Tag Archives: Malacosoma americanum

Malacosoma americanum (Fabricius): Eastern Tent Caterpillar

California Pest Rating for
Photo Credit: Pennsylvania Department of Conservation and Natural Resources - Forestry , Bugwood.org
Photo Credit: Pennsylvania Department of Conservation and Natural Resources – Forestry , Bugwood.org
Malacosoma americanum (Fabricius): Eastern Tent Caterpillar
Lepidoptera: Lasiocampidae
Pest Rating: A

Initiating Event:

Malacosoma americanum is frequently intercepted by CDFA.  A pest rating proposal is required to support its pest rating.

History & Status:

BackgroundMalacosoma americanum is a foliage-feeding moth.  The moth overwinters in egg masses of 150-424 eggs on tree branches1,2.  Egg hatch coincides with bud break.  The caterpillars from each egg mass stick together and spin a silky tent in a crotch of the tree1.  During evening, night, and early morning hours the caterpillars emerge from the tent to eat leaves1.  They feed for 4-6 weeks then move individually to protected places to pupate1.  Moths emerge about 3 weeks later to mate and lay eggs1.  There is only one generation per year1.  Preferred hosts include: Rosaceae: cherry, peach, and plum (Prunus spp.2), apple and crabapple (Malus spp.2), hawthorns (Crataegus spp.2), and sometimes pear (Pyrus spp.2).  In addition to the preferred hosts the caterpillars will also feed on a wide variety of hardwoods2.  A similar moth Malacosoma californium (western tent caterpillar) is native to and widespread in California; however, its host preferences are aspens, willows, cotton, and mountain mahogany4.  Eastern tent caterpillar poses a distinct threat to California’s specialty crops.  It may be transported into the state as a contaminating pest (cocoons or larvae) on a wide variety of objects or as egg masses on potted trees or fresh tree branches.

Worldwide Distribution: Malacosoma americanum is native to the eastern United States and Canada1.  It is not known to have invaded any other nations.

Official Control: Malacosoma americanum is listed as a harmful organism by Japan, the Republic of Korea, Mexico, Morocco, the Russian Federation, Taiwan, and Turkey3.

California DistributionMalacosoma americanum has never been found in the environment of California.

California Interceptions:  Between January 1, 2000 and December 31, 2014 Malacosoma americanum was intercepted 108 times by CDFA’s border stations.  Interceptions typically occur on outdoor items and goods associated with household moves.

The risk Malacosoma americanum (eastern tent caterpillar) would pose to California is evaluated below.

Consequences of Introduction:

 1) Climate/Host Interaction: Malacosoma americanum is native to a wide variety of climates in eastern North America and its host plants are widely grown in California. Eastern tent caterpillar can be expected to establish a widespread distribution in this state and receives a High (3) in this category.

Evaluate if the pest would have suitable hosts and climate to establish in California.  Score:

Low (1) Not likely to establish in California; or likely to establish in very limited areas.

Medium (2) may be able to establish in a larger but limited part of California.

High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Preferred hosts of Malacosoma americanum are limited to seven varieties of plants in one family.  However, it can also feed on other hardwood trees.  It receives a Medium (2) in this category.

Evaluate the host range of the pest. Score:

Low (1) has a very limited host range.

Medium (2) has a moderate host range.

High (3) has a wide host range.

3) Pest Dispersal Potential: Malacosoma americanum has only one generation per year and each female lays an average of 293 eggs2.  Adult moths may fly short distances and caterpillars can crawl between trees.  The moths may also be transported long distances as a contaminating pest or when infested plants or plant parts are moved.  Eastern tent caterpillar receives a High (3) in this category.

Evaluate the natural and artificial dispersal potential of the pest. Score:

Low (1) does not have high reproductive or dispersal potential.

Medium (2) has either high reproductive or dispersal potential.

High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Eastern tent caterpillar is not expected to lower crop yields.  In their native range the moths often defoliate entire trees.  They may increase crop production costs in California as orchards and nurseries treat to mitigate damage.  Malacosoma americanum is listed as a harmful organism by several of California’s trading partners.  The moth could disrupt exports of nursery stock but it is not likely to follow fresh fruit pathways.  Eastern tent caterpillar is not expected to change cultural practices or vector other organisms.  Exposure to the caterpillars is thought to cause the equine disease mare reproductive loss syndrome (MRLS) which is responsible for abortions in horses5Malacosoma americanum is not expected to disrupt water supplies.  Eastern tent caterpillar receives a High (3) in this category.

Evaluate the economic impact of the pest to California using the criteria below. Score:

A. The pest could lower crop yield.

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural practices.

E. The pest can vector, or is vectored, by another pestiferous organism.

F. The organism is injurious or poisonous to agriculturally important animals.

G. The organism can interfere with the delivery or supply of water for agricultural uses.

Low (1) causes 0 or 1 of these impacts.

Medium (2) causes 2 of these impacts.

High (3) causes 3 or more of these impacts.

5) Environmental Impact: Malacosoma americanum is not expected to lower biodiversity, disrupt natural communities, or change ecosystem processes.  It is not expected to affect threatened or endangered species or disrupt critical habitats.  Eastern tent caterpillar often completely defoliates ornamental trees.  This damage is likely to trigger treatment programs by residents, orchards, and the nursery industry.  Malacosoma americanum receives a High (3) in this category.

Evaluate the environmental impact of the pest on California using the criteria below.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered species.

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment programs.

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

High (3) causes two or more of the above to occur.

Consequences of Introduction to California for Malacosoma americanum (Eastern Tent Caterpillar):  High (14)

Add up the total score and include it here.

Low = 5-8 points

Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Malacosoma americanum has never been found in California and receives a Not established (0) in this category.

Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included.

Not established (0) Pest never detected in California, or known only from incursions.

Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).

Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: High (14)


Traditionally residents have treated outbreaks by dousing tents with a flammable liquid and setting them on fire during the afternoon when all of the caterpillars are inside, although this is no longer recommended1.  This treatment could cause additional significant environmental impacts in California due to the drier climate.  Existing IPM programs in orchards may preclude damage from Malacosoma americanum.

Conclusion and Rating Justification:

Malacosoma americanum has not been found in California and is expected to have significant economic and environmental impacts if it were to establish in the state.  An “A” rating is justified.


1 Bessin, Ric. 2013. Eastern Tent Caterpillar. University of Kentucky ENTFACT-423.  http://www2.ca.uky.edu/entomology/entfacts/ef423.asp

2 Hyche, L.L. 1996. The Eastern Tent Caterpillar: A Guide to Recognition and Habits in Alabama. Auburn University. http://www.ag.auburn.edu/enpl/bulletins/easterntentcaterpillar/easterntentcaterpillar.htm

3 USDA Phytosanitary Certificate Issuance & Tracking System (PCIT) Phytosanitary Export Database (PExD).  https://pcit.aphis.usda.gov/pcit/

4 Forest Insect Defoliators. Field Guide to Insects and Disease of Arizona and New Mexico Forests. http://www.fs.fed.us/r3/resources/health/field-guide/fid/tent-caterpillar.shtml

5 Webb, Bruce A., W.E. Barney, D.L. Dahlman, S.N. DeBorde, C. Weer, N.M. Williams, J.M. Donahue, K.J. McDowell. 2003. Eastern tent caterpillars (Malacosoma americanum) cause mare reproductive loss syndrome. Journal of Insect Physiology 50(2-3):185-193. http://www.sciencedirect.com/science/article/pii/S002219100300249X

Responsible Party:

Jason Leathers, 1220 N Street, Sacramento, CA, 95814, (916) 654-1211, plant.health[@]cdfa.ca.gov.

Comment Format:

♦  Comments should refer to the appropriate California Pest Rating Proposal Form subsection(s) being commented on, as shown below.

Example Comment

Consequences of Introduction:  1. Climate/Host Interaction: [Your comment that relates to “Climate/Host Interaction” here.]

♦  Posted comments will not be able to be viewed immediately.

♦  Comments may not be posted if they:

Contain inappropriate language which is not germane to the pest rating proposal;

Contains defamatory, false, inaccurate, abusive, obscene, pornographic, sexually oriented, threatening, racially offensive, discriminatory or illegal material;

Violates agency regulations prohibiting sexual harassment or other forms of discrimination;

Violates agency regulations prohibiting workplace violence, including threats.

♦  Comments may be edited prior to posting to ensure they are entirely germane.

♦  Posted comments shall be those which have been approved in content and posted to the website to be viewed, not just submitted.

Pest Rating: A

Posted by ls