Lake residents concerned about caterpillars

This article submitted by Linda Stelling on 4/25/01.

Forest tent caterpillars can eat the leaves off trees in a few days time, which is what landowners around Rice Lake would like to prevent from happening this spring.

Last year the forest tent caterpillars were thick around Rice Lake, stripping many trees of leaves.

"June looked exactly like April on Rice Lake," said Tim Lane who lives on the southeast shore of Rice Lake. "There were no leaves on the trees. I don't want to repeat that," he added.

The problem hasn't been as bad on Lake Koronis in recent years, said Bud McMillan, who watches for the caterpillars for the Koronis Lake Association. He is concerned about one area on the east end of the lake.

Last year some landowners by Rice Lake sprayed their trees from the ground. That helped, but not like an aerial spraying would have, so a group this year is already organizing an aerial spraying campaign.

There are alternatives to spraying. Homeowners can pick the egg masses off the tree branches before they hatch or brush the cocoons off the house siding, according to the Department of Natural Resources (DNR).

If homeowners choose not to spray, they will have to tolerate the nuisance of having caterpillars in their yards.

Either way, though, homeowners must be prepared to fight forest tent caterpillars in May, when they hatch, and are small, as this is the optimal time for alternative methods and for spraying.

Rice Lake spraying
About 20 property owners from Rice Lake met Thursday night to discuss an aerial spraying campaign.

Doug Jackson, Glenwood, informed the landowners he uses Foray 48B, a biological insecticide, to kill the caterpillars. Unlike some chemicals, it kills only leaf-eating caterpillars and nothing else. It has no harmful effects on birds, mosquitoes, honeybees, or lady beetles, he said.

Foray 48B contains bacillus thuringiensis (Bt). According to the Department of Natural Resources (DNR), this chemical is found in nature, but not in sufficient concentrations to kill anything. The worms need to eat the leaf with the chemical on it for them to die. It does not kill on contact, Jackson said.

McMillan said the Koronis Lake Association used Bt in 1997 when that lake was last sprayed. "There is very little an individual can do. The time to spray is when the eggs hatch and the caterpillars start eating," said McMillan.

"The worms have to be there before the chemical can do any good," Jackson added. The best time to spray is when the worms are in the eating stage, measuring one-half inch to three-fourths of an inch. Once they are an inch long, they are too big to kill.

"Timing is everything. If you spray too early, you will probably only kill the ones eating, but not those that hatch four or five days later," Jackson said.

He cautioned that all areas of the lake won't hatch at the same time, so the entire lake won't need spraying at the same time.

The insecticide will have no harmful effect or buildup in the soil, or result in harm caused by runoff in waterways, Jackson stressed.

Jackson said his plane will cover a 70-foot wide area with each pass over the lake. Jackson charges $22 per acre.

After the meeting, Lane said about three-fourths of Rice Lake could be sprayed this summer. There are alternatives to spraying, but probably not as effective. See related box listing alternatives.

Anyone interested in further information about spraying for forest tent caterpillars on Rice Lake, should contact Lane in the evening at 320-453-2525.

Property owners will need to give permission and pay for the expenses before their property will be sprayed. Those wanting their property sprayed need to contact Lane.

If their prevalence increases by Lake Koronis, McMillan may want to coordinate a spraying for Lake Koronis at the same time as Rice Lake.

The egg masses have been wrapped around tree branches since last fall. Forest tent caterpillar larvae will start emerging in early to mid-May, according to the Department of Natural Resources (DNR).

Caterpillars feed actively on broadleaf trees for five to six weeks. In June they become restless and move around to find more food. Near the end of June, full-grown caterpillars wander from where they are feeding to search for protected places to spin cocoons. Full-grown caterpillars are about two inches long, mostly blue and black, with a row of white, footprint shaped markings on their backs, and many hairs along the edge of the body.

Trees at risk
Forest tent caterpillar's favorite trees are the hardwood trees such as birches, aspen, and oaks, according to the DNR.

Newly planted woody ornamentals and tree saplings are very vulnerable to any type of stresses; and with the loss of leaves, some may be killed.

The production from fruit trees, raspberries, strawberries, and other fruit and vegetable crops will be greatly reduced if the plants suffer moderate defoliation.

According to the DNR, the most important thing a person can do is keep the defoliated trees and shrubs well-watered. Supply one inch per week if there isn't much rainfall from May 1 to Sept. 1.

Do not fertilize trees or use a weed and feed product on your lawn during an outbreak. Heavy nitrogen fertilization encourages the tree to produce more leaves, which may deplete energy reserves and put additional stress on the tree.

Alternate methods for controlling caterpillars
Forest tent caterpillars are found throughout most of the United States wherever hardwood trees are found.

There are many methods of controlling the caterpillars, the most popular being chemical spraying. The Minnesota Department of Natural Resources provides the following alternatives:

•Before they hatch (any time up to early May) hand pick the egg masses off valuable perennial plants. The masses are wrapped tight on a tree branch.

•Hand pick caterpillars off plants and put them in a soapy water solution to kill them.

•Caterpillars can be brushed off the house with a stiff broom or knocked down by a stream of water. If possible, do this daily. Avoid squashing caterpillars on the house. You can also treat lawn furniture, patios, decks, screens, etc., with either of these two methods. The longer caterpillars sit on painted surfaces, the more difficult it is to wash away any staining that may occur.

•Build a 24-inch high polyethylene wall enclosing the area you want to protect. Spray the plastic with vegetable oil to prevent the caterpillars from climbing on the wall. Repeat oil application as needed. This method has not been proven, but has been recommended by many homeowners.

•Use a product like Tanglefoot on the trunks of shrubs and trees. This prevents caterpillars from adjacent areas climbing up treated trees. However, caterpillars already in the tree are totally unaffected and small caterpillars can be wind transported on silken threads they secrete from glands in their heads.

•Dispose of dead caterpillars by burying them or mixing them into the compost pile.

•Cocoons may be difficult to remove by water pressure. They can be brushed off the house with a stiff broom. Bag, burn, bury, or compost the cocoons.

Return to Archives