The ecological significance of the big-headed ant in mealybug wilt disease of pineapple

Abstract

When ants are managed in pineapple fields, the mealybug population rapidly declines. Why ants are so vital to mealybug survival in pineapple is the subject of this dissertation. Historically, three explanations were given for the importance of ants to mealybugs in pineapple. One hypothesis was that ants protect mealybugs from natural enemies. Another hypothesis was that ants perform an essential task for the mealybugs by consuming the mealybug honeydew, suggesting that mealybugs would drown in their own waste material were it not for the sanitation activities of the ants. According to another hypothesis ants are the primary or sole means of mealybug dispersal in pineapple. In other words, the ants actually bring the mealybugs into the field and distribute them from one pineapple plant to another. These explanations are frequently presented in the literature as facts, not hypotheses; although little supporting evidence has been published on the topic. Through a series of field and laboratory experiments I have provided evidence that: 1. Big-headed ants (BHA), Pheidole megacephala (F.), are essential for the survival of gray pineapple mealybugs (GPM), Dysmicoccus neobrevipes Beardsley, in pineapple fields. 2. BRA suppress predator populations. 3. BRA do not aid GPM through sanitation. 4. GPM are not dispersed by BRA. 5. First instar GPM are dispersed by the wind. In short, the data presented in this dissertation do not support the dispersal or sanitation hypothesis, but do support the hypothesis that BRA protect GPM from natural enemies. Thus, when ants are eradicated from a pineapple field, the resulting increase in predation (and possibly parasitization) of mealybugs brings wilt disease under control. The pineapple industry has unintentionally been relying on biological control for wilt management. Once the most efficient species of natural enemies are identified it should be possible to artificially augment their populations and reduce the need for insecticides. The discovery that GPM are dispersed by the wind will be useful for monitoring populations with sticky traps. Wind barriers and traps may be also be used to prevent, or decrease the severity of GPM infestations.