Parasites also have little effect on boll weevil populations. Several native parasitic wasps do attack weevil larvae but rarely in numbers high enough to reduce populations. An exotic parasitic wasp, Catolaccus grandis , is effective, but it cannot overwinter at these latitudes. It may be useful in release programs, though. Disease organisms do kill some boll weevils, but again, normally not at levels that will control infestations.
Suicidal emergence can be a significant factor.
In some years, many boll weevils come out of diapause long before cotton squares are available. Because the boll weevil is a tropical insect that traveled north, it is not well-adapted to the climate of much of the United States. Because overwintering boll weevils are insulated by leaf litter, air temperatures usually have to drop lower than the values mentioned above. Snow or ice cover also insulate effectively and protect boll weevils from lethal temperatures. In most years in Missouri, less than 10 percent of the boll weevils that enter wintering habitat survive to spring.