Abstract
AbstractAbiotic stresses such as drought and nutrient availability can affect invertebrate herbivores feeding on plants, and potentially cascade up to impact their predators and parasitoids. Although these two factors separately been the subject of many studies, there are few tests of their combined effects in the context of pest species and their natural enemies on cultivated plants. Climate change models predict an increase in the frequency and severity of droughts, while the type and amount of fertiliser applied to crops is more under the control of growers. Understanding how these two abiotic factors may interact is key to utilising the potential of natural enemies to control pests under a future climate. To address this, a range of drought and fertiliser type treatments were applied to a model Brassica system in a factorial design, and the performance of two ubiquitous aphid species and their parasitoids was assessed. One aphid species was a specialist on Brassicas (Brevicoryne brassicae, with parasitoid Diaeretiella rapae) and the second a generalist aphid species (Myzus persicae, with parasitoid Aphidius colemani). The performance of both aphid species responded in a similar way to the treatments, and was maximised on plants growing in organic fertilisers under medium levels of drought stress. The strongest effects of drought and fertiliser cascaded up to affect parasitoids. Parasitoid performance responded in a broadly similar way to their aphid host performance in relation to fertiliser type. Some of the smaller effects of fertiliser treatments on aphid performance were not found for parasitoid performance. Aphid performance was greatest on plants under medium drought stress, but the parasitoids only responded consistently to the high drought stress treatment, on which their performance was reduced. Interactions between the drought and fertiliser did not have a large effect on aphid or parasitoid performance, compared with the strong main effects found for each treatment. These results are discussed in the context of previous and future research on the impacts of abiotic stresses on invertebrate herbivores and their natural enemies.
Highlights
Abiotic stresses such as water availability (Tariq, Wright, Rossiter, & Staley, 2012), and the type and amount of nutrients supplied to a plant (Rowen, Tooker, & Blubaugh, 2019), have been shown to separately alter the performance and abundance of invertebrate herbivores feeding on plants, and can cascade up to affect their predators and parasitoids (Tariq, Wright, Bruce, & Staley, 2013a)
These results support our first hypothesis above, that performance of both aphid species will be increased when feeding on plants under medium drought stress
In relation to the type of fertiliser supplied to their host plant, performance of both aphid species was overall greatest on the two organic fertilisers (CM and SMAS), lowest on the unfertilized control plants and intermediate on plants fertilised with conventional ammonium nitrate (AN) fertiliser
Summary
Abiotic stresses such as water availability (Tariq, Wright, Rossiter, & Staley, 2012), and the type and amount of nutrients supplied to a plant (Rowen, Tooker, & Blubaugh, 2019), have been shown to separately alter the performance and abundance of invertebrate herbivores feeding on plants, and can cascade up to affect their predators and parasitoids (Tariq, Wright, Bruce, & Staley, 2013a). An experiment on potted B. oleracea growing under controlled conditions showed the performance of individual aphids of the two species responded in a similar way to the treatments as their abundance responded in the field trial (Stafford et al, 2012) These responses may be because of effects of fertiliser on the host plant chemistry, as B. oleracea var capitata fertilised with animal manure produced more glucosinolates, a group of chemicals which the specialist B. brassicae is well adapted to, than those plants growing in conventional AN fertiliser (Kazana et al, 2007; Staley et al, 2010). Aphidius colemani performance will be reduced under high drought stress
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