Cankerworm
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Cankerworms have been documented as native North American tree pests since colonial days. Spring and fall cankerworms are similar in distribution, appearance, and the damage caused. The fall cankerworm in lays its eggs in November and early December, and the spring cankerworm in late February and March. The female moths of both are wingless. Cankerworm infestation have been reported in the Canadian Maritimes, southern Canada, and the Eastern Midwestern, and Western United States. The spring cankerworm tends to range of hosts that include deciduous forest, shade, and fruit trees. Apple and elm seem to be preferred, but many oaks, maples, hickories, ashes, and beech are also fed upon. The fall cankerworm lays about 100 eggs in an aligned, compact, single-layered, reddish-brown mass, which becomes gray as it weathers (fig. 8). The masses can be found directly on small branches, at the juncture of a branch and a tree trunk. Occasionally a single egg will be laid. It is possible to distinguish old eggs from new by the absence of a covering on eggs. The eggs of both species of cankerworms hatch in the spring, and it is not unusual to find them feeding together on expanding buds and foliage. Larval feeding lasts 4 to 6 weeks. Feeding by young larvae produces a shothole appearance on the leaves. Older larvae material but the major veins. Full-grown fall cankerworm larvae vary from light to dark green and are about 25 mm (1 in.) long (fig. 9). Mature spring cankerworm larvae are 18 to 30 mm (0.8 to 1.3 in) long and vary considerably in color, ranging from reddish to yellowish green, or black. The head is light and mottled with a yellowish stripe along each side of the body, and a broad, greenish-yellow stripe runs the length of the undersurface of the body. Some older fall cankerworm larvae have a dark stripe running lengthwise down the back; light-green larvae have white lengthwise lines. Larvae of the fall cankerworm have three pairs of hind legs or claspers. One pair is smaller, sometimes indistinct, and in front of the two obvious hind legs (fig. 10). The spring cankerworm lacks this additional pair of hind legs. |
Figure 8. – Female laying eggs. |
Figure 9. – Fall cankerworm larva. |
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Figure 10. – Side view of fall cankerworm larvae showing the three pair of hind legs. |
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Figure 11. – Adult male fall cankerworm. |
When the larvae finish feeding in early July and are ready to pupate, they crawl or spin down on silken threads to the ground and prepare cocoons in the soil. Fall cankerworm adults emerge in November and early December, frequently after some freezing weather; spring cankerworm adults emerge in late February and March. Fall cankerworm male moths are brownish gray with a wingspread of 25 to 35 mm (1 to 1.4 in) (fig. 11) the wingless female are about 12 mm (0.5 in) long.
Spring cankerworm adults are similar to the fall cankerworm. Male moths have a wingspan of 21 to 30 mm (1.0 to 1.3 in). The forewings are silky brownish gray, crossed by three jagged dark lines. The forewings are pale ashy gray with a darkened central spot. Females are wingless, generally white, with brown or black white with brown or black hairs, with a dark stripe down the back . Both have two transverse rows of reddish spines on the back. These are more prominent in the female and give the back a reddish cast.
