Chemically mediated tritrophic interactions: opposing effects of glucosinolates on a specialist herbivore and its predators

Abstract

Summary1. 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, an alternate 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 matter and in cabbage aphids feeding on each food source. Aphid development, aphid fecundity, predation 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 glucosinolates 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 effective biological control. The issue of pest control must be considered from the bottom up as well as the top down.

Phyllotreta striolataflea beetles use host plant defense compounds to create their own glucosinolate-myrosinase system

“…The mechanism by which the beetle controls the hydrolysis of its stored glucosinolates to avoid autointoxication remains to be determined but is likely to be actively controlled. In contrast, the "mustard-oil bomb" in aphids is activated when an individual is attacked by a predator, which benefits the survival of the clonal aphid colony by repelling the predator (17,19,47). Although we cannot rule out the participation of sequestered glucosinolates in a similar defense in Phyllotreta, an additional signaling role is also suggested by previous studies showing that ITCs enhance the response to the male-produced aggregation pheromone (26)(27)(28)(29).…”

Dealing with food shortage: larval dispersal behaviour and survival on non‐prey food of the hoverfly Episyrphus balteatus

“…;Vanhaelen et al, 2002;Kos et al, 2011). For example, the mortality of hoverfly larvae fed with the specialist cabbage aphid Brevicoryne brassicae increased drastically when aphids had been reared on a glucosinolate-rich host plant as compared with hosts that contain fewer glucosinolates ( Vanhaelen et al, 2002;Chaplin-Kramer et al, 2011). Pea aphids reared on M. sativa were shown to have a higher caloric content than pea aphids reared on V. faba, which results in better survival and faster development of lacewings and ladybirds (Giles et al, 2000(Giles et al, , 2001(Giles et al, , 2002.…”

Detecting pest control services across spatial and temporal scales

“…The presence of alternate hosts in complex landscapes has been shown to be a counterbalancing force to increased top-down control, in aphids in particular Thies et al, 2005). In our system, the Mediterranean climate means that aphids do not need to overwinter (Hughes, 1963), but they may build up populations on the weedy mustard that commonly occurs in unmanaged habitats near agricultural fields (Chaplin-Kramer et al, 2011a).…”