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Forest management to mitigate the risk of pest damage

forest heterogeneity affects bottom-up and top-down mechanisms influencing pest insect population dynamics

Bellone, Davide (2018). Forest management to mitigate the risk of pest damage. Diss. (sammanfattning/summary) Uppsala : Sveriges lantbruksuniv., Acta Universitatis Agriculturae Sueciae, 1652-6880 ; 2018:21
ISBN 978-91-7760-182-1
eISBN 978-91-7760-183-8
[Doctoral thesis] (Submitted)

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Mixed forests are proposed as a management strategy that enables high levels of wood production while also supporting ecological and social benefits that are weakened by traditional monoculture strategies. An ecological benefit of mixed forestry is that it is expected to reduce plant damage caused by specialist insect pests because theory suggests that heterogeneous habitats such as mixed forests should limit fluctuations in pest insect population dynamics. Two main hypotheses have been proposed to explain this reduction in plant damage. The first is based on a bottom-up effect whereby plants grown in a diverse habitat have more effective defences against herbivores. The second is based on a top-down effect whereby plant species diversity creates habitat diversity that supports a higher abundance of pests’ natural enemies. However, the mechanisms underlying these bottom-up and top-down effects are not fully understood.
To fill this gap, I investigated how heterogeneity affects the bottom-up and top-down mechanisms influencing pest insect population dynamics. As a model organism, I used the European pine sawfly (Neodiprion sertifer), an herbivorous insect that frequently reaches outbreak densities and whose larvae cause severe damage when feeding on pine needles. Sawfly performance is known to be affected by pine needle chemistry – particularly the concentration of di-terpenes, which affect larval survival, and nitrogen levels, which affect cocoon weight and thus adult fecundity. Generalist and specialist natural enemies have been showed to cause significant mortality in sawfly larvae and cocoons. Despite their importance, the effects of plant chemistry and natural enemies on insect herbivores have not been studied in the context of forest heterogeneity.
I found that the variation in cocoon weight within groups of sawfly larvae feeding on Scots pine needles increased with the needles’ contents of di-terpenes, and that this trend was stronger in mixed forests than monocultures. Additionally, the rate of generalist predation on sawfly larvae was higher in more densely planted spots within forest stands. Predation on sawfly cocoons was favoured by tree diversity, but the presence of dead wood mitigated the negative effect of low tree diversity. In addition, forest heterogeneity had no discernible effect on specialist enemies, which were able to locate their hosts equally well in simple and complex habitats.
This thesis shows that bottom-up effects on sawfly larvae cannot explain the observed reduction in tree damage in heterogeneous habitats. It may be that the higher variation in cocoon weight observed in mixed stands increases the average fecundity of adult sawflies, resulting in higher likelihood of outbreaks. Conversely, heterogeneity promoted top-down effects because it favoured generalist predators without negatively affecting specialists. These results will be useful to forest managers seeking to understand how mixed forestry can be used to mitigate anticipated increases in insect pest damage due to future climate change.

Authors/Creators:Bellone, Davide
Title:Forest management to mitigate the risk of pest damage
Subtitle:forest heterogeneity affects bottom-up and top-down mechanisms influencing pest insect population dynamics
Series Name/Journal:Acta Universitatis Agriculturae Sueciae
Year of publishing :2018
Depositing date:23 March 2018
Number of Pages:43
IBellone, D., Klapwijk, M.J., Björkman C. Effect of stand type on host plant quality and pine sawfly performance. (Manuscript)
IIBellone, D., Björkman, C., Klapwijk, M.J. Combined top-down pressure from generalist and specialist predators facilitated by habitat heterogeneity. (submitted)
IIIBellone, D, Klapwijk, MJ, Björkman, C. (2017) Habitat heterogeneity affects predation of European pine sawfly cocoons. Ecol Evol. 7:11011–11020
IVBellone, D., Klapwijk, M.J., Björkman, C. Plant position but not environmental complexity affects aphid parasitism rates. (Manuscript)
Place of Publication:Uppsala
Publisher:Department of Ecology, Swedish University of Agricultural Sciences
ISBN for printed version:978-91-7760-182-1
ISBN for electronic version:978-91-7760-183-8
Publication Type:Doctoral thesis
Full Text Status:Public
Agris subject categories.:H Protection of plants and stored products > H10 Pests of plants
K Forestry > K10 Forestry production
Subjects:(A) Swedish standard research categories 2011 > 1 Natural sciences > 106 Biological Sciences (Medical to be 3 and Agricultural to be 4) > Ecology
(A) Swedish standard research categories 2011 > 4 Agricultural Sciences > 401 Agricultural, Forestry and Fisheries > Forest Science
Agrovoc terms:biological control, mixed forests, information processing, pest insects
Keywords:Biological control, habitat heterogeneity, bottom-up, top-down, pest, outbreaks
Permanent URL:
ID Code:15384
Faculty:S - Faculty of Forest Sciences
Department:(NL, NJ) > Dept. of Ecology
(S) > Dept. of Ecology
External funders:FORMAS
Deposited By: Davide Davide Bellone
Deposited On:26 Mar 2018 13:29
Metadata Last Modified:09 Sep 2020 14:17

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