Mechanism of Dispersal

The probability that suitable resources can be found and colonized depends on the mode of dispersal. Three general mechanisms can be identified: random, phoretic, and directed.

Random dispersal direction and path is typical of most small insects with little capacity to detect or orient toward environmental cues. Such insects are at the mercy of physical barriers or wind or water currents, and their direction and path of movement are determined by obstacles and patterns of air or water movement. For example, first instar nymphs of a Pemphigus aphid that lives on the roots of sea aster growing in salt marshes climb the sea asters and are set adrift on the rising tide. Sea breezes enhance movement, and successful nymphs are deposited at low tide on new mud banks where they seek new hosts (Kennedy 1975). Aquatic insect larvae often are carried downstream during floods. Hatching gypsy moth, Lymantria dispar, and other tussock moth larvae (Lymantriidae), scale insect crawlers, and spiders (as well as other arthropods) disperse by launching themselves into the airstream. Lymantriid and scale insect adults have poor (if any) flight capacity. The wind-aided dispersal by larval Lepidoptera and spiders is facilitated by extrusion of silk strands, a practice known as "ballooning." Western spruce budworm, Choristoneura occidentals, adults aggregate in mating swarms above the forest canopy and are carried by wind currents to new areas (Wellington 1980).

The distance traveled by wind- or water-dispersed insects depends on several factors, including flow rate and insect size or mass. Jung and Croft (2001) measured falling speeds, relative to morphology and activity, of several wind-dispersed mite species. Heavier mites fell more rapidly than did lighter mites, as expected. However, anesthetized mites fell more rapidly than did active mites, indicating mite ability to control buoyancy and landing to some extent.

The probability that at least some insects will arrive at suitable resources depends on the number of dispersing insects and the predictability of wind or water movement in the direction of new resources. Most individuals fail to colonize suitable sites, and many become part of the aerial or aquatic plankton that eventually "falls out" and becomes deposited in remote, unsuitable locations. For example, J. Edwards and Sugg (1990) documented fallout deposition of many aerially dispersed insect species on montane glaciers in western Washington.

Phoretic dispersal is a special case in which a flightless insect or other arthropod hitches a ride on another animal (Fig. 2.15). Phoresy is particularly common among wingless Hymenoptera and mites. For example, scelionid wasps ride on the backs of female grasshoppers, benefiting from both transport and the eventual opportunity to oviposit on the grasshopper's eggs. Wingless Mallophaga attach themselves to hippoboscid flies that parasitize the same bird hosts. Many species of mites attach themselves to dispersing adult insects that feed on the same dung or wood resources (Krantz and Mellott 1972, Stephen et al. 1993). The success of phoresy (as with wind- or water-aided dispersal) depends on the predictability of host dispersal. However, in the case of phoresy, success is enhanced by the association of both the hitchhiker and its mobile (and perhaps cue-directed) host with the same resource.

Mesostigmatid Mites

| Phoretic mesostigmatid mites on coxae of scarab beetle. Photo courtesy of A. Tishechkin.

| Phoretic mesostigmatid mites on coxae of scarab beetle. Photo courtesy of A. Tishechkin.

Directed dispersal provides the highest probability of successful colonization and is observed in larger, stronger fliers capable of orienting toward suitable resources (see Chapter 3). Many wood-boring insects, such as wood wasps (Siricidae) and beetles (especially Buprestidae), are attracted to sources of smoke, infrared radiation, or volatile tree chemicals emitted from burned or injured trees over distances of up to several kilometers (W. Evans 1966, Gara et al. 1984, R.G. Mitchell and Martin 1980, Raffa et al. 1993, Wickman 1964). Attraction to suitable hosts often is significantly enhanced by mixing with pheromones emitted by early colonists (see Chapters 3 and 4). Visual or acoustic cues also may aid orientation. For example, masking the silhouette of tree boles (with white paint) substantially reduced numbers of attracted southern pine beetle, Dendroctonus frontalis (Strom et al. 1999), Ips engraver beetles, and some bark beetle predators (Goyer et al. 2004).

Migration is an active mass movement of individuals that functions to displace entire populations. Migration always involves females, but not always males. Examples of migratory behavior in insects include locusts, monarch butterflies, Danaus plexippus, and ladybird beetles. Locust, Schistocerca gregaria and Locusta migratoria, migration depends, at least in part, on wind patterns. Locust swarms remain compact, not because of directed flight, but because randomly oriented locusts reaching the swarm edge reorient toward the body of the swarm. Swarms are displaced downwind into equatorial areas where converging air masses rise, leading to precipitation and vegetation growth favorable to the locusts (Matthews and Matthews 1978). In this way, migration displaces the swarm from an area of crowding and insufficient food to an area with more abundant food resources. Monarch butterfly and ladybird beetle migration occurs seasonally and displaces large numbers to and from overwintering sites, Mexico for monarch butterflies and sheltered sites for ladybird beetles.

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