The geometry of search movements of insects in plant canopies

The aim of this study was to provide a framework for describingand understanding the geometry of movement of insects foragingwithin complex plant canopies where the insect is exposed tovarying stimuli. We used the apple maggot fly, Rhagoletis pomonella(Walsh) (Diptera: Tephritidae), foraging in apple trees devoidof fruit as our model system. The framework provides the nullhypothesis required for inferring the influence of externalstimuli, such as fruit color and odor, on the paths of foragingflies. We mapped trees into cells, released preconditioned fliesin caged trees, and recorded their behavior and location. Fliesmoved mainly to the nearest neighbor cells, but displacementwithin a wide range of distances was observed. The model closestto observations is a random walk with a position-dependent biasin the vertical component of movement. Four other models werebuilt, spanning a range of simplifications in the rules determiningthe vertical component of movement We used the concept of avoidanceof self-crossing in a searching path for defining efficiencyof movement Flies were quite efficient at visiting almost asmany different cells as possible. Comparisons of assumptionsand predictions of the five models revealed that this efficiencyis due to the small number of steps, the location of the startingcell, and a strong tendency to move upward in its vicinity.We discuss the selection pressures on movement rules: pressurefrom predators may explain the short hops, while the sensoryecology of fruit finding and the avoidance of sites alreadyvisited by other flies or by the same fly may explain the position-dependentupward bias. Strong similarities between the rules for the verticalcomponent of movement of one simplified model and the observationslead us to believe that canopy architecture influences insectmovement not only by defining the set of locations that theinsect can visit using predefined rules for movement but alsoby defining the rules of movement