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HOST LEARNING BEHAVIOR OF APPLE MAGGOT FLIES 



Ronald J. Prokopy 

 Department of Entomology 



During the past decade, I have written numerous articles in FRUIT NOTES 

 about the natural history and economic importance of apple maggot flies. 

 The more we have studied the biology of this major apple pest, the more we 

 have become fascinated with its behavior. Here, I will describe briefly 

 the results of some of our recent experiments that demonstrate that apple 

 maggot flies, like humans, have an ability to learn. 



In Massachusetts and elsewhere, the apple maggot fly has two principal 

 hosts: hawthorn fruit (the native host) and apple (first introduced to the 

 United States by the Pilgrims about 1620 and first colonized by apple maggot 

 flies about 1850). In 1981, we discovered that apple maggot flies which had 

 been maintained in laboratory cages without fruit since emergence (and thus 

 were naive) showed a high level of acceptance of hawthorn fruit for egg- 

 laying and a moderate level of acceptance of apples. When we went to the 

 same apple trees from which the pupae of these flies had been collected and 

 then tested flies that had been laying eggs for about a week in the apples 

 there, we discovered that the flies still showed a moderate level of accep- 

 tance of apples but failed altogether to accept any hawthorn fruit for 

 egglaying. This finding caused us to wonder whether the flies might be 

 capable of learning with respect to host fruit acceptance. 



We then proceeded to "train" groups of flies in the lab by providing 

 them apples for several ovipositions and then testing their responses to 

 hawthorns. We also did the reverse: training on hawthorns and offering 

 apples. In all tests, most flies trained on one fruit type (termed the 

 familiar type) refused to oviposit in the other fruit type (termed the novel 

 type). The training proved reversible. That is, flies trained on apple and 

 rejecting hawthorn could, after three days without any fruit, be trained on 

 hawthorn and found to reject apple. To our surprise, we recently found that 

 flies that have had familiarity with one fruit type do not accept that fruit 

 type to any greater degree than naive flies that have never laid an egg. 

 Rather, the former flies simply reject a novel or unfamiliar fruit type to a 

 greater degree than do naive flies. It turns out that flies reject novel 

 chemical fruit stimuli (e.g., fruit having a rather different aroma) as well. 

 as novel physical fruit stimuli (e.g., fruit of a rather different size). 

 This appears to be the first proven case in any animal including humans that 

 the nature of learning can be of a type in which a familiar stimulus does 

 not become increasingly acceptable with experience but rather that a novel 

 stimulus becomes decreasingly acceptable. 



How does the ability of apple maggot flies to learn to reject novel 

 fruit stimuli affect the flies' foraging behavior for fruit in nature? 

 Presently, we are attempting to gain a partial answer to this question 

 through experiments we are conducting in a very large field cage that con- 

 tains mixtures of potted apple and hawthorn trees. First, we release onto 

 one of the trees a single female whose previous egglaying experience is 



