starting at the tips. On beans and strawberries there is a marked curling under of the leaf edges, and there is severe dwarfing. Controlling leafhopper early is very important because once the phloem is disturbed the plant does not recover. Scout your beans and potatoes frequently and pay attention to the Pest Report. The best way to scout is with a sweep net, but you can just brush the tops of the plants with your hand and if the leaf hopper adults are there they will fly up. You can see them land and get a look at them. Look for nymphs on the underside of the leaves. They will not fly but will run around.

The best control of them that I know is the concentrated pyrethrum formulations that have about 6% pyrethrum in them. Johnny's sells one. The ones with PBO work much better, but PBO will not be permitted next year so it does not make sense to buy any now.

Cucumber beetle

SCOUT CABBAGE TRANSPLANTS FOR SYMPTOMS OF BLACK ROT! When buying into transplants, you may be buying into black rot. Black rot of crucifers is caused by the bacterium Xanthomonas campestris pv. campestris. The bacteria can infect plants at any growth stage. Initial symptoms consist of localized wilting at the leaf margin followed by a yellowing of the tissue. The most characteristic symptom is a yellow "V-shaped" lesion on the edge of the leaf, with the base of the "V" toward the leaf center. Within the yellow tissue, leaf veins become black. The blackened veins may extend from the leaf to the main steam.

The bacteria can survive in infected plant debris and on or in the seed. Infection occurs through pores on the leaves (stomata & hydathodes), wounds or insect injuries. The bacteria need moisture (rain, dew, etc.) to infect and multiply. So the weather conditions we've had the first weeks in June are perfect for disease development. Warm days promote growth of the bacteria and symptom development, and morning dews provide the moisture needed by the bacteria. Black rot is very difficult to control. Here are a few suggestions on how to avoid or minimize the problem:

· Scout greenhouses and seedbeds on a weekly basis. Characteristic V-shaped lesions are expressed on seedlings. The wilting and blackened veins distinguish the disease from symptoms of water stress or other physiological disorders.
· Keep varieties separated in the greenhouse and in seedbeds. The bacteria are rapidly spread in water, and close spacing in seedbeds and in the greenhouse are ideal for rapid disease development. Keeping the varieties separate will help you identify problematic varieties. · Transplants should only be handled when the foliage is dry.
· If black rot is detected in a seedbed, consider all plants at the location to be contaminated. Do not attempt to separate healthy plants from diseased plants…many plants will be contaminated, but will not be showing symptoms.
· Destroy all remaining plants in a seedbed as soon as transplanting operations are completed.
· Clean all transplanting equipment before and after each use.
· Do not plant transplants showing black rot symptoms or transplants suspected of being contaminated.
· Flea beetles must be controlled in seedbeds and in production fields.
Research has shown that flea beetles can transmit the bacteria from infected plants to healthy plants. Insect wounds provide a large window of opportunity for infection.
· Avoid using overhead irrigation, but if necessary irrigate during the time of day when the plants will dry quickly.
· Contaminated equipment, people, animals, overhead irrigation, and wind-driven rain will spread the disease. Always work in the diseased fields last and restrict activities in fields until late in the day when plants are completely dry to reduce the potential spread of the disease.
· A three-year rotation is recommended. Pathogenic bacteria will survive in the crop tissue until the tissue breaks down and rots.
· Infected cabbage heads should not be placed in storage.

If black rot shows up in your field, copper sprays are legal to use for disease control. Unfortunately, copper only does a fair job of controlling black rot.

· Helene R. Dillard, Department of Plant Pathology, Geneva Experiment Station

Sweet corn pests

The ECB damage that you will see first is pinholes in the leaves as they come out of the whorl. If feeding injury is found on 30% or more of your corn, you should spray Bt right down into the whorl. If you have corn in the pre-tassel stage, the control threshold is lowered to 15% because larvae feeding on the later stages is more likely to move into the ears of the plant. Later feeding will be tunneling into the tassel and stalk. At that time it is difficult to reach with a spray, so scout now. Early plantings of corn are at risk of ECB getting into the ear through the silk channel. Silking corn in areas where ECB is flying should have the silk treated. The oil/Bt mix that we have been experimenting with with Ruth Hazzard is working pretty well.

It's time to be sure that you have a corn earworm trap set up in your silking corn! We recommend the Heliothis Scentry net trap baited with the Hercon corn earworm lure. Place the trap within a block in fresh silk with the base at about ear height. Two traps in the same field will give you a more accurate count. Getting weekly information from this pest report is helpful, but the best insurance for effective corn earworm control is to have a trap on your OWN FARM! It is possible for flights to arrive overnight, and those new eggs can hatch before you get word from this network -- or from your neighbors.