Corn Flea Beetle and Bacterial Wilt
Flea beetles overwinter as adults, thus winter temperatures have been useful in determining spring population pressure of adults. The 1997-98 winter was unusually warm, and high densities of flea beetles caused severe damage to the 1998 sweet corn crop. Some stands lost 40% of the crop due to a bacterial disease transmitted by corn flea beetles!
Corn flea beetles transmit a bacteria that causes bacterial wilt (also known as Stewart's wilt) of sweet corn. The bacteria is called Erwinia stewarti . The best cultural practice that can prevent wilt problems is to use resistant varieties when possible. This is more important for the early plantings, which typically sustain higher densities of corn flea beetles in sweet corn.
The 1999 Commercial Vegetable Production Guide provides three abbreviations to help you determine how the variety responds to Stewart's wilt. The abbreviations are: BWR - bacterial wilt resistant BWMS - bacterial wilt moderately susceptible BWS - bacterial wilt susceptible This notation is listed next to the cultivar. Also, the source of the seed may be able to tell you if the cultivar is resistant to Stewart's wilt.
Be prepared to treat susceptible varieties with an insecticide if there are 6 or more beetles per 100 plants. Be prepared to act quickly - you need to look for these beetles when the plants are in the spike stage, just coming out of the ground. A banded application just over the plants should work. Keep scouting even after a spray, to make sure more beetles do not move in. The overwintering adults will feed on weeds and move into corn plants throughout May and June. Beetles tend to be more abundant on outer rows. Scouting on calm, sunny days works best - the beetles are most active then, and although they will jump away from you, it will be easier to spot them because of this activity.
While a systemic may help, it cannot be totally relied upon. I am aware of one field in 1998 where the flea beetle was a significant problem even though a systemic soil insecticide was applied. A systemic insecticide applied at planting sometimes retains efficacy long enough to control flea beetles, but this is less likely if soil temperatures are cool or if immigration occurs over a long time. For example, if you have 7 days from planting to plant emergence, and then the beetles move in over a 3-week period, you would need the systemic activity to be maintained for a month, which is difficult.
There are several species listed as causing this damage. They include the corn flea beetle, pale striped flea beetle, the western black flea beetle, the toothed flea beetle, the sweetpotato flea beetle, and the smartweed flea beetle. All have enlarged femora on their hind legs (in other words, the thigh of their hind legs are a bit enlarged) which serves for the large muscles attached there. This gives them their ability to jump rapidly. Larvae feed on roots. But their damage is generally only associated with the early stages of corn, with disease transmission as the most important issue. The corn plant, once it gets going, can easily outgrow the larval feeding or adult feeding on older corn plants. But once the bacteria start multiplying inside the corn plant, that plant, if it is a susceptible variety, will get progressively worse.
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