The horn fly is a widespread pest of cattle whose bites cause blood loss and annoyance, resulting in reduced weight gain. The immatures develop in dung, and the horn fly females are the first insects to lay eggs in freshly deposited dung. Predators that feed on the immatures in dung can reduce horn fly populations, but they must arrive quickly. The most important predators are rove beetles. Five species of Philonthus that were commonly associated with horn fly in cattle dung in northern Florida were evaluated as predators of this pest.
In laboratory feeding trials, there were significant differences in the number of horn fly immatures consumed by the five species and their preference for horn fly eggs or larvae.
The most voracious was P. longicornis, an immigrant to North America, first recorded here in 1840 (but not deliberately introduced) and now widespread in the United States. Both adults and larvae of this species consumed more eggs than maggots when given a choice.
In second place was another immigrant species, P. ventralis, which was reported from the U.S. in 1802 and now occurs throughout the southern U.S. and Mexico. This beetle devoured more larvae than eggs.
The remaining three species, believed to be native to North America, are smaller than the other two, perhaps accounting for their similar lower predation rates. These species ate equal numbers of eggs and maggots in choice tests.
Other lab tests were conducted with single adult beetles caged with 100 horn fly eggs in simulated cow pies. The containers were maintained until the flies completed their development. The reduction in emergence of horn flies varied from 33-80%. When two beetles were used per cage, the reduction was 70-96%. Although some of the beetles killed substantial numbers of horn fly immatures in these tests, in the field they may be feeding on other prey, or they may not be able to find the dung fast enough, so that their offspring may not have an impact on the horn flies.
Horn fly eggs hatch in less than a day, and the maggots complete their first two instars in about 1½ days. After this stage the maggots are too big for the rove beetles to feed on. However, it takes 2-3 days for the rove beetle eggs to hatch; by the time the larvae hatch, horn fly larvae are too far advanced for the beetle larvae to feed on. It appears they survive on other, later-arriving fly larvae. This research shows these five species of rove beetles can consume large numbers of immature horn flies, but their impact on populations in the field has not been determined.
Hu, G. Y. and J. H. Frank. 1997. Predation on the horn fly (Diptera: Muscidae) by five species of Philonthus (Coleoptera: Staphylinidae). Environ. Entomol. 26(6): 1240-1246.
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