This project aims to compare populations of the green spruce aphid Elatobium abietinum inhabiting spruce forests that have been managed using alternative silvicultural systems. The project tests the hypothesis that increasing the structural diversity of forest stands can reduce the impact of pest species, in particular by supporting a greater diversity and abundance of invertebrate natural enemies.
Specific research objectives are to analyse datasets collected during field studies in Wales, to:
- compare spruce aphid populations in forests managed by clear felling and replanting (even-aged forest stands) with populations in continuous cover forests managed using group selection or shelterwood silvicultural systems (mixed-aged forest stands)
- analyse the abundance and distribution of invertebrate natural enemies of the spruce aphid in forest stands managed using different silvicultural systems
- provide recommendations on forest design that may increase the resilience of commercial plantation forests to damage by the spruce aphid
Results so far
Spruce aphids and invertebrate natural enemies were sampled in 24 Sitka spruce stands in Wales from 2007 to 2011, using a variety of techniques at ground level and in the canopy. Preliminary analyses show that spruce aphid densities increased with tree age in even-aged stands, but on average were higher in group selection and shelterwood stands. Aphid densities above 2.8 aphids per 100 needles reduce volume increment of Sitka spruce by 6% (Straw et al., 2011) and this threshold was exceeded in mid and mature even-aged stands and on trees of all ages in the mixed-age stands.
Invertebrate predators showed a wide range of responses to forest management and stand structure. Amongst the even-aged stands, some taxa, notably lacewings (Hemerobiidae) and smaller soldier beetles (Malthodes, Cantharidae) increased in abundance with stand age and attained their highest densities in mature stands, in a manner similar to the increase in spruce aphid populations. Other predators however, such as ladybirds (Coccinellidae), hoverfly larvae (Syrphidae) and the larger soldier beetles (Rhagonycha, Cantharidae) decreased in abundance in the oldest stands and attained maximum population densities in mid-rotation stands. Populations of natural enemies in group-selection and shelterwood stands were either similar to or lower than those observed in even-aged stands.
This analysis started in 2012 and is ongoing.
Straw, N.A., Fielding, N., Green, G., Price, J. & Williams, D. (2011) Defoliation and growth relationships for mid-rotation Sitka spruce attacked by the green spruce aphid, Elatobium abietinum (Walker) (Homoptera: Aphididae). Forest Ecology and Management 262, 1223–1235.
Funders and partners
This research is unded by the Forestry Commission under the programme, Advice and Scientific Support for Tree Health
Forestry Commission policy
This research underpins the evidence base for the delivery of healthy and resilient forests and wider ecosystems which is part of the Tree Health and Plant Biosecurity Action Plan