Tent Caterpillars and Webworms

 

Eastern Tent Caterpillars (Malacosoma americanum)
and web on Black Cherry (Prunus serotina). 
Rowan County, North Carolina.

There are about 12,000 species of moths in North America, and each one comes from caterpillars.  In some species this larval stage is tiny and inconspicuous, others are large and impressive.  While all caterpillars can make silk, a select group weaves a large web to house and protect the larvae.   

Two caterpillars in our area make impressive webs on trees, the Eastern Tent Caterpillar (Malacosoma americanum) and the Fall Webworm (Hyphantria cunea).

A large group of Eastern Tent Caterpillars swarming on
their web.  This tent was made in an ornamental cherry tree (Prunus sp.).
Rowan County, North Carolina. 

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Eastern Tent Caterpillars hatch in spring and build large, tent-like webs at the forks of tree branches. The caterpillars are usually found on trees in the Rose family, the Rosaceae.  We see their webs most often on Wild Cherry (Prunus serotina).  Each web may host several hundred caterpillars.  Eastern Tent Caterpillars go through 6 larval stages called instars. Each instar is larger than the last and the caterpillar molts between these stages.

A sixth instar Eastern Tent Caterpillar feeding on
Black Cherry leaves.
Rowan County, North Carolina. 

The Eastern Tent Caterpillars shelter in the web and crawl out the tree branch to feed on leaves.  The caterpillars eat several times a day and then return to the web. The web controls the temperature and humidity to give the best environment for caterpillar development.   The final instar is about two inches long, dark brown with a white stripe down the middle of the back and blue, black and yellow markings on the side.  Caterpillars of all stages have hairs called setae that help in thermoregulation and may deter predators. Birds are the main predators of Eastern Tent Caterpillars.  The birds prefer the early instars, probably because of the large, irritating setae of later stages.  The final larval stage may wander from the host tree and pupate in a sheltered location.  The moth stage of the Eastern Tent Caterpillar emerges in early spring, lays eggs on a tree and the cycle begins again. 

A sixth instar Tent Caterpillar wandering from the web.  
This one was on a metal fence post. 
Rowan County, North Carolina. 

As their name suggests, Fall Webworms appear in late summer and are present through the fall.  The Fall Webworm Moth lays eggs on the host tree in summer and when the caterpillars hatch, they immediately begin spinning their webs. The webs enclose whole tree branches, including leaves.  The caterpillars eat the soft tissue of the leaves, leaving behind the veins.  Fall Webworms remain within their web to feed and as leaves are consumed, they expand the web to include fresh leaves. Fall Webworms go through 5 larval instars in their development with the final instar being about 1½ inches long, yellow in color with long white setae. 

Fall Webworms (Hyphantria cunea) in their web. 
The host tree of this web was
an Eastern Redbud (Cercis canadensis).
Rowan County, North Carolina. 

Detail of Fall Webworms and web.
Rowan County, North Carolina.

A fifth instar Fall Webworm surrounded by
 earlier larval stages. 
Rowan County, North Carolina. 

Fall Webworms use dozens of deciduous tree species as their host. This fall in our yard they took over branches of Eastern Redbud (Cercis canadensis) and Red Mulberry (Morus rubra).  Birds are major predators of Fall Webworms. Yellow-billed Cuckoos (Coccyzus americanus) on fall migration will tear open the webs and eat dozens of the caterpillars. Fall Webworms are native to North America but have been introduced to Europe and Asia where they are considered invasive species. 

A Yellow-billed Cuckoo (Coccyzus amerianus)
eating fall Webworms.
Orange County, North Carolina. 

 

A fifth instar Fall Webworm outside the web on
Red Mulberry (Morus rubra).
Rowan County, North Carolina. 

While Eastern Tent Caterpillars and the Fall Webworms are superficially similar, they are easy to tell apart.  Eastern Tent Caterpillars appear in early spring and make their webs at the fork of tree branches.  Fall Webworms show up later in the year with webs that enclose branches including leaves.  Both these caterpillars do some damage to trees but do not kill them.  I think these two caterpillars are bookends of the growing seasons of the year.  Eastern Tent Caterpillars to open the spring and Fall Webworms to close out the warm months.  

Video of Eastern Tent Caterpillars 

and Fall Webworms.

Rowan County, North Carolina