How Do Apple Maggot Flies Search for 

 Food on Leaf Surfaces? 



Jorge Hendrichs, Joshua Prokopy, and Ronald Prokopy 



Department of Entomology, University of Massachusetts 



Brian Fletcher 



CSIRO, Canberra, Australia 



Our field observations (first article in this series) 

 showed that apple maggot flies spend considerable 

 time foraging for diffuse food sources on leaf surfaces of 

 host and non-host trees. We concluded that this "graz- 

 ing" type of feeding may be typical of flies when concen- 

 trated food sources that are presumably detectable by 

 odor from a distance (see second article in this series) 

 are scarce or lacking. 



How do flies find diffuse food sources on leaf sur- 

 faces that are not readily detectable by odor? Field 

 observations indicated that the searching behavior for 

 food on foliage occurs during stereotyped hopping from 

 leaf to leaf: a fly walks diagonally across the top of a leaf 

 in an approximate straight line, jumps to the under 

 side of a leaf above it, moves to the upper side of that leaf 

 and walks again across the leaf surface, before hopping 

 once more to another leaf above it. Occasionally, the fly 

 lowers its mouthparts (proboscis) on an upper leaf 

 surface, probably in response to a food substrate de- 

 tected initially with the tarsi (terminal extensions of 

 the legs). Depending apparently on food quantity and 

 quality as perceived during touching of the proboscis, 

 the fly is either (a) arrested and feeds at length, (b) 

 stimulated to probe and feed periodically while con- 

 tinuing to walk slowly, or (c) not stimulated to probe or 

 feed, but rather it continues walking without altering 

 its approximately straight-line course across the leaf 

 surface. In the first 2 cases, after feeding or while 

 feeding-walking, the fly switches from a rather unidi- 

 rectional walk to a convoluted searching pattern of 

 walking and turning. If, during this localized search, 

 the fly detects more food it again undertakes area-con- 

 centrated search on the leaf. The following experi- 

 ments addressed the effect of food quality and quantity 

 on fly foraging and feeding behavior on leaf surfaces. 



Methods Used 



Tests were carried out in field cages in Amherst, 

 MA, in the summer of 1988. Apple twigs with ten leaves 

 each were used for the foraging studies. Before mount- 



ing twigs on their poles about 1 m tall, twigs were 

 washed thoroughly with detergent and handled with 

 gloves. This procedure was necessary to remove any 

 trace of existing food and to insure that flies had access 

 only to food deposited (i.e., yeast or sucrose) as a droplet 

 on the upper surface of the leaf upon which they were 

 released. For each trial a "new release leaf ' was pinned 

 to the twig and a droplet of a food solution placed on it. 

 Leaves with dry food were prepared in advance to allow 

 the droplet to dry. The position of the release leaf was 

 always between the second and third lowermost leaves 

 of the twig. 



We worked with apple maggot females of wild 

 origin (5 to 7 days old), thatwere maintained in labora- 

 tory cages and were deprived of protein from emer- 

 gence. They had access only to sucrose and water. In 

 trials where we tested fly reaction to sucrose, flies were 

 deprived of sucrose 12 to 18 hours before testing. For 

 each trial, a single fly was placed on the release leaf near 

 the food droplet. The fly was transferred to the release 

 leaf on a piece of filter paper dipped into a 0. 1% solution 

 of the food resource on the release leaf. Observations 

 commenced the moment the fly found the food drop, 

 and lasted until the fly left the twig or if it did not leave, 

 for a maximum observation period of 30 minutes. Each 

 treatment was replicated 30 times. 



Fly Foraging, Food Concentration, 

 and Acceptance Thresholds 



A series of foraging tests was conducted, both with 

 yeast hydrolysate and with sucrose, in which we varied 

 either the concentration of food per droplet, the 

 amount of food solutes, or the total droplet volume. In 

 the first test, we report those findings that illustrate 

 the effect of food concentration and quantity on fly for- 

 aging behavior. Flies were presented with a constant 

 droplet size of decreasing amount of food and hence of 

 decreasing nutritional value. 



As shown in Table 1, for both yeast and sucrose, fly 

 foraging time on apple twigs as well as the number of 



Fruit Notes, Summer, 1990 



