Elm Spanworm Head Capsule Widths and Instars

Abstract

Series of individual larvae of elm spanworm, Ennomos subsignarius (Hübner), were reared on 3 host species: northern red oak, Quercus rubra L.; white oak, Q. alba 1,.; and pignut hickory, Carya glabra (Mill.) Sweet. On the rod oak and pignut hickory the larvae had 5 distinct instars, based upon head capsule studies. Most of the larvae reared on white oak had 5 instars, but 1 ♂ and 3 ♀ had 6. The possibility and frequency of a 6th instar in the field is unknown. Measurements taken on field material did not resolve this problem. These rearings on 3 common spanworm hosts indicate the general utility of the instar-head capsule relationship, but certainty beyond the 3rd instar should be avoided. “Dyar's Rule” did not apply to the elm spanworm larvae, but use of Taylor's modification of the Rule showed that there were no missing instars among the 5-instar group. Head capsule widths did not overlap among the 5- instar larvae, even though a significant difference in size dependent upon sex was found. In the 3rd, 4th, and 5th instars the mean widths of female head capsules were greater than those of corresponding males.