STLM is of European origin, but has been in North America for many decades. Prior to the outbreaks of the 1970s and 80s, the insect was largely undetected in commercial orchards, primarily because frequent usage of synthetic organic insecticides for primary pests, such as plum curculio, codling moth, and apple maggot, were also controlling STLM. However, the insect developed widespread resistance to common orchard insecticides, primarily the organophosphates. However, these same insecticides were effectively eliminating the multitude of natural enemies of STLM. Therefore, neither chemical nor natural controls were effective, so STLM became a typical example of "secondary pest upset."
In wild apple trees and abandoned orchards, STLM populations are generally very low, indicating that natural controls are effective if not interfered with. There have been several studies on the natural enemies of STLM and its relatives. Predators are occasionally important, especially spiders and ground beetles that feed on overwintering pupae on the orchard floor. Some generalist predators such as lacewing larvae feed on STLM larvae in the leaf mines. But by far the most important natural enemies are parasitic wasps. In the Midwest there are several species of parasitoids that kill STLM larvae. Although the importance of individual species varies from location to location, I will discuss two that are common in the Midwest and important in Wisconsin and adjoining states.
The eulophid wasp Sympiesis marylandensis is a generalist species that attacks many types of leafminers and other small caterpillars. It is an ectoparasite, meaning that the Sympiesis larva is attached to the outside of the STLM larva. Sympiesis has a very short generation time, only about one third that of STLM. It prefers to attack older (tissue-feeder) leafminer larvae. In lab studies, each Sympiesis female can parasitize up to 500 STLM larvae. In addition, Sympiesis adults host feed, meaning that the adult female wasps are predators that kill and feed from the blood of additional STLM larvae. Again in laboratory studies, individual Sympiesis females killed an additional 150 STLM larvae by host feeding. Therefore, under optimum conditions and very high host populations, each Sympiesis female has the capability of killing as many as 650 STLM larvae by a combination of parasitism and host feeding. It is likely that this high level of leafminer mortality is rarely realized in nature.
Because of its rapid generation time, the life cycle of Sympiesis is not closely synchronized with that of STLM. In Wisconsin, Sympiesis heavily parasitizes the second generation of STLM, but is much less important during the first and third generations of the host. In a study of unsprayed situations in Wisconsin, Sympiesis parasitism of STLM was 35, 60, and 18% in first, second, and third generations, respectively. Parasitism rates are substantially lower in orchards frequently sprayed with broad spectrum insecticides.
Because adult Sympiesis are very tiny and resemble many other small parasitic wasps, it is difficult to monitor the adult stage. However, monitoring the larval stage as they parasitize the STLM larvae is relatively easy. A 10-power hand lens will be helpful, as will be fine-pointed forceps or a pin. Use the forceps or pin to carefully tear back the lower covering of a late-stage STLM mine. The leafminer larva should be relatively easy to see. Check to see if it appears to be active and has fed recently. If not, it may be parasitized. Look closely to see if there appears to be a tiny maggot-like larva attached to the outside of the leafminer larva. If so, this is probably Sympiesis or another ectoparasitic eulophid. If the STLM larva has been killed and there is a shiny tan or black pupa inside the mine, this is likely a eulopid pupa, from which the adult wasp will emerge. It may be necessary to dissect several mines before you see a parasite.
In the September issue, I will discuss a second important parasitoid of STLM, and discuss how these natural enemies can be conserved in an orchard for more effective biological control.
- Dan Mahr,University of Wisconsin - Madison
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