Know Your Friends

Natural Enemies of Spotted Tentiform Leafminer on Apple, Part II

In the July issue I briefly described spotted tentiform leafminer (STLM), a pest of apples, and discussed one of its major parasitoids in the Midwest, the eulophid wasp Sympiesis marylandensis. In this issue I will describe a second major parasitoid, and discuss how these natural enemies can best be used in biological control of STLM.

Pholetesor ornigis is a small braconid wasp. It parasitizes younger (mostly sap-feeder) stages of the STLM larva. It is an endoparasitoid, meaning that its larva develops entirely inside the leafminer larva. However, when done feeding it does emerge from its host's body to spin a football-shaped, tightly woven silken cocoon, usually next to the host cadaver, within the mine. The cocoon is pale in color with a dark band around the middle. The STLM larva is able to continue to feed and grow somewhat before being killed by the parasite within. You can monitor for parasitism by Pholetesor by carefully opening mines, as described for Sympiesis (see MBCN Vol 3. No. 7). A leafminer killed by Pholetesor will appear as a dead and shrunken larva, with the parasite pupa nearby. Adult Pholetesor are attracted to the color yellow, and yellow sticky traps, such as those used for monitoring apple maggot, can be used to monitor the adult flight period of this important parasite.

The life cycle of Pholetesor is very closely synchronized with that of STLM, making it a very efficient parasite during all three STLM generations. In studies in Wisconsin, it appears to contribute less parasitism than Sympiesis during the second leafminer generation. However, we have found that Sympiesis will parasitize STLM 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.

Pholetesor adults do not host-feed as do Sympiesis. But they are highly fecund; under laboratory conditions of high leafminer density, Pholetesor females parasitized an average of 370 leafminer larvae. They require a carbohydrate source such as flower nectar or insect honeydew.

Conserving leafminer natural enemies. Successful biological control of spotted tentiform leafminer requires conservation of its natural enemies, especially the parasitoids. In laboratory studies we have shown that the adult wasp stage of Pholetesor is highly susceptible to the traditional organophosphate, carbamate, and synthetic pyrethroid insecticides commonly used in apple orchards. Sympiesis is also susceptible to these materials. Of the two parasitoids, Pholetesor has the greatest synchrony with its host, and its own generations are more discrete. Therefore, I believe that conservation biological control of Pholetesor is more manageable than that of Sympiesis. Pholetesor adults are flying during the young sap-feeder stages of STLM. First generation flight of Pholetesor occurs during the petal fall period, and for several days after. Although the petal fall insecticide has long been considered critical for managing early season apple pests, we know that 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 Wisconsin growers have found that it is not necessary to use the petal fall insecticide every year. By eliminating this spray when possible, they are not only saving money, but are conserving Pholetesor and improving on biological control. The other flight periods of Pholetesor can also be determined by using yellow sticky cards, as described above, or by monitoring when the STLM generation is in the sap feeder stage. If possible, avoid spraying during these flight periods also.

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. In these cases, we are seeing a reduction in the use of broad spectrum insecticides that are harmful to beneficials such as Sympiesis and 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 in Midwestern orchards.

I need to acknowledge the contributions of my former graduate students Nino Ridgway and Robert Dahl, who contributed significantly to the understanding of the natural enemies of spotted tentiform leafminer.

- Dan Mahr, University of Wisconsin - Madison


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