St. John's Wort Inchworm
(one synonym is: Anaitis scotica Richardson, 1952)
CHESIADINI, LARENTIINAE, GEOMETRIDAE, GEOMETROIDEA
This is a popular Caterpillar, as it used to control its food plant :
which is a poisonous perennial noxious weed in many places, although it is grown as a medicinal herb elsewhere. The caterpillar is also found on :
The early instar caterpillars avoid being poisoned by the plant by avoiding the leaf glands that contain the poison. Later instars can eat these glands, as the caterpillars appear to become immune. The caterpillars are fawn to reddish-brown inchworms. When disturbed, they sit still and mimic dead twigs. Unlike many other caterpillars, they can feed in direct sunlight, as their skin is substantially opaque to ultra-violet light. The caterpillars are also able to withstand high and low temperature extremes, and are found particularly in dry habitats. The caterpillars grow to a length of about about 2 cms.
To pupate, they burrow into the soil. In moist soil, the pupae are prone to fungus infection, so this may be why the species appears to prefer dry habitats.
The adult moths are grey with three dark grey bands across the forewings, and pale brown hind wings. They have a natural posture with the hindwings covered, making a triangular shape. The wingspan is about 3.5 cms.
The eggs are white and laid singly or in small groups, on flowers and leaves. A female moth can lay about 300 eggs.
The species can have two generations per year. They may be purchased for release in the USA each September, typically at a rate of 100 per acre.
The species originated in the northern hemisphere, including :
The moths were released in Australia in 1936-1938, but seem to have failed to propagate, perhaps due to predation of the eggs and caterpillars by ants.
Further reading :
Edition 10, Volume 1 (1760), Class 5, Part 3, p. 526, No. 174.
(updated 9 July 2007, 26 February 2017)