Eastern Tent Caterpillars, Extrafloral Nectaries, and Ants

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Overwintered eastern tent caterpillar (ETC) (Malacosoma americanum) moth eggs are hatching in southwest Ohio; a sure sign that spring has sprung.  As their common name implies, the caterpillars are accomplished tent-makers displaying their silk handiwork in branch forks.

 

ETC

 

ETC spends the winter in shiny, blackish-brown egg masses wrapped around twigs on their host plants.  A close examination will reveal that the eggs are encased in a structure that resembles bubble-wrap perhaps for winter protection.

 

ETC

 

ETC

 

The accumulated Growing Degree Days (GDD) that predicts ETC egg hatch is 92.  The full bloom of Corneliancherry dogwood (Cornus mas) is usually a pretty good phenological indicator.

 

Cornelian Cherry Dogwood

 

The caterpillars begin producing their highly visible silk nests immediately upon hatching.  Indeed, egg-hatch may be revealed by observing the tiny, hairy first instar caterpillars clinging to small, silk nests that surround their egg mass.

 

ETC

 

The caterpillars prefer to feed on trees in the family Rosaceae, particularly those in the genus Prunus, such as cherry.  They also occasionally feed on birch, maple, and oaks.  Healthy large trees can handle the early season defoliation by producing a new flush of leaves.  However, newly planted trees may not fare as well.  Leaves lost to caterpillar feeding this spring must be replaced using energy stored from last season. 

 

ETC

 

Small nests can be eliminated digitally using five-fingered "smash and/or smear" techniques.  Less hands-on methods include applications of the naturally occurring bacterium, Bacillus thuringiensis serotype kurstaki (Btk), applied to early instar stages, as well as standard insecticides labeled for general caterpillar control.

 

 

 

Sweet Relief

 

Nectar is the currency used by plants to pay insects and other animals to do their bidding.  It costs plants nothing to photosynthesize this sweet medium of exchange using energy from sunlight to rearrange atoms that constitute water and carbon dioxide.  The recent rise in awareness of the importance of pollinators has highlighted this nectar-based economy. However, flowers are not the only place where nectar serves as coinage in exchange for services.

 

Extrafloral nectaries (EFNs) are plant glands that produce nectar but are not associated with flowers.  You may be aware of the EFNs found on the leaf petioles of trees in the genus Prunus because they are such a handy feature used to identify plants in the genus, particularly cherry.  Indeed, the glands first served only as taxonomic signposts before research studies eventually revealed their ecological function.

 

Extrafloral Nectaries

 

The EFNs on cherries can vary considerably in size and shape from species to species.  Some appear as small red or green "bumps" or even "dots" on the petiole at the base of the blade while others look like well-defined deep red donuts.  Of course, a lot of things look like donuts to me; I have a highly developed search image.

 

Extrafloral Nectaries

 

Extrafloral Nectaries

 

In 1978, University of Minnesota ecologist Dave Tilman published a paper in the journal Ecology titled, "Cherries, Ants and Tent Caterpillars: Timing of Nectar Production in Relation in Relation to Susceptibility of Caterpillars to Ant Predation."  The title says it all.

 

Extrafloral Nectaries

 

Tilman found that the EFNs on black cherry (P. serotina) commonly attract western thatching ants (Formica obscuripes) which are predaceous on many caterpillars including ETC.  His research showed that the number of ants visiting the EFNs was directly correlated with the distance between ant colonies and cherry trees and ETC survivorship was positively related to those distances.

 

He also found that the highest number of ants visiting the trees occurred just after bud break and decreased as the number of active EFNs decreased.  This time frame coincided with the development of ETC with caterpillars large enough to escape ant predation appearing after EFNs ceased their nectar-payment activity.  He concluded that "… the ant-cherry relationship is a facultative mutualism and that nectar production is timed so as to maximize the chance of successful ant predation on tent-caterpillar colonies."

 

Extrafloral Nectaries

 

The term "myrmecophile" means "ant lover."  It is derived from the Greek "myrmex" = ant, and "phlos" = loving.  The term applies to the special relationship some plants and animals, including insects (e.g. aphids), have with ants.  Obviously, many plants in the Prunus genus are myrmecophiles with their sweet love expressed through EFNs.