Mimosa and the Web

Top Photo: Mimosa leaves with mimosa webworm infestation.

Both mimosa the tree and the webworm are non-native and considered invasive species. The tree was introduced into the United Sates during the mid 18th century. Most sources quote 1745 as the year of introduction as an ornamental.

Mimosa is a legume and produces copious amounts of long, seed containing pods. The seeds are very hardy and stay viable for years. New trees pop up all around the mother tree, even sprouting some distance from the parent.

It seems people either love mimosa or they hate it. Most people who’re in the “love” camp like the tree for its feathery twice compound, or bipinnately compound, leaves and the silky pink and white flowers. It is an attractive tree.

Feathery, bipinnate leaf of mimosa.
Hummingbird visits silky mimosa flower for nectar.

Besides the invasiveness of mimosa, the “haters” don’t like the mess made by the falling spent flowers in spring and summer, and leaf liter later in the fall.

Mimosa is also susceptible to various diseases and insect infestations. I reported in 2010 here on this blog, about a psyllid which turned out to be Acizzia jamatonica and which attacks mimosa leaves. Just two days ago I mentioned a moth species whose caterpillars feed on the mimosa’s leaves.

Adult psyllids are more than twice the size of the nymphs (adult here about 2mm). This adult sits atop a mimosa leaflet.

Both the psyllid and moth (mimosa webworm) are non-native (East Asian) which would seem to even the score since they feed on a non-native species of tree which is also from that part of the world. But, the moth caterpillars also feed on honey locust, which is native.

The psyllid is a recent arrival, being first detected in 2007, but the moth has been around since at least the 1940s when it was discovered on honey locust trees in Washington DC planted to replace American elms killed by Dutch Elm Disease (Asian in origin).

Mimosa webworms wrap the leaves of the mimosa in silk, classic webworm style, and feed on the leaves from within the safety of the web. If you carefully peel back some of the silk you may get a glimpse of one or two of the caterpillars. They’re greenish-brown and have five light colored longitudinal stripes, two on each side, one dorsally. The head is usually darker and more brown than the body.

Web surrounds leaves and caterpillars within. Dark specks are frass (caterpillar poop).
Peeling back silk of web may reveal caterpillar. That’s the tip of my index finger at top of photo.

When disturbed the caterpillar moves rapidly and mechanically to escape danger. Their movements appear jerky, claymation-like. They grow to about 15 mm before pupating.

A very small, early instar caterpillar (about 8mm).

It’s curious why the webworm hadn’t become known in the States until the mid 20th century when the mimosa tree had already been in country for some 200 years prior.

Mimosa webworm.


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