Journal of Applied Ecology 2011, 48, 880–887 doi: 10.1111/j.1365-2664.2011.01990.x Chemically mediated tritrophic interactions: opposing effects of glucosinolates on a specialist herbivore and its predators Rebecca Chaplin-Kramer1*, Daniel J. Kliebenstein2, Andrea Chiem3, Elizabeth Morrill1, Nicholas J. Mills1 and Claire Kremen1 1Department of Environmental Science Policy & Management, University of California, Berkeley, 130 Mulford Hall #3114, Berkeley, CA 94720, USA; 2Department of Plant Sciences, University of California, Davis, One Shields Ave., Davis, CA 95616, USA; and 3Department of Integrative Biology, University of California, Berkeley, 3060 Valley Life Sciences Bldg #3140, Berkeley, CA 94720, USA Summary 1. The occurrence of enemy-free space presents a challenge to the top-down control of agricultural pests by natural enemies, making bottom-up factors such as phytochemistry and plant distributions important considerations for successful pest management. Specialist herbivores like the cabbage aphid Brevicoryne brassicae co-opt the defence system of plants in the family Brassicaceae by sequestering glucosinolates to utilize in their own defence. The wild mustard Brassica nigra,analter- nate host for cabbage aphids, contains more glucosinolates than cultivated Brassica oleracea,and these co-occur in agricultural landscapes. We examined trade-offs between aphid performance and predator impact on these two host plants to test for chemically mediated enemy-free space. 2. Glucosinolate content of broccoli B. oleracea and mustard B. nigra was measured in plant mat- ter and in cabbage aphids feeding on each food source. Aphid development, aphid fecundity, preda- tion and predator mortality, and field densities of aphids and their natural enemies were also tested for each food source. 3. Cabbage aphids growing on high glucosinolate plants like B. nigra contained more glucosino- lates than aphids on lower glucosinolate B. oleracea. Aphids on B. nigra had shorter generation times and greater daily fecundity, while their predators (Diptera: Syrphidae) had lower feeding and higher mortality rates. Lower syrphid densities were found on B. nigra than on B. oleracea in the field. 4. Synthesis and applications. This study presents physiological and field evidence to suggest that weedy B. nigra may provide enemy-free space from an important predator. Habitat near crops should be examined for its potential to provide enemy-free space and compromise otherwise effec- tive biological control. The issue of pest control must be considered from the bottom up as well as the top down. Key-words: biological control, Brassicaceae, Brevicoryne brassicae, chemical defences, enemy-free space, parasitism, pest management, physiological trade-off, syrphid & Lawton 1984). Although the best examples of enemy-free Introduction space have been documented in natural systems and Plants can provide an ecological refuge for herbivores by through experimental host-plant shifts (Denno, Larsson & allowing them to chemically or physically escape their natu- Olmstead 1990; Gratton & Welter 1999; Murphy 2004), the ral enemies, sometimes called an ‘enemy-free space’ (Jeffries concept also has useful application in the context of biologi- cal control for agricultural systems. Our understanding of biological control is generally focused on the top-down *Correspondence author. California Institute for Energy & Envi- ronment, University of California, 2087 Addison Street – 2nd pressures of predators on their prey, but the occurrence Floor, Berkeley, CA 94704, USA. E-mail: [email protected] of enemy-free space reveals important subtleties in the Ó 2011 The Authors. Journal of Applied Ecology Ó 2011 British Ecological Society Chemically mediated tritrophic interactions 881 relationship between these trophic groups. Successful imple- produces volatile toxic isothiocyanates just as it does in plants mentation of top-down control by natural enemies may (Francis et al. 2001; Francis, Lognay & Haubruge 2004). hinge on incorporating bottom-up factors such as the distri- The degree to which a plant with high glucosinolate content bution of plants providing refuge to pests. Identifying which like B. nigra serves as a viable predator refuge depends on the plants could provide enemy-free space for different crop trade-off between enemy-free space and herbivore perfor- pests could help guide management to reduce or eliminate mance. Cabbage aphids are completely dependent on their an important potential source of pests to crop fields. food source for the acquisitionofthemustardbomb.They Many wild plants contain higher levels of defence com- cannot synthesize their own glucosinolates (Kazana et al. pounds than their domesticated congeners (Baker 1972; Cole 2007) and therefore can only acquire defences through gluco- 1997a; Gols & Harvey 2009), and specialized herbivores can sinolate consumption (Francis, Haubruge & Gaspar 2000; often utilize those toxins to compile their own chemical arsenal Vanhaelen, Gaspar & Francis 2002; Olmez-Bayhan, Ulusoy & to escape enemies (Nishida 2002; Hopkins, van Dam & van Bayhan 2007; Pratt et al. 2008). Thus, differences in the con- Loon 2009). If this chemical refuge allows pests to escape their centration of glucosinolates within the aphids’ diet may influ- enemies more effectively than when feeding on crops, the ence their ability to deter predation. However, any benefit of occurrence of such plants around farmland could be support- predator refuge derived from glucosinolates may be offset by ing populations of pests that are then unregulated by their slower aphid development or reproduction, because of poten- enemy community. The presence of refugia could allow pests tial energetic costs of sequestering these toxic compounds to build to greater levels than if such refuges were absent from (Cole 1997a). While not framing their study specifically in landscapes. The invasive mustard Brassica nigra (L.) Koch has terms of glucosinolate content, Ulusoy & Olmez-Bayhan much higher glucosinolate levels than the domesticated Bras- (2006) found that aphids grown on another wild mustard, sica oleracea (L.) (Rodman, Kruckeberg & Alshehbaz 1981; Sinapis arvensis (L.), had lower reproductive rates than those Mithen, Raybould & Giamoustairs 1995), and weedy patches reared on domesticated B. oleracea cultivars. Aphids on of B. nigra are commonly found in field margins or edges S. arvensis also had shorter generation times, however, result- around B. oleracea fields (cole crops, such as broccoli, cauli- ing in no net difference in intrinsic rate of increase for aphids flower, kale, cabbage). The co-occurrence of these two Brassica on wild mustard vs. cole crops in the absence of predation. species provides an excellent opportunity to test whether an However, these herbivore development studies have not been invasiveweedcanserveasenemy-freespaceforcroppests. conducted on the same plant species as the studies investigating Plants in the family Brassicaceae are a good study system for effects on their predators, and therefore,norealconclusions chemically mediated enemy-free space, because of their sophis- regarding the potential trade-offs of enemy-free space can be ticated two-part defence system involving a glucosinolate com- drawn from the current literature. pound and myrosinase protein complex that has been It is possible that wild Brassica species could serve as sources described as a ‘mustard-oil bomb’ (Ratzka et al. 2002). When of pests to nearby crops if they provide enemy-free space with- plant tissue is damaged, glucosinolates come into contact with out compromising the pest’s own population growth. We myrosinase enzymes, which remove glucose from the glucosin- simultaneously measured herbivore performance and preda- olates, leading to the formation of toxic hydrolysis products tion risk on two host-plant species to assess the potential for such as isothiocyanates. The cabbage aphid Brevicoryne brassi- chemically mediated trade-offs. Specifically, we quantified the cae (L.) is among a small group of insects that have found a glucosinolate content of two Brassica species (B. oleracea and strategy for exploiting Brassicaceae, a resource toxic to most B. nigra), measured the glucosinolate content in aphids reared herbivores. Generalist herbivores feeding on brassicaceous on each of the two plant species, assessed aphid development plants, including the lepidopterans Mamestra brassicae (L.), and fecundity while growing on these two food sources, and Spodoptera eridania (Cramer) and Trichoplusia ni (Huebner), evaluated syrphid development and predation of aphids raised tend to rely on direct metabolic detoxification of the glucosino- on each host plant. Subsequently, we addressed the overall lates, receiving little or no predator defence benefit (Li et al. effects of these trade-offs by estimating the abundance of 2000; Lambrix et al. 2001; van Leur et al. 2008). In contrast, aphids, syrphids, and aphid parasitoids on the two Brassica specialist herbivores, such as the cabbage aphid, the sawfly species at a series of locations in the field. We tested the Athalia rosae (L.), and the harlequin bug Murgantia histrionica hypotheses that (i) high glucosinolate content places a physio- (Hahn), co-opt toxicity from glucosinolates through sequestra- logical burden on herbivores utilizing these compounds for tion, escaping many of their enemies as a result (Francis et al. their own defence, (ii) high glucosinolate content confers an 2001; Muller et al. 2001; Aliabadi, Renwick & Whitman advantage