“Using pyrethroids and carbamates can lead to outbreaks of wooly apple aphid, San Jose scale, and spider mites.”
Vedalia beetle and the cottony cushion scale: http://www.biocontrol.entomology.cornell.edu/predators/Rodolia.php
Lynx and hares:
Up until the mid-19th century, the Arctic hare (Lepus arcticus bangsii) was widespread though not extraordinarily abundant on the Island of Newfoundland (in Canada). In the later 19th century, the Snowshoe hare (Lepus americanus) was introduced to Newfoundland as a game animal; Snowshoe hare have a higher rate of reproduction than Arctic hare, and they have a more “pugnacious disposition” than the more “timid” Arctic hare.
Shortly thereafter, lynx (Lynx canadensis), which eat hare and had previously been somewhat rare on the island, saw a drastic increase in population.
Prior to the introduction of the Snowshoe hare, Arctic hare were found in a variety of habitats, including wooded sections of the island. After the introduction of the Snowshoe hare, the Arctic hare was no longer found in those wooded habitats; competition was the presumed cause of this.
It was discovered later, however, that Arctic hare were also no longer in habitats without trees, and also without Snowshoe hare. They (the Arctic hare) did, however, persist in boulder fields, where there was cover for them to escape predators in the crevices between exposed rocks; they did not persist in areas nearby to wooded sites.