Grape leaffolder—Desmia funeralis
Grape leaffolder (family Pyralidae) is a chewing pest of grape leaves in the Central and Southern San Joaquin Valley. It also feeds on evening primrose, redbud, and Virginia creeper.
Adults (moths) are about 1 inch long with a dark brown or gray to blackish body and wings. Each forewing has two white blotches and there are two white bands on the abdomen. On the hind wings females have two white blotches and males have one or two white blotches.
Eggs are elliptical, about 1/30 inch (0.8 mm) long, iridescent, and translucent. If a leaf is turned in the sun, both hatched and unhatched eggs can be observed to glisten. Unhatched eggs are convex, and hatched eggs are concave. Eggs can occur singly on either side of the leaf, most commonly on the underside adjacent to a leaf vein. On some leaves several eggs are laid together partly overlapping.
Larvae are translucent to pale brown, greenish, orangish, pinkish, or whitish. Larvae are generally green when they have fed on leaves and brownish if they have fed on berries. They are 1/20 inch (1.2 mm) long just after hatching, grow up to 1 inch, and have a brown to yellowish head.
Older instars have black spots on the thorax that distinguish them from the similar larvae of omnivorous leafroller that also feed on grape. When omnivorous leafroller larvae are disturbed, they wriggle and drop attached to a silken thread. Grape leaffolder larvae when disturbed wriggle vigorously and drop to the ground generally without a silken thread.
Pupae of grape leaffolder are brown to orange, elongate, and about 2/3 inch long. Pupae and the black fecal pellets of the larvae commonly occur within the fold of the leaf created by the larvae.
Grape leaffolder develops through 4 life stages: egg, larva, pupa, and adult. Overwintering is as a pupa. The first generation of adults emerge in April or May, mate, and the females lay eggs. The hatching larvae develop though 5 increasingly larger instars. Young larvae feed mostly where leaves touch or between 2 leaves webbed together with silk for about 2 weeks. Sometimes several young larvae feed in a group. Third instar and older larvae roll a leaf and feed on the leaf edge inside this shelter. At maturity larvae construct a new leaf roll and pupate inside it.
Grape leaffolder has 3 generations a year. The adults are flying and laying eggs during about April to May, mid-June to mid-July, and August to early September. Egg to adult development time is about 7 weeks for the first generation and 4-1/2 weeks for the second generation. The third generation overwinters as prepupae (mature larvae) or pupae from which adults emerge in spring.
Grape leaffolder larvae can reduce the photosynthetic capacity of grapevines and potentially reduce fruit yield. If abundant during summer, leaf damage from the third generation and sometimes the second generation can be severe enough to cause extensive or complete premature defoliation. This leads to sunburned and soft berries as well as damage from direct berry feeding by the larvae.
Young larvae feed by scraping the leaf surface, then the injured tissue turns brown to whitish. Beginning in the third instar, larvae roll leaves to a width like that of a pencil. Larvae roll leaves only during the night, so this behavior is generally not observed. Larvae chew and feed within the leaf roll, usually only one older larva per roll. Each larva rolls at least two leaves before it pupates.
Parasitoid (parasitic) flies and wasps are the most important natural enemies and during some years appear to be the reason grape leaffolder populations are low. The larval parasitoid Habrobracon (=Bracon) cushmani is generally the most common natural enemy of grape leaffolder. The adult wasp is 1/8 inch long with a blackish head and thorax and a fat, pale orangish abdomen. It primarily parasitizes third through fifth instars. After stinging a leaffolder larva and injecting venom that paralyzes the caterpillar, the adult female H. cushmani lays one to several elongate whitish eggs on the body of leaffolder larvae. The pale, maggotlike wasp larvae then feed attached to the outside of the grape leaffolder. After completing their feeding, the wasp larvae pupate in silken, white cocoons near the shriveled body of the caterpillar they killed. This parasitism frequently reduces the abundance of the second and third generation to low levels so that leaffolder damage is not a pest problem. See Bracon cushmani for more discussion and photos of this wasp.
Other wasp parasitoids and at least two species of tachinid flies, Erynnia tortris and Nemorilla pyste, also parasitize leaffolder larvae. Small larvae are parasitized by E. tortris and older larvae are attacked by N. pyste. Predators including assassin bugs, larvae of brown lacewings and green lacewings, and spiders prey on and kill grape leaffolders.
If grape leaffolders have caused significant defoliation during previous seasons, insecticide applications may be warranted during the second or third generation, especially if monitoring plants reveals that parasitism is low. If vines are sprayed, do so when first and second instars are the predominate life stage before they become older larvae that roll leaves. When warranted thoroughly, spray infested foliage with Bacillus thuringiensis (Bt) or spinosad. If applying Bt make a second application 7 to 10 days after the first. Add horticultural oil to the mix to increase the persistence of spinosad and kill caterpillar eggs and other potential pests such as leafhoppers and mites.
Adapted from Grape Pest Management Third Edition from the University of California Agriculture and Natural Resources (also available as an eBook) and Pest Management Guidelines: Grape and Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, University of California Statewide Integrated Pest Management Program (UC IPM).
Rolled grape leaf with brown chewing damage of grape leaffolder.
Numerous leaves rolled by larvae of grape leaffolder.
Larva of grape leaffolder and its dark fecal pellets.
Pupa of grape leaffolder.
Adult of grape leaffolder.