Mimosa webworm is active in landscapes in Delaware. The following is more information.
Mimosa webworm is an introduced pest that effects both honeylocust and mimosa. Trees in grown in nurseries may be more susceptible. There are usually two generations per year. The overwintering pupal stage gives rise to very small, gray moths that lay tiny, rose-colored eggs in late spring. Young larvae strip/skeletonize upper surfaces of leaves and produce large amounts of silk. This silk sandwiches together leaves and small branch tips. Eventually, webby masses are quite visible on the tree. Caterpillars vary in color from pinkish gray to green to dark brown and grow to about 5/8”. After feeding, larvae use silk threads to lower themselves into bark crevices or to the ground and pupate in small, white cocoons. The second generation is produced quickly in mid-late summer and there are reports of a third generation during warm years in southern mid- Atlantic locations. The earliest observed date for larvae is June th 15 in Northern DE. A GDD range of 750-3216 (avg. 1643) and full bloom of Kalmia latifolia can also be used.
Research at Purdue University indicates that isolated honeylocust trees are more commonly affected than trees grouped closely together; and observations in this region seem to confirm this. Early research indicates resistance amongst certain thornless honeylocust cultivars. 'Sunburst' is quite susceptible, while 'Moraine', 'Shademaster', and 'Imperial' were touted as less susceptible. Manual web removal is not a practical control method.
Sprays of Bacillus thuringiensis 'kurstaki' (Dipel, Bt Worm Killer, many others) may be used very early in the season if the eggs or young larvae are found during scouting and before webbing. Neem products containing azadirachtin such as Azatin, Ornazin, and Azatrol provide some feeding repellency along with larval knockdown. Conserve (a.i. spinosad) and pyrethroids are used once webs are widely observed to protect against greater damage by subsequent generations.
Mimosa webworm webbing. Both mimosa and honeylocust are attacked by the mimosa webworm. Photo by John A. Weidhass, Virginia Polytechnic Institute and State University, Bugwood.org.
Information from Casey Sclar, IPM Coordinator, Longwood Gardens