Arthropod Predators and Parasitoids

Predators

  1. Spiders can take a heavy toll of small flying insects or young larvae. However, little is known of impact of spiders on forest insects and whether they can limit pest numbers to low densities.
  2. Clerid beetles are important predators of bark beetles in both adult and larval stages. They seem to be particularly effective against bark beetles that feed in the outer bark of pines, such as the western pine beetle, Dendroctonus brevicomis, and southern pine beetle, D. frontalis (Coleoptera: Scolytidae). Some believe that cyclic outbreaks of the southern pine beetle are the result of interaction with clerid populations (Reeve 1997).
  3. Ladybird beetles (Coleoptera: Coccinellidae) are predators in both adult and larval stages on soft-bodied insects such as aphids. Ladybird beetles often migrate in large numbers into aphid infested areas and completely wipe out their prey. They have been shown to limit aphid populations to very low densities in orchard situations.
  4. Hover flies (Diptera: Syrphidae) are predators in the larval stages of soft-bodied insects such as aphids.
  5. Ground beetles (Coleoptera) are scavengers and predators of many arthropods on the forest floor. They have been implicated as important predators of gypsy moth larvae and pupae.
  6. Ants (Hymenoptera) can be important predators of caterpillars and have been implicated in the control of spruce budworm populations.

Arthropod predators are mainly generalists in that, although they may specialize on particular groups of insects (caterpillars, aphids, wood borers, etc.), they do not specialize on particular insect species. Thus, they tend to switch from one prey species to another as the densities of their prey changes. This, plus the high mobility of some predators like ladybird beetles, leads them to aggregate at places where their prey are abundant. Besides this, arthropod predators sometimes increase by reproductive numerical responses to the density of their prey. There is some evidence that arthropod predators can limit prey populations to very sparse densities.

EXAMPLES:

Cottony cushion scale (Homoptera) was introduced into California from Australia in the late 19th century and almost destroyed the newly formed citrus industry. The farmers were saved by the introduction of a ladybird beetle, the vedalia, which quickly reduced the scale populations to inconsequential levels.

Winter moth (Lepidoptera: Geometridae) was introduced into Canada from Europe. Although introduced insect parasitoids suppressed winter moth populations to low densities, it is thought that they are now regulated by native insect predators feeding on the pupae in the ground.

Parasitoids

  1. Parasitic wasps (Hymenoptera: Ichneumonoidea and Chalcidoidea) attack all groups of forest insects but are particularly important on those exposed on the foliage (defoliators) or which live in thin-walled mines in leaves or needles (needle and leaf miners). Caterpillars and sawflies seem to be especially vulnerable to parasitoids (Slide). Hymenopterous parasitoids usually use their long ovipositors to lay eggs within the body cavity of their hosts. Parasitoids that develop within their hosts are called endoparasitoids. On the other hand, ectoparasitoids lay their eggs on or near their hosts and the parasitic larvae feed externally (Slide). Ectoparasitoids usually paralyze their hosts prior to oviposition. Endoparasitoids do not normally paralyze their hosts but they may inject chemicals into their hosts that alter their development or castrate them.
  2. Parasitic flies (Diptera: Tachinidae and Sarcophagidae) attack many forest insects but seem to be particularly important on hairy caterpillars like gypsy moths and tent caterpillars. Lacking a penetrating ovipositor, parasitic flies lay their eggs on their host (Slide) or scatter them at random on the food of their hosts with the hope that they will be devoured. Some of these parasitoids, particularly those that scatter their eggs, can lay huge numbers of eggs (2000 or more). First instar larvae gain entrance to the body cavity of their host by boring through the exoskeleton or the stomach lining.

Insect parasitoids are often more specialized than other natural enemies, some even feeding on a single host species. Because of this, they often have strong reproductive numerical responses to the density of their hosts, and this can lead to time delays in the negative feedback and resultant population cycles (4th principle is evoked). However, a few parasitoids are generalists and these can have strong switching and aggregating responses to host densities.

EXAMPLES:

Gypsy moth (Lepidoptera: Lymantriidae) outbreaks in Europe tend to be cyclic. The collapse of the outbreak is usually associated with the build-up of specialist tachinid parasitoids. In the periods between outbreaks, however, generalist tachinids seem to limit gypsy moths to very low numbers by their aggregation at potential epicenters (Montgomery and Wallner 1988).

Black-headed budworm (Lepidoptera: Tortricidae) populations tend to cycle continuously but peak numbers rarely reach densities that do serious damage to the forest. Populations of fairly specialized hymenopterous parasitoids are closely associated with the rise and fall of the budworm cycles which suggests that the cycles are caused by interactions with parasitoid populations (Berryman 1986). Another species that is thought to cycle because of delayed negative feedback with populations of parasitoids is the Douglas-fir tussock moth (Lepidoptera: Lymantriidae) (see figure) (Mason 1988).

References

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© 1998 Alan A. Berryman