Effects of air pollution on natural enemies of the leaf beetle Melasoma lapponica

1. Air pollution might have differential effects on herbivores and their na tural enemies, thus changing population dynamics. Therefore, from 1993 to 1 998 we studied mortality caused by parasitoids and predators to the willow- feeding leaf beetle Melasoma lapponica in the impact zone of the Severonike l nickel-copper smelter (Kola Peninsula, north-western Russia). 2. Densities of M. lapponica were very low at clean forest sites (below fiv e beetles per 10-min count) but higher in polluted areas (10-340 beetles pe r count). There were, however, variations between study years. 3. Egg predation, mainly by syrphid larvae and zoophagous bugs, was higher at relatively clean sites (55.3%) than at polluted sites (22.2%). Similarly , predation on larvae by zoophagous bugs and wood ants was higher at clean sites (68.4%) than at polluted sites (32.9%). 4. In contrast to predation, mortality caused by the parasitoid flies Megas elia opacicornis (Phoridae) and Cleonice nitidiuscula (Tachinidae) was lowe r at clean sites (12.3%) than at polluted sites (35.3%). Total parasitism l evels increased significantly with pollution load. 5. Total mortality caused by natural enemies was higher at clean sites (93. 7%) than at polluted sites (79.4%) due to higher predation rates, which may partly explain increased leaf beetle density within the smelter's impact z one. The effects of predators in clean forests were confirmed by the extinc tion of adults of M. lapponica introduced to one of the forest sites. 6. Although some individual sources of mortality appeared to be density dep endent (direct or inverse), the joint effect of all natural enemies was not . 7. Our data show that a decrease in predation can contribute to increased l eaf beetle density at polluted sites. However, the overall effects of natur al enemies in this case were not sufficient to account for all density vari ations between sites. To our knowledge this is the first study to assess ho w pollution affects the partitioning of mortality in herbivorous insects be tween predators and parasitoids.