Elsevier

Animal Behaviour

Volume 77, Issue 4, April 2009, Pages 901-909
Animal Behaviour

Multivariate statistics coupled to generalized linear models reveal complex use of chemical cues by a parasitoid

https://doi.org/10.1016/j.anbehav.2008.12.016Get rights and content

Understanding how animals integrate multiple cues, in particular complex mixtures of volatile chemicals, is a subject of current interest. Insect behavioural responses to volatile blends have traditionally been analysed separately to the changes in blends themselves, making it difficult to link behaviour directly to volatile cues. We coupled principal components analysis (PCA) and generalized linear models (GLMs) to link volatiles released by plants in response to pea aphid, Acyrthosiphon pisum, feeding directly to the behaviour of the aphid parasitoid Aphidius ervi. We used choice bioassays to compare parasitoid response to odours from aphid-damaged and undamaged plants, then collected volatiles from the same plants used in bioassays and analysed volatile profiles with PCA. The principal components explaining 99% of the variation in the data set were used as explanatory variables in a GLM to analyse the behavioural response. This was done for two plant species: alfalfa, Medicago sativa, and broad bean, Vicia faba. For each species, a single principal component was important in explaining insect attraction. In both cases it explained a small amount of variability in the volatile data set (7.0% and 0.5% in M. sativa and V. faba, respectively). For both plants, the analysis revealed the presence of volatiles that supported or inhibited parasitoid attraction. Compositional analysis of the blend revealed no major changes in either plant, highlighting that A. ervi can detect minor changes, ignoring the major variability in the blend. The approach could be valuable for behavioural studies on multisensory orientation by foraging animals.

Section snippets

Plants and Insects

Medicago sativa seeds (Herbiseed, Twyford, U.K.) were germinated on damp filter paper. When the cotyledons started expanding, they were transplanted to plastic pots (8 × 8 cm) filled with garden compost and placed in a greenhouse at 18:6 h light:dark (LD) and 21 ± 2 °C. Vicia faba (variety Hangdown Grunkernig, Svalöf Weibull, Svalöf, Sweden) seeds were sown individually in plastic pots (8 × 8 cm) with compost and placed in the greenhouse.

Acyrthosiphon pisum colonies were started from approximately 50

Aphid-induced Volatiles

The total amount of volatiles produced did not differ between AD and UD plants in M. sativa (treatment effect: F1,17 = 2.93, P = 0.105; date: F9,17 = 0.71, P = 0.697; date*plant pair: F8,17 = 1.27, P = 0.322) or in V. faba (treatment effect: F1,20 = 1.20, P = 0.287; date: F11,20 = 15.30, P < 0.001; date*plant pair: F9,20 = 1.32, P = 0.289).

The volatile blend of M. sativa did not differ significantly between UD and AD plants when analysed by compositional analysis (Wilk's λ = 8.21 × 10−2, approximate F17,1 = 0.66, P = 0.766).

Discussion

We have presented an analysis that directly links behavioural cues to which foraging insects were exposed with the insects' response in a simplified environment (olfactometer). When the mean response was tested, A. ervi responded significantly to volatiles from V. faba but not M. sativa. The response to V. faba is in accordance with previous studies showing the attraction of this parasitoid to A. pisum-damaged plants (Guerrieri et al., 1993, Guerrieri et al., 1999, Du et al., 1996). Previous

Acknowledgments

We are grateful to M. Torrance (Rothamsted Research) for providing A. ervi, to G. Piaggio for statistical advice and comments on the manuscript and to two anonymous referees for very helpful comments on the manuscript. This work was supported financially by Carl Tryggers Foundation for Scientific Research, the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning (Formas) and Mistra through the PlantComMistra program. Rothamsted Research receives grant-aided

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    A. Mohib, M. A. Birkett and S. Dufour are at the Centre for Sustainable Pest and Disease Management, Rothamsted Research, Harpenden AL5 2JQ, U.K.

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