Responses of the willow beetle Phratora vulgatissima to genetically and spatially diverse Salix spp. plantations.

Abstract

Phratora vulgatissima is the most serious pest of willow grown as short-rotation coppice in Britain. The effects of genetically diverse plantations of willow on beetle density, damage and oviposition were studied in 1996-98, in a field trial in southern England containing monocultures of willow varieties with different willow beetle susceptibility, and mixtures which differed in number and spatial configuration of varieties (2 varieties of Salix viminalis, S. dasyclados, S. burjatica and S. stipularis). In addition, P. vulgatissima adults were introduced to potted willows in cages in a glasshouse as either a monoculture or a mixture of 3 varieties, and damage and oviposition were recorded. Beetle density, damage and oviposition in the field were generally lower in mixtures than in monocultures, especially with 5 varieties rather than 3 and random structural composition rather than regular. In the caged experiment, where beetles could not migrate, there were no significant differences between the monoculture and the mixture for mean damage or oviposition on the 2 susceptible varieties. Over time, the average beetle density per tree in the field increased in the monocultures of susceptible varieties, yet decreased in the mixtures. The behavioural response of P. vulgatissima to vegetational diversity was consistent with the resource concentration hypothesis, with lower colonization in, and increased emigration from, mixtures compared with monocultures. These results indicate that genetically and spatially diverse willow mixtures provide a sound basis for pest management due to the propensity for movement by the beetles. As each movement necessitates the location of a new host plant, it is more difficult for beetles to find favourable varieties in a genetically diverse mixture than in a monoculture. Adult P. vulgatissima occur in mobile aggregations, so pest and damage surveys must be intensive and cover a wide time interval. An option for pest management is a mixture of 5 willow varieties, at least one of which has low susceptibility to the willow beetle, grown in a random planting of varieties. The choice of variety is important, especially as regards susceptibility to rust disease (Melampsora spp.), as are physiological interactions between varieties that might affect beetle ecology.

Key words

• arthropod pests
• coppice
• forest damage
• forest pests
• forests
• fungal diseases
• insect pests
• mixtures
• monoculture
• oviposition
• pathogens
• pest resistance
• pests
• plant pathogenic fungi
• plant pathogens
• plant pests
• population density
• rust diseases
• short rotation forestry
• species trials
• trees
• varieties
• variety trials
• willows
• woody plants