Parasitism affects the host (prey) population in ways that are similar to predation and can be described using predation models. However, whereas predation involves multiple prey killed and consumed during a predator's lifetime, parasites feed on living prey. Parasitoidism is unique to insects, especially flies and wasps, and combines attributes of both predation and parasitism. The adult par-asitoid usually deposits eggs or larvae on, in, or near multiple hosts, and the larvae subsequently feed on their living host and eventually kill it (Fig. 8.5). Parasites must be adapted to long periods of exposure to the defenses of a living host (see Chapter 3). Therefore, parasitic interactions tend to be relatively specific associations between co-evolved parasites and their particular host species and may involve modification of host morphology, physiology, or behavior to benefit parasite development or transmission. Because of this specificity, parasites and par-asitoids tend to be more effective than predators in responding to and controlling population irruptions of their hosts and, therefore, have been primary agents in biological control programs (Hochberg 1989). In fact, release from parasites may largely explain the rapid spread of invasive plants and animals (Torchin and Mitchell 2004).

Parasitic interactions can be quite diverse and complex. Parasites can be assigned to several categories (van den Bosch et al. 1982). Ectoparasites feed externally, by inserting mouthparts into the host (e.g., lice, fleas, mosquitoes, ticks), and endoparasites feed internally, within the host's body (e.g., bacteria,

Parasitism: a parasitoid (sarcophagid fly) ovipositing on a host caterpillar at Nanjinshan Long Term Ecological Research Site, Taiwan.

Parasitism: a parasitoid (sarcophagid fly) ovipositing on a host caterpillar at Nanjinshan Long Term Ecological Research Site, Taiwan.

nematodes, bot flies, and wasps). A primary parasite develops on or in a nonpar-asitic host, whereas a hyperparasite develops on or in another parasite. Some parasitic species parasitize other members of the same species (autoparasitism or adelphoparasitism), as is the case for the hymenopteran, Coccophagus scutellaris. The female of this species parasitizes scale insects and the male is an obligate hyperparasite of the female (van den Bosch et al. 1982). Superparasitism refers to more individuals of a parasitoid species occurring in the host than can develop to maturity. Multiple parasitism occurs when more than one parasitoid species is present in the host simultaneously. In most cases of superparasitism and multiple parasitism, one dominant individual competitively suppresses the others and develops to maturity. In a special case of multiple parasitism, some parasites preferentially attack hosts parasitized by other species (cleptoparasitism). The clep-toparasite is not a hyperparasite but usually kills and consumes the original parasite as well as the host.

Insects are parasitized by a number of organisms, including viruses, bacteria, fungi, protozoa, nematodes, flatworms, mites, and other insects (Hajek and St. Leger 1994, Tanada and Kaya 1993, Tzean et al. 1997). Some parasites cause sufficient mortality that they have been exploited as agents of biological control (van den Bosch et al. 1982). Epidemics of parasites often are responsible for termination of host outbreaks (Hajek and St. Leger 1994, Hochberg 1989). Parasites also have complex sublethal effects that make their hosts more vulnerable to other mortality factors. For example, Bradley and Altizer (2005) reported that monarch butterflies, Danaus plexippus, parasitized by the protozoan, Ophryocystis elek-troscirrha, lost 50% more body mass per kilometer flown and exhibited 10% slower flight velocity, 14% shorter flight duration, and 19% shorter flight distance, compared to uninfected butterflies. These data, together with much higher infection rates among nonmigrating monarchs (Altizer et al. 2000), suggest that longdistance migration of this species may eliminate infected individuals and reduce rates of parasitism.

Some parasites alter the physiology or behavior of their hosts in ways that enhance parasite development or transmission. For example, parasitic nematodes often destroy the host's genital organs, sterilizing the host (Tanada and Kaya 1993). Parasitized insects frequently show prolonged larval development (Tanada and Kaya 1993). Flies, grasshoppers, ants, and other insects infected with fungal parasites often climb to high places where they cling following death, facilitating transmission of wind-blown spores (Tanada and Kaya 1993) (Fig. 8.6).

Insects have evolved various defenses against parasites (see Chapter 3). Ants stop foraging and retreat to nests when parasitoid phorid flies appear (Feener 1981, Mottern et al. 2004, Orr et al. 2003). Hard integument, hairs and spines, defensive flailing, and antibiotics secreted by metapleural glands prevent attachment or penetration by some parasites (e.g., Hajek and St. Leger 1994, Peakall et al. 1987). Ingested or synthesized antibiotics or gut modifications prevent penetration by some ingested parasites (Tallamy et al. 1998, Tanada and Kaya 1993). Endocytosis is the infolding of the plasma membrane by a phagocyte engulfing and removing viruses, bacteria, or fungi from the hemocoel. When the foreign

Parasitism: stinkbug infected and killed by a parasitic fungus in Louisiana,

United States.

Parasitism: stinkbug infected and killed by a parasitic fungus in Louisiana,

United States.

particle is too large to be engulfed by phagocytes, aggregation and adhesion of hemocytes can form a dense covering around the particle, encapsulating and destroying the parasite (Tanada and Kaya 1993). However, some parasitic wasps inoculate the host with a virus that inhibits the encapsulation of their eggs or larvae (Edson et al. 1981, Godfray 1994).

Many insects and other arthropods function in the capacity of parasites. Although parasitism generally is associated with animal hosts, most insect herbivores can be viewed as parasites of living plants (Fig. 8.7). Some herbivores, such as sap-suckers, leaf miners, and gall-formers, are analogous to blood-feeding or internal parasites of animals. Virtually all terrestrial arthropods and vertebrates are parasitized by insect or mite species. The majority of insect parasites of animals are wasps, flies, fleas, and lice, but some beetle species also are parasites (e.g., Price 1997). Parasitic wasps are a highly diverse group that differentially parasitize the eggs, juveniles, pupae, or adults of various arthropods. Spider wasps (e.g., tarantula hawks) provision burrows with paralyzed spiders for their parasitic larvae. Flies parasitize a wider variety of hosts. Mosquitoes and other biting flies are important blood-sucking ectoparasites of vertebrates. Oestrid and tachinid flies are important endoparasites of vertebrates and insects. Fleas and lice are ectoparasites of vertebrates. Mites, chiggers, and ticks parasitize a wide variety of hosts.

Insect parasites can significantly reduce growth, survival, reproduction, and movement of their hosts (J. Day et al. 2000, Steelman 1976). Biting flies can reduce

Puerto Rico.

growth and survival of wildlife species through irritation, blood loss, or both (J. Day et al. 2000). DeRouen et al. (2003) reported that horn fly control resulted in significantly reduced numbers of horn flies on treated cattle (14% of horn fly numbers on untreated cattle) and a significant 14% increase in cattle weight but no effect on reproductive rate. However, Sanson et al. (2003) found that control of horn flies, Haematobia irritans, resulted in significantly reduced horn fly abundance but was associated with significantly increased weight of cattle in only 1 of 3 years of study. Other studies of the effects of arthropod parasites of livestock also have shown that direct effects of parasites on host productivity may be more variable. Amoo et al. (1993) reported that a range of acaricide treatments to reduce tick, primarily Amblyomma gemma, parasitism of cattle had little effect on growth, reproduction, or milk production in the most and least intensive treatments. Although tick abundance in the most intensive treatment was only 14% of the abundance in the least intensive treatment, the lowest weight gain was observed in the most intensive treatment group, suggesting that reduced exposure to ticks may have prevented acquisition of resistance to tickborne diseases.

Many arthropod parasites also vector animal pathogens, including agents of malaria (Plasmodium malariae), bubonic plague (Yersinia pestis), and encephalitis (arboviruses) (Edman 2000). Some of these diseases cause substantial mortality in human, livestock, and wildlife populations, especially when contracted by nonadapted hosts (Amoo et al. 1993, Marra et al. 2004, Stapp et al. 2004, Steelman 1976, Zhou et al. 2002). Human population dynamics, including invasive military campaigns, have been substantially shaped by insect-vectored diseases (Diamond 1999, R. Peterson 1995).

Generally, parasitoids attack only other arthropods, but a sarcophagid fly, Anolisomyia rufianalis, is a parasitoid of Anolis lizards in Puerto Rico. Dial and Roughgarden (1996) found a slightly higher rate of parasitism of Anolis ever-manni, compared to Anolis stratulus. They suggested that this difference in parasitism may be the result of black spots on the lateral abdomen of A. stratulus that resemble the small holes made by emerging parasites. Host-seeking flies may tend to avoid lizards showing signs of prior parasitism.

Nicholson and Bailey (1935) proposed a model of parasitoid-prey interactions that assumed that prey are dispersed regularly in a homogeneous environment, that parasitoids search randomly within a constant area of discovery, and that the ease of prey discovery and parasitoid oviposition do not vary with prey density. The number of prey in the next generation (us) was calculated as follows:

where p = parasitoid population density, a = area of discovery, and ui = host density in the current generation.

Hassell and Varley (1969) showed that the area of discovery (a) is not constant for real parasitoids. Rather, log a is linearly related to parasitoid density (p) as follows:

where Q is a quest constant and m is a mutual interference constant. Hassell and Varley (1969) modified the Nicholson-Bailey model to incorporate density limitation (Q/pm). By substitution, pa = loge (ui/u, ) = Qp1-m (8.7)

As m approaches Q, model predictions approach those of the Nicholson-Bailey model.

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