Sterictiphora cellularis Say Hymenoptera Argidae

Natural History

Distribution. This native species occurs throughout the eastern United States, occurring as far west as eastern Nebraska and Texas. It has been known as a serious pest only in the Chesapeake Bay region, specifically Maryland, Virginia and North Carolina, and near the mouth of the Mississippi River, in Louisiana and Mississippi.

Host Plants. Sweetpotato sawfly is known only from Ipomoea spp., and rarely has been collected from anything other than sweet potato.

Natural Enemies. A larval parasitoid, Schizocero-phaga leibyi Townsend (Diptera: Tachinidae) is reported to be an effective biological control agent, having been reared from 60-70% of sweetpotato saw-fly cocoons. Chapman and Gould (1929) attributed the small size of the autumn generation principally to the actions of this fly. A wasp parasitoid, Boethus schizoceri (Riley and Howard) (Hymenoptera: Ichneumonidae) also is known from sweetpotato sawfly.

Life Cycle and Description. Apparently there are three generations annually in Virginia, with larvae present in July, August, and September. A generation can be completed in about 28 days. This insect is presumed to overwinter in the pupal stage.

Egg. The eggs are inserted within the leaf tissue in rows along the principal veins on the lower surface of the leaf. They cause the leaf tissue to bubble or blister, and each blister contains a single egg. Rows of 6-12 eggs are common, and a single leaf may contain over

200 eggs. The egg is whitish, and has a somewhat flattened oval shape. The egg measures about 1.6 mm long at hatching, but because sawfly eggs enlarge before hatching they must be smaller when first deposited. Eggs hatch in 6-7 days.

  1. The larva is yellowish-green and adorned with numerous black, blunt spines. The larva bears seven pairs of abdominal prolegs, but they are short and obscure, the terminal prolegs especially cryptic. Apparently there are five instars. Duration of the larval stage is about 10 days. At maturity the larva measures about 12 mm long.
  2. When the larva completes its growth and is ready to pupate it drops to the soil and constructs a brownish silken cocoon at, or near, the soil surface. The cocoon is oval in shape and measures about 14 mm long and 5 mm wide. The cocoon frequently has soil and leaf debris attached to the outside. The pupal stage apparently has not been described. The adult emerges from the cocoon about 9-12 days after cocoon formation.
  3. The adults are reported to be short-lived and weak fliers. The body of adult males is shiny black, and the wings dusky. The female is similar in color except that her abdomen is orange-red. In both sexes the legs are dark basally but lighter or colorless distally. Females measure about 7 mm long, with a wingspan of about 12 mm. The male is slightly smaller

Sweetpotato sawfly larva.

Sweetpotato sawfly larva.

Sawfly Adult Male
Adult male sweetpotato sawfly.

in size. The antenna of the male is branched whereas that of the female is unbranched.

The biology of sweetpotato sawfly was described by Chapman and Gould (1929).

Damage

The larvae are defoliators of sweet potato, consuming leaf tissue but leaving the stems and basal portion of the principal veins. As adults are not very dispersive, they tend to deposit many eggs in small areas. This leads to severe defoliation in very localized sections of fields, and sometimes death of numerous larvae as they exhaust the food supply. Repeated defoliation by sawfly may reduce yield by 50%.

Management

This species is infrequent, and apparently has caused no large-scale damage since the early 1900s. Observations from early in the 1900s suggested that damage was limited to the sweet potato cultivar "Big Stem Jersey," and changes in cultivar selection by sweet potato growers might have accounted for the demise of this insect as a pest. Foliar insecticides are effective, and crop rotation is recommended due to the poor dispersive ability of the adults.

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