Spraying for forest tent caterpillars

Planning an insecticide treatment to control forest tent caterpillar defoliation

What are the reasons to control forest tent caterpillars (FTC)?

  1. To protect valuable ornamental foliage and perennial plants from defoliation. Examples would be fruit trees, small fruit crops and gardens. These plants or areas are often small in size and are suitable for treatment from the ground.
  2. To reduce caterpillar populations in order to reduce the nuisance to people. This could include resorts and private campgrounds which might suffer economically during the June activity of the FTC. Also groups of homeowners or lakeshore associations may wish to preserve the aesthetics of their land. Due to the size of the trees and the acreage involved, aerial spraying will likely be necessary.
  3. To prevent defoliation of high value oaks that suffered drought stress and previous FTC defoliation or two-lined chestnut borer attack during the past two years. These already weakened trees may die from the additional stress of defoliation and continued drought.

Will spraying this year reduce the FTC problem next year?

No. Spraying will only affect this year's population of caterpillars. In July, FTC moths from outside the spray area will fly into the treated area and lay eggs which will hatch next year and cause defoliation.


Preplanning: Winter and early spring

  1. Organize a neighborhood, lakeshore or town meeting to discuss the spray operation. Have participants review the Forest tent caterpillar Web siteand MDA’s Pesticide Unit: State Statute 18b.07
  2. Plan to use a product containing Bacillus thuringiensis var kuristaki (Btk) as the active ingredient. These recommendations only pertain spray operations using Btk.
  1. Treatment will likely need to occur before or after Memorial Day weekend.
  2. Obtain written permission to spray from each landowner involved. Spraying a landowner's property who has not granted permission is a violation of state law. Many people have concerns about spraying pesticides regardless of the reason or type of pesticide involved. Be aware of your neighbor’s wishes or their reactions regarding plans to spray and actual spraying activities. If the spray drifts off of the targeted land, it is a violation of state law, too. Review MDA State Statute 18b.07
  3. To be effective, the spray block should be at least 10 acres in size, and 80 percent of the block needs to be treated. Large forested tracts directly adjacent to the area being protected may also require treatment of a border strip along the edge to prevent movement of caterpillars into the sprayed area. The width of the strip should be at least 100 yards.
  4. Obtain funding for the treatment. Possible funding could include fees from landowners based on acres treated or feet of lakeshore to be treated.
  5. Determine number of acres to be treated. Prepare maps or make a mosaic of aerial photos to inform the licensed commercial pesticide applicator.
  6. Contact aerial applicators. A list of licensed aerial applicators can be obtained through the MN Department of Agriculture on their interactive website. Applicators should be licensed for forest spraying and either aerial or ground treatment, depending on your needs.
  7. Provide your pesticide applicator with maps and acreage estimates. Obtain bids for supplying Btk (and adherent, if recommended) and all application costs on a per acre basis. Ask for bids for a single application and for two applications with the understanding that the second application is optional at your determination. One application is usually sufficient to protect tree foliage, particularly if the forest composition is made up of only a few different tree species. Two sprays may be necessary to reduce the numbers of nuisance caterpillars or to protect foliage when forest composition is diverse in species or diverse in tree heights.
  8. Negotiate and develop a contract with the aerial applicator of your choice.
  9. Provide the name, address, and phone number for the local contact person who will act as the local supervisor and liaison between the contracted applicator and the landowner group.


  1. Based on the contract with the pesticide applicator, timing of spraying can be determined by the contractor or by the local supervisor. If the latter is the case, then the local supervisor notifies the pesticide applicator that he has five days to complete spraying before leaves and caterpillars reach optimal size.

    The best time to spray forest tent caterpillars with Btk:

    Caterpillars: average ¼ to ½ inch in length
    Foliage: leaves are less than of their normal length

    Caution: During very high populations, the young caterpillars may eat the foliage off as it appears. If this occurs, time the spraying based on the size of the caterpillars.

  2. The pesticide applicator notifies landowners in the spray block by direct notification of each owner or resident. This can include mailings, broadcast emails, public meetings, posted placards and neighborhood newsletters, among other methods. MDA State Statute 18b.07
  3. The pesticide applicator is also responsible for publicizing the pesticide treatment in a local newspaper of general circulation at least 24 hours prior to the actual application. MDA State Statute 18b.07
  4. The local supervisor should be on hand to assess weather conditions and to monitor application. The local supervisor has the authority to stop the spray operation if weather conditions change (see label recommendations), the contract specifications are not being followed, the operator is acting in an unsafe or illegal manner or pesticide drift is occurring.
  5. If a second spray is needed, it should be scheduled to occur three–five days after the first.
  6. The landowner group should complete their contract responsibilities with the pesticide applicator.