Photo-oriented Aerial-dispersal Behavior of Tetranychus urticae (Acari: Tetranychidae) Enhances Escape from the Leaf Surface

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Abstract
In response to wind and light, twospotted spider mites, Tetranychus urticae Koch, manifest an aerial-dispersal behavior that facilitates their escape from the leaf-surface boundary layer and enables them to become airborne. This behavior, which is manifest only in the presence of wind, involves the mites orienting away from the light source and raising their forelegs and forebodies. Seventy percent of the adult females removed from a green host plant held on a dry test arena for 8 h, and exposed to a 1.5-m/s wind velocity, manifested this aerial-dispersal behavior. Mites do not adopt a dispersal posture in the absence of wind. As wind velocity increased the dispersal response increased until the response curve leveled at 1.5 m/s. All life stages, except adult males, manifested the aerial-dispersal behavior. Nymphal stages manifested the behavior less frequently than adult females. Larvae rarely adopted a dispersal posture. The low percentage of immature stages found in an aerial-dispersal posture can be partially explained by the sharp reduction in boundary-layer wind velocity at heights <0.5 mm above the leaf surface. On deteriorating corn plants in the field, a positive photokinetic response of “dispersal-phase” mites leads them to the upper periphery of the plant where aerial-dispersal behavior increases the probability of mites being carried aloft on updrafts.