Predaceous Ants in Biological Control of Insect Pests of North American Forests
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Predaceous Ants in Biological Control of Insect Pests of North American Forests Lorna C. Youngs nts are common and usually ig- large and aggressive, colony populations budworm survival. The conclusions of nored members of forest com- frequently reach the hundreds of thou- Campbell et al. (1983) on the importance A munities, yet their activities af- sands, a wide variety of prey is taken, and of ant predation are contrary to those of fect forests in many ways. Their nesting colony activity can extend up to 200 days Markin (1979), who examined the in soil aids soil aeration, movement of soil per year in favorable climates (Finnegan number of budworm larvae and pupae particles to the surface, accumulation and 1974). A comprehensive review of the and the amount of defoliation on trees breakdown of organic matter, and nu- usefulness of ants in European forest pro- where ants were excluded and on trees trient recycling (Petal 1978). Ants influ- tection can be found in Adlung (1966). routinely foraged by ants. Except at very ence vegetation patterns by dispersing Until recently, predaceous ants have re- high ant densities, he found no significant Downloaded from seed while foraging, and by creating fa- ceived little attention from North Amer- differences between trees, in budworm vorable or unfavorable microhabitats for ican forest entomologists. A few investi- numbers or defoliation. Because preda- plant growth near ant mounds. Ant for- gators have encountered species of both tion by birds was not prevented in his cx- aging, whether for plant or animal mate- Fonnica and Camponotus feeding on the periments, the compensatory mortality rial, can affect both the population dy- larvae or pupae of defoliating caterpillars from bird predation demonstrated by namics and the distribution of species and sawflies, but have not quantified the Campbell et al. (1983) may have masked http://besa.oxfordjournals.org/ commonly included in the ants' diet. Al- effects of ant predation on forest pests differences in budworm mortality be- though ants feed on a wide range of or- (Silver 1960, llnitzky and Mcleod 1965, tween ant-free and ant-foraged trees. ganic material, my discussion is confined Bradley and Hinks 1968, Allen et al. 1970, to those ant species that habitually prey Jennings 1971, Otvos 1977). History of the Use of Ants in on other insects-species that can play Research in the western United States Biological Control of Forest Pests an important role in the biological control on the population dynamics of the European efforts to use ants in biolog- of forest insect pests. western spruce budworm, Choristoneura ical control of forest pests have concen- The development of sociality has en- occidentalis Freeman, has revealed an im- trated on augmenting native species of the by guest on February 5, 2016 abled ants to interact with their environ- portant role for predaceous ants (Camp- F. rufa group. Considerable research has ment in ways unavailable to solitary in- bell and Torgersen 1982, Campbell et al. been devoted to methods of quantifying sects. Group foraging and food sharing 1983). Using exclusion techniques, the the minimum number of colonies per supply ant colonies with sufficient re- investigators measured the predation hectare necessary to promote forest pro· sources to support large populations, rates of insectivorous birds, predaceous tection and on methods of mass rearing, which in turn, through division of labor, ants (both Camponotus spp. and Fonnica transplantation, and protection of key ant permit the simultaneous performance of spp.), and both groups on budworm species (Gosswald 1951). Using the cri- all the tasks a solitary insect usually per- larvae and pupae across a range of bud· teria of large aggressive workers, large forms sequentially. Ants can modif)' the worm densities. At low budworm densi- colony size, colonial nest formation, and microhabitat at the nest site, thus reo ties, budworm numbers were 10- to 15- multiple queens per colony, researchers ducing losses of the vulnerable brood fold greater on trees protected from ant have selected F. polyctena (Foerster) as stage to environmental stress. The Ion· and bird predation, compared with trees the primary candidate for augmentation gevity of ant queens and the presence of accessible to both predator groups. At (Cotti 1963). Another species of the F. supplementary reproductives, in many high budworm densities, protected trees rufa group, F. lugubris Zetterstedt, has species, confers a kind of immortality on had twice as many budworms as accessible been augmented to a lesser extent, pri- ant colonies. Ant colonies are long-term trees. Either birds or ants caused similar marily in alpine forests where F. polyc- residents of forest stands. budworm mortality in the absence of the tena is less common and possibly less suc- other group, and predation by either or cessfui (Pavan 1960). both groups was the major source of mor- Research on the Role of Ant In North America, researchers have tality to the budworm in the late larval Predation in Forest Protection closely followed European techniques in and pupal stages (Campbell et al. 1983). their attempts to use ants in biological Research on ant predation on forest The work of Campbell and Torgersen pests has been conducted in Europe for quantifies the effect of ant predation on well over 100 years (Cotti 1963). This budworm survival and thus verifies the I O.G. Bain.Unpublishedreport."Twowood research has concentrated almost exclu- earlier investigation of Bain.I He found a ant speciesattackingwesternsprucebudworm sively on the mound-building species of high rate of budworm larval predation by Choristoneura occidentalis (Lepidoptera:Tor· the Fonnica tufa group, commonly called both Formica obscuripes Forel and F. tricidae) in western Montana."Undated.On red wood ants. This group of species has criniventris Wheeler, although he did not file,PacificNorthwestForestand RangeExper- been singled out because the workers are quantify the effect of ant predation on imentStation,Corvallis,Oreg.8 pp. WINTER1983 47 control of forest pests. In the early 1950s, ants were also effective in reducing pop- (1971 a,b) found a reduction of sycamore colonies of three Formica species, F. ulations of pine-defoliating caterpillars in sapling growth and a reduction of root fusca L.,F subnuda Emery, and F inte- southern Sweden (Wellenstein 1980). growth in lime samplings associated with groides Emery, were transplanted in New Studies frequently document the number high aphid populations. He did not, how- Brunswick forests to assess their effects of insects captured by ant colonies ever, link aphid numbers to the degree of on the spruce budworm, Choristoneura (Horstman 1970, 1972) or the amount of ant tending. fumiferana (Clemens) (Morris 1963). defoliation occurring at various distances The association of ants with aphids may The colonies did not become established, from ant mounds (laine and Niemela indirectly benefit forest protection. Hon- and the experiments were not repeated. 1980). eydew is a fairly stable food source that Finnegan (1974), in Quebec, has been ant colonies can exploit in periods of low a strong supporter of using ants in biolog- arthropod abundance. Other insect pred- Possible Disadvantages to Using Ants ical control. Using the European criteria ator-prey systems demonstrate that hon- in Biological Control for potentially effective ant species, he eydew provides an alternate food source (1971) surveyed the indigenous ant fauna Those that question the value of ant that sustains predator populations during of Quebec and concluded that none pos- predation in the control of forest pests do low prey density (Price et a!. 1980). sessed the necessary characteristics for not deny that ants eat large numbers of The effects of aphids cannot be casily augmentation programs like those in Eu- pest species. They point out, however, evaluated, because detrimental effects on rope. Therefore, Finnegan (1975) im- that ants do not feed exclusively on pests. tree growth depend not only on aphid ported F. lugubris from Italy and success- Beneficial insects such as parasitic wasps density, but also on the time of feeding fully established colonies in two sites in and flies are included in their diet (Adlung and the part of the tree under attack. The Quebec. Subsequently, Finnegan (1978) 1966). The impact of ant activity on other relation between ants and aphids can be found up to 20% less defoliation by C beneficial insects is likely to be localized complex. Aphids themselves may become Downloaded from fumiferana on trees near F. lugubris near ant colonies, however, and would ant prey (Ayre 1959, Horstman 1970, mounds than on distant trees. McNeil et depend on the extent that the trees in the Skinner 1980), or one aphid species may al. (1978) studied the seasonal predatory area are foraged on by ants, and the extent be protected and another consumed activity of F lugubris and found that C that ant activity interferes with other ben- (Skinner and Whittaker 1981). Ant pre- fumiferana was the most common prey eficial insects. dation on other insects may also be a di- http://besa.oxfordjournals.org/ of this ant during peak budworm abun- The tendency for ant colonies to be rect result of aphid protection. Forest tent dance. clumped rather than randomly distributed caterpillars were attacked by Formica and Transplantation of indigenous ant spe- has supplied critics with an additional Camponotus species when larvae ven- cies was attempted later. Formica obscu- reason to question their usefulness in bi- tured too ncar ant-tended aphid colonies ripes colonies were successfully moved ological control of forest pests. Ants es- (Green and Sullivan 1950). from Manitoba into Quebec (Finnegan tablish foraging territories near their The habit of some carpenter ant species 1977). Previous work in Manitoba had nests, and they are frequently faithful to of nesting in both living and dead wood shown that colonies of F.