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Forest tent caterpillars prey on hardwood trees found in west-central Minnesota woods. (Jupiter Images - Thinkstock)
Forest tent caterpillars prey on hardwood trees found in west-central Minnesota woods. (Jupiter Images - Thinkstock)

Pests or pesticides, which is the lesser of two evils?

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news Alexandria, 56308

Alexandria Minnesota 225 7th Ave E
P.O. Box 549

The fuzzy little caterpillar takes a nap in a cute cocoon and emerges a beautiful butterfly. Ah. Why then does the tale turn into a horror story when a forest tent caterpillar turns into a moth?


May is usually the month people start seeing aircraft spraying pesticides. In the Alexandria lakes area, pesticide is sprayed to ward off forest tent caterpillars. The fine mist drifting through the air everyone shares has raised questions about Foray 48B, the biological insecticide sprayed to eradicate the threat.

As much as people don't like the sickly trees left behind and the squish of guts beneath their feet, they have concerns about how Foray 48B can affect human health. If it's killing insects, it must be dangerous, right?

The Lake Latoka Property Owners Association board opted to not pursue spraying since reports on the effects of Foray 48B seem to be conflicting, said board president Greg Peterson.

"Some of the lakeshore owners were adamant that their property not be sprayed, while the neighbors on both sides of that person wanted to be, so logistically it was impossible," Peterson explained.

Planes applying the solution aerially must fly lower than usual to strategically place the pesticide for maximum effectiveness, according to the Minnesota Department of Agriculture, which regulates licensed pesticide applicators.

"There are a limited number of licensed aerial pesticide applicators in this area and they are booked solid for the season each year," explained Robin Trott, University of Minnesota horticulture extension educator. Spraying is limited to the availability of the pesticide, time and conditions being right for application.

"[Foray 48B] is only toxic to caterpillars."

Robin Trott


Foray 48B, which contains Bacillus thuringiensis kurstaki (Bt), is a biological insecticide that kills lepidopterous larvae, caterpillar babies. The product has been used since the 1920s to control populations of approximately 25 caterpillar and moth species.

Endotoxin crystals and spores in Bt are poisonous once eaten by larvae. The insects usually die within two to five days after ingestion. Aerial applications may need to be repeated every three to 14 days to maintain control and protect new plant growth.

"Bt is one of the safest pesticides to use," Trott said. "It is only toxic to caterpillars."

Trott further explained that Foray 48B is not toxic to people, animals, plants, fish, birds or other insects. Although residue is not toxic, she advises washing all produce prior to consumption.

A precautionary statement on the insecticide label states it can cause moderate eye irritation in humans and domestic animals. Contact with eyes or clothing is cautioned against. Washing with soap and water after handling and before eating, drinking, chewing gum, using tobacco or using the toilet is advised. When used in an agricultural setting, workers are required to wear personal protective equipment and may not enter a treated area for four hours after application.

The product is not intended to be used directly on water or in areas where surface water is present unless under a forest canopy. In consideration for the environment, Foray 48B cannot be used in an area within one-quarter mile of endangered lepidoptera. It is a violation of federal law to use the product inconsistent with its labelling.

Valent BioSciences Corporation manufactures Foray48B. According to the company's product label, five to 10 gallons of the product should be used per acre in the Western United States and two to three gallons in the East, which puts the Midwest somewhere in the middle.

The amount of water needed for dilution prior to aerial application is dependent on crop size, weather conditions, spray equipment and local experience. Based on information in the 2011 Minnesota Forest Health Annual Report, the average cost per acre of Foray 48B application is $35.

Aerial sprayers are hired by townships, cities and lake associations to spray specific areas, Trott said. If property owners are concerned, they should contact their local entity. Associations should secure permission from all property owners within the spray zone to inform them of the timeframe in which spraying will occur.

In an instance where a property is not to be sprayed, the plane will make a wide berth and avoid overhead spraying of that property, Trott explained. Being that it is applied into the air, there could be some drift. If a property owner wishes to file a complaint, they can do so in writing with the Minnesota Department of Agriculture.

The forest tent caterpillar (Malacosoma disstria) preys on aspen, oak, birch and other hardwood trees found in Minnesota woods. In 2010, a peak defoliation rate was reported and more than 70,000 acres in central Minnesota were infested by the intrepid insects.

A possible decline in 2011 put damage at 61,149 acres. But, that number may not be correct as the state shutdown in July 2011 occurred during aerial survey season. The Department of Natural Resources noted that populations of forest tent caterpillars are far from declining in the west-central counties. Basswood and oaks are primary targets that might suffer in 2013 due to local outbreaks.

Statewide, 274,000 acres of defoliation were mapped during the summer of 2012 aerial detection survey.


Forest tent caterpillars (Malacosoma disstria) are not armyworms.

Armyworms (Pseudaletia unipuncta) feed in fields; forest tent caterpillars feed primarily on trees.

Forest tent caterpillars do not turn into gypsy moths (Lymantria dispar). Gypsy moths are hairy and dark colored with raised blue and red dots along their backs. Forest tent caterpillars are blue and black with white-colored footprint shapes lining their backs. Armyworms are brownish-black and tan.

DeyCrystal Dey Crystal Dey is a staff reporter for the Echo Press. Originally from Minnesota's Iron Range, Dey worked for newspapers in North Dakota, Florida and Connecticut before returning to her home state to join the Echo Press in October 2011. Dey studied Mass Communications at Minnesota State University Moorhead with an emphasis in Online Journalism. Follow Staff Reporter Crystal Dey on Twitter at @CrystalDey_Echo.