P. terebrans adult. R.M. Trimble ©Her Majesty in Right of Canada.

Pholetesor ornigis (Hymenoptera: Braconidae)

Pholetesor ornigis is a small braconid wasp native to North America.

Appearance

P. ornigis can be seen and monitored by carefully opening leafminer mines: use a fine-pointed forceps or pin and a 10-power hand lens. Carefully tear back the lower covering of a late-stage spotted tentiform leafminer mine. The leafminer larva should be relatively easy to see. Parasitized leafminer larvae remain active until the fifth instar or prepupal stage. A leafminer killed by Pholetesor ornigis will appear as a dead and shrunken larva with a football-shaped, tightly woven silken parasitoid cocoon nearby. The cocoon is pale in color with a dark band around the middle.

Habitat

Northeastern United States and eastern Canada.

Pests Attacked (Host Range)

Spotted tentiform leafminer (Phyllonorycter blancardella), apple blotch leafminer (P. crataegella), and other gracillarids.

Life Cycle

P. ornigis overwinters as a prepupa within a cocoon in the leaf mine of its host. This occurs concurrent with leaf fall in southeastern Canada. First generation flight of Pholetesor occurs during the petal fall period and for several days after. There are 3-4 generations per year and four to five distinct adult flight periods.

Adult females oviposit primarily in sap feeding, 3rd-5th instar larvae of leafminers. When 1st instars are parasitized, P. ornigis development time is about 5 days longer than on other instars. This suggests that a delay in parasitoid development takes place until the host reaches the second or later instar. The same delay is reported in other braconids, as well, and allows parasitoid oviposition in hosts that are too small to support development but are also too weak, immunologically and physically, to reject it.

Total development time from egg to adult is about 23 days at 23°C.

Relative Effectiveness

In the laboratory with high leafminer densities, P. ornigis females parasitized an average of 370 leafminer larvae, and they are highly fecund. In southern Canada (Guelph), the parasitism rate of spotted tentiform leafminer by P. ornigis was found to be 10%.

Because of the closely synchronized life cycle of P. ornigis with that of spotted tentiform leafminers, it is a very efficient parasitoid during all three leafminer generations. P. ornigis females live two times as long and produce three times as many offspring as its host, the spotted tentiform leafminer.

In Wisconsin studies, P. ornigis appeared to contribute less parasitism during the second leafminer generation than the other major parasitoid of spotted tentiform leafminer, Sympiesis marylandensis. However, it was found that Sympiesis will parasitize leafminer larvae that have already been internally parasitized by Pholetesor. Therefore, it is likely that Pholetesor would have an even greater impact on the second leafminer generation in the absence of Sympiesis.

Conservation

Successful biological control of spotted tentiform leafminer requires conservation of its natural enemies, especially the parasitoids. Phloetesor adults are flying during the young sap-feeder stages of the leafminer. Flight periods other than the one occurring during petal fall can be determined by using yellow sticky cards or by monitoring when the leafminer generation is in the sap feeder stage. Spraying should be avoided during all flight periods, if possible.

Although the petal fall insecticide has long been considered critical for managing early season apple pests, it is not always necessary. If leafrollers are a major concern, they can frequently be managed with conventional insecticides before bloom, or with microbial insecticides based on Bacillus thuringiensis before bloom or at petal fall.

Codling moth activity often doesn't peak until after petal fall; this can be monitored with pheromone traps. Plum curculio is the remaining major apple pest that can cause damage at and after petal fall. By careful visual monitoring, you can determine if it is necessary to control plum curculio.

Many growers in Wisconsin found that it was not necessary to use the petal fall insecticide every year. By eliminating this spray when possible, not only can money be saved, but Pholetesor can be conserved and biological control improved.

Apple growers continue to adopt Integrated Pest Management practices, including careful monitoring of pests and natural enemies and the use of alternative pest control practices. We are seeing a reduction in the use of broad spectrum insecticides that are harmful to beneficials such as Pholetesor and an overall decrease in secondary pest problems such as leafminers, aphids, and spider mites. With a little effort and an understanding of the natural enemies and their requirements, conservation biological control of spotted tentiform leafminer can be a reality.

Adults require carbohydrate food such as honeydew from aphids and coccids, tree sap ooze, or floral nectar.

For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, or the Volume II, No. 1Feature Article on conservation in the Midwest Biological Control News Online.

Pesticide Susceptibility

In laboratory studies, the adult wasp stage of Pholetesor was highly susceptible to the traditional organophosphate, carbamate, and synthetic pyrethroid insecticides commonly used in apple orchards.

Commercial Availability

Not available commercially.

Taken from:

Mahr, Dan (1996) Know Your Friends: Natural Enemies of Spotted Tentiform Leafminer on Apple, Part I & Part II, Midwest Biological Control News OnLine. Vol.III, Nos.7 & 9.

Additional References

Laing, J.E. and J.M. Heraty (1987) Overwintering of Phyllonorycter blancardella (Lepidoptera: Gracillariidae) and its parasites, Pholotesor ornigis and Pholotesor pedias (Hymenoptera: Braconidae) in Southwestern Ontario. Env. Ent. 16: 1157-1162.

Ridgway, N.M. and D.L. Mahr (1990) Reproduction, development, and longevity of Pholetesor ornigis (Hymenoptera: Braconidae), a parasitoid of spotted tentiform leafminer (Lepidoptera: Gracillariidae), in the laboratory. Ann. Ent. Soc. Am. 83: 790-794.

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