Although quantifying the impact of a new species is an unsolved challenge, it is safe to say that most introduced species do not generate major impacts. However, some are enormously damaging, whereas others are highly beneficial. The variety of impacts is staggering.

Many introduced insects prey on natives. This activity can be useful, as in biocontrol introductions such as that of the Australian vedalia (Rodolia cardinalis) to attack the cottony cushion scale (Iceryapurchasi). Predation can also be extremely damaging; on Christmas Island, the introduced yellow crazy ant Anoplolepis gracilipes has locally devastated populations of the dominant red crab Gecarcoidea natalis. Because the crab controls seedling recruitment and litter breakdown, the entire community is affected. Parasites can also be beneficial or harmful. The wasp A. melinus has effectively controlled California red scale in parts of California. Alternatively, sheep blowfly (Lucilia cuprina), introduced to Australia from Africa, caused staggering losses. Herbivory can similarly be beneficial or detrimental. The South American flea beetle, Agasicles hygrophila, effectively controls alligatorweed in Florida. However, phytophagous insect crop pests impose staggering costs. The alfalfa weevil (Hypera postica) caused $500 million in losses in the United States in 1990 alone.

Resource competition is subtler than predation, parasitism, and herbivory, but many introduced insects outcompete natives. The European sevenspotted lady beetle (Coccinella septempunctata), introduced to the United States for control of the Russian wheat aphid (Diuraphis noxia), has locally outcompeted several native lady beetles. European honey bees outcompete the native bee Osmia pumila for pollen in New York State. Introduced insects can even outcompete vertebrates. The introduced wasps Vespula germanica and V. vulgaris in New Zealand outcompete an endemic parrot for honeydew produced by a scale insect (Ultracoelostoma assimile) and have locally lowered parrot populations.

Introduced insects can transmit or be reservoirs of diseases of humans, domestic animals, cultivated plants, and wild animals and plants. The spread of yellow fever and dengue as the vector mosquito A. aegypti dispersed throughout the tropics, and of malaria to Brazil with the introduction of its vector Anopheles gambiae, are notable examples of human disease organisms transmitted by introduced insects. Cat fleas (Ctenocephalides felis) and dog fleas (C. canis), introduced to Australia with their hosts, are intermediate hosts for the dog tapeworm (Dipylidium caninum). The southern house mosquito, Culex quinquefasciatus, was accidentally introduced to the Hawaiian islands in 1826. Subsequently it transmitted the disease organism causing avian malaria, introduced with resistant Eurasian songbirds, to susceptible native birds and helped to exclude them from low elevations. The mosquito A. japonicus transmits West Nile virus to both birds and humans in the northeastern United States. Animal disease vectors may be useful biocontrol introductions. For example, the rabbit flea (Spilopsyllus cuniculi), the main vector of the organism causing myxomatosis in Europe, has been introduced (so far unsuccessfully) in Australia to attempt to boost disease transmission. In the lab, it also transmits calicivirus.

Among plants, Dutch elm disease was dispersed to and through North America with the European elm bark beetle. Beechbark disease spread throughout northeastern North America after the causal fungus was introduced from Europe around 1890 with its vector, the beech scale (Cryptococcus fagisuga). The wine industry in California is threatened by the recent introduction of the glassy-winged sharpshooter (Homalodisca coagulata) from the southeastern United States. The sharpshooter spreads an incurable bacterial disease of grape vines, a malady long present but rarely a problem until this vector arrived.

Introduced species often exacerbate one another's impacts, a process termed "invasional meltdown." Sometimes this interaction occurs when coevolved mutualists invade a region separately. In south Florida, over 60 species of ornamental figs were not invasive because their obligatory pollinating wasps were absent. Since the 1970s, three such wasps (Parapristina spp.) have arrived, and three formerly innocuous fig species have begun spreading in natural areas. However, invasional meltdown need not involve coevolved species. In California citrus orchards, the Argentine ant (Linepithema humile) tends and protects the Asian California red scale, thereby exacer bating its impact. Similarly, in Hawaii, the African bigheaded ant (Pheidole megacephala) protects the tropical American gray pineapple mealybug (Dysmicoccus neobrevipes) from coccinellids introduced for biological control.

Relative to the numbers of species introduced, insects rarely cause enormous ecological (as opposed to economic) damage. Introduced species whose impacts ripple through entire communities usually do so by changing the habitat dramatically, and such change agents are mostly plants (which become structural dominants or modify fire regimes) or pathogens, which attack dominant plants. Occasionally mammals can generate an enormous ecosystem impact by trampling or grazing. A recent list of the world's 100 worst introduced species included 15 insects, but at most one would qualify as having a huge ecosystem-wide impact: the yellow crazy (or long-legged) ant, which removes the keystone red crab species on Christmas Island. Of the 15 insects, five are ants. In addition to the yellow crazy ant, the Argentine ant, the bigheaded ant, the little fire ant (Wasmannia auropunctata), and the red imported fire ant (Solenopsis invicta) all affect other ants greatly, and sometimes other insects, but to date none has had the dramatic impact of certain plants and mammals. Some species among the 15 transmit organisms that cause human diseases (Aedes albopicta and Anopheles quadrimaculatus) and others are agricultural pests, such as the sweetpotato whitefly (Bemisia tabaci) and several of the ants. The Formosan termite (Coptotermes formosanus shiraki) has caused enormous damage to housing in New Orleans. With respect to ecosystem-wide damage to natural areas, however, the only members of the list that might qualify, aside from the yellow crazy ant, are the gypsy moth in North America, by virtue of its devastating impact on dominant trees, and the Argentine ant, because it has greatly lowered densities of native seed-carrying ants in the fynbos of South Africa. Other insects may have ecosystemic impacts by removing dominant plants. The beetle transmitting the Dutch elm disease organism has already been noted. The Asian balsam woolly adelgid (Adelges piceae) has eliminated the dominant Fraser fir throughout the high southern Appalachians, whereas the hemlock woolly adelgid (A. tsugae) has locally killed large fractions of hemlocks in much of eastern North America.

Impacts of an introduced species can occur after a substantial lag period during which the species can seem to be innocuous. For example, the beetle Chrysolina quadrigemina, introduced to Australia in 1939 to control St. John's wort, seemed to die out but resurfaced and spread in 1942. Such lags are mysterious; they are often attributed to favorable changes in the environment or to evolution of the invader, but evidence for these phenomena is generally lacking. Some introduced insects achieve great numbers and appear to have a major impact, but the population suddenly crashes, again for reasons poorly understood. The European browntail moth (Euproctis chrysorrhoea) followed this trajectory in New England and eastern Canada.

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