Ecological Traps and Weed Biological Control
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Proceedings of the XIV International Symposium on Biological Control of Weeds, pp 105-111. F.A.C. Impson, C.A. Kleinjan and J.H. Hoffmann (eds). 2-7 March 2014, Kruger National Park, South Africa. Ecological traps and weed biological control R.A. Casagrande*1, F.S. Chew2 and R.G. Van Driesche3 1Department of Plant Sciences and Entomology, University of Rhode Island, Kingston, RI 02881 USA 2Department of Biology, Tufts University, Medford, MA 02155 USA 3Department of Environmental Conservation, University of Massachusetts, Amherst, MA 01003 USA Keywords: Pieridae, Monarch butterfly, swallow-worts, milkweeds, garlic mustard Abstract In a phenomenon sometimes called an ecological trap, native insects may select an invasive plant as a suitable oviposition site even though their larvae may perish on these non-hosts. Two North American native butterflies (Pieris oleracea and Pieris virginiensis. Lepidoptera: Pieridae) readily oviposit on garlic mustard (Alliaria petiolata, Brassicaceae), and monarch butterflies (Danaus plexippus, Lepidoptera: Nymphalidae) readily oviposit on non-host swallow-worts (Vincetoxicum nigrum and Vincetoxicum rossicum, Apocynaceae: Asclepiadoideae) where these non-host plants coexist with milkweeds. Oviposition on swallow-worts is not generally listed among the major factors in the extensive decline of monarch populations. However, milkweeds are increasingly being eliminated through agricultural practices and are being displaced by swallow-worts in unmanaged areas. The balance between suitable hosts and ecological traps is therefore shifting and this may result in further decline of monarchs in future. An effective swallow-wort biological control program might assist in efforts to restore monarch populations. Garlic mustard is widely distributed throughout the range of P. virginiensis which is a habitat specialist of forests. Females lay more readily on garlic mustard than on the normal hosts even though there are no signs of adaptation to the adopted host and P. virginiensis populations are in decline. This native butterfly might benefit from a garlic mustard biological control program but since North American garlic mustard populations are now known to decline on their own, the outcome of a potential biocontrol release is uncertain. In a third case, garlic mustard biocontrol appears to be unnecessary to protect populations of P. oleracea. A population of this butterfly in western Massachusetts, USA appears to be avoiding this potential ecological trap by being able to survive and develop on garlic mustard. This adaptation, combined with escape from attack from the parasitioid Cotesia glomerata (Hymenoptera: Braconidae) and the introduction and spread of a new host plant, the exotic cuckoo flower (Cardamine pratensis, Brassicaceae) may allow populations to recover to historical levels. Although difficult to assess on a regional population scale, the negative impacts of exotic plants on native insects should be considered in developing, evaluating, and regulating weed biological control programs. 1. Introduction they give an example of mayflies ovipositing on an asphalt road that had the same reflective properties as a pond. This In environments subjected to rapid anthropogenic concept is expanded to include introduced species as change, organisms may become ‘trapped’ by responses to ecological traps in a subsequent paper where Schlaepfer et environmental cues that had served them well through al. (2005) describe inappropriate oviposition by three evolutionary time. A newly introduced element can mimic butterfly species on exotic non-host plants that mimic the a stimulus used by an organism in selecting a suitable olfactory stimuli of their traditional hosts. The North habitat, thus inducing an inappropriate response. American native monarch butterfly Danaus plexippus (L.) Schlaepfer et al. (2002) termed this an evolutionary trap and *Corresponding author: [email protected] 105 Casagrande et al. (Lepidoptera: Nymphalidae), which normally reproduces 2.1. Monarch decline on milkweed species, also oviposits (and their larvae perish) Populations of eastern monarch butterflies have on exotic swallow-worts (Haribal and Renwick, 1998; experienced population increases and declines in the past, Casagrande and Dacey, 2007). Similarly, the native North particularly related to weather conditions in overwintering American butterflies Pieris oleracea Harris and Pieris sites and during migratory periods, but since the 1976 virginiensis W.H. Edwards (Lepidoptera: Pieridae), which discovery of overwintering populations in Mexico, these traditionally oviposit on native Brassicaceae, are stimulated populations have greatly declined (Brower, 1995). In the to oviposit on garlic mustard, Alliaria petiolata (M. Bieb.) past decade, an estimated winter population covering 11.12 Cavara & Grande (Brassicaceae), which is toxic to their ha in 2003/4 was reduced to a mere 1.29 ha in 2012/13. larvae (Courant et al., 1994; Porter, 1994). In addressing This population shrank to 0.67 ha in 2013/14 (Wines, 2014) management issues related to ecological traps, Schlaepfer et following the precipitous decline (near absence) of al. (2005) suggest novel approaches that assist organisms in monarchs throughout the upper Midwest and northeast parts adapting to the novel environment instead of pursuing of the United States in the 2013 season (Robbins, 2013). “futile attempts to restore ‘pristine’ or ‘ancestral’ conditions.” Biological control programs under There are no good historical data on abundance of development for both garlic mustard and swallow-worts are North American monarch butterflies. Brower (1995) unlikely to restore a pristine environment, but they may mentions several reports of enormous migratory flights of reduce the impact of these invasive plants on butterfly monarchs in the midwestern United State before 1880. The populations, perhaps allowing time for them to adapt to subsequent spread of A. syriaca throughout the Midwest these plants and other environmental constraints. and into the previously forested, eastern states certainly expanded the range of the monarchs, but it is unknown what In the United States it currently takes a decade or more effect this had on the overall size of the North American of research and regulatory consideration before a novel monarch population. There is some (disputed) evidence biological control agent for an invasive plant is permitted that larger monarch populations in North America resulted for introduction. The continuing vulnerability of in colonization of distant islands and continents around P. virginiensis populations and precipitous decline of 1880 (Brower, 1995). The most widely accepted counts are monarchs argue for population-level assessments of the provided by estimates of overwintering colonies1. impacts of these and other ecological traps and possible reconsideration of regulatory processes. A reduction in size of the fir forest and thinning of trees in some monarch overwintering sites in Mexico has exposed monarchs to unfavorable weather conditions, resulting in winter mortality as high as 42% in 1981 and 80% in 1992 2. Monarch butterflies (Brower, 1995). Brower also reported surprisingly high The monarch butterfly, native to North America, is overwintering predation by birds and mice on adult well known for its seasonal migrations. Eastern populations butterflies that should have been protected by cardiac exhibit the longest flights, overwintering in Mexico. glycosides accumulated as larvae. He attributes this to Western populations fly shorter distances to overwintering production of these overwintering adults on A. syriaca with sites in coastal California, and Florida populations do not variable levels of cardiac glycosides instead of more toxic migrate at all (Altizier et al., 2000). The eastern monarch prairie milkweeds that were their primary hosts over a population primarily overwinters in Mexico and migrates century ago (Brower, 1995). Recent factors considered north in early spring. Before large-scale conversion of the important to the decline of eastern monarch populations plains grassland ecosystems to agricultural lands, monarchs include the loss of farm conservation land to corn reproduced on native Asclepias species, with successive production for biofuels and the widespread use of herbicides generations moving northward as host plants senesced and on corn and soybeans (Robbins, 2013). These factors have then retreating in great mass flights to Mexico for the winter resulted in 60-90% loss of milkweed in a decade in Iowa (Brower, 1995). The late 1800s witnessed two changes in (Hartzler, 2010; Brower et al., 2012). An increased monarch behavior – both related to native common incidence of the protozoan Ophryocystis elektroscirrha milkweed (Asclepias syriaca L., Asclepiadaceae). As the McLaughlin and Myers (Neogregarinida: Ophryocystidae) plains were farmed, native milkweeds were largely in overwintering colonies in recent years might also be eliminated and replaced by the weedy A. syriaca which also contributing to the decline of eastern monarchs and to a moved into eastern farm fields newly created from forest. male-skewed shift in sex ratios in overwintering sites and Monarchs switched to A. syriaca in the midwestern United throughout the breeding range of these butterflies (Davis States and followed this host plant into the eastern states and Rendón-Salinas, 2010). In addition to these problems, where the first large-scale migrations were noted in the which have