Tree species richness around urban red maples reduces pest abundance but does not enhance biological control

Urban trees often host greater insect pest abundance than trees in rural
forests. This may be due, in part, to differences in tree diversity and
canopy cover between these settings. Urban trees are often planted in
isolation or monoculture, which favors pest accumulation. The gloomy
scale, Melanaspis tenebricosa Comstock, is a pest of urban red maples
(Acer rubrum L.) that is abundant where impervious surfaces dominate the
local landscape. Increasing tree diversity and canopy cover around urban
red maples may reduce gloomy scale abundance by supporting natural enemy
communities. We investigated the effect that surrounding tree species
richness and tree canopy cover had on gloomy scale abundance, natural
enemy abundance, and biological control in red maple trees in Raleigh, NC,
USA. We collected scales and natural enemies from red maples that spanned
a gradient of tree species richness, canopy cover, and impervious surface
values. We also measured gloomy scale parasitism and predation of sentinel
prey in red maple canopies. Greater tree species richness and canopy cover
were associated with lower gloomy scale density. Red maples in diverse
settings also hosted fewer scales per natural enemy. Parasitoids were less
common in maples in diverse settings, but generalist predator abundance
was unaffected by tree diversity. Finally, tree species richness and
canopy cover did not increase biological control of scales or sentinel
prey. Our findings suggest that higher tree diversity and greater canopy
cover may reduce gloomy scale density, but this is not entirely explained
by the effects of natural enemies and biological control.

城市树木相较于乡村森林中的林木，通常承载更高的昆虫害虫种群丰度。该现象的部分成因，在于两类生境中树木多样性与冠层覆盖度（canopy cover）的差异。城市树木常以孤立单植或单优群落的形式布设，这会促进害虫种群的积累。暗盔蚧（Melanaspis tenebricosa Comstock）是城市红花槭（Acer rubrum L.）的主要害虫，在当地以不透水面（impervious surfaces）为主的景观中种群丰度极高。提升城市红花槭周边的树木多样性与冠层覆盖度，或可通过支撑天敌群落的途径降低暗盔蚧的种群丰度。本研究针对美国北卡罗来纳州罗利市的红花槭展开调查，探究周边树木物种丰富度与冠层覆盖度对暗盔蚧种群丰度、天敌种群丰度以及生物防治（biological control）的影响。研究人员在沿树木物种丰富度、冠层覆盖度与不透水面值梯度分布的红花槭上，采集了蚧虫与天敌样本。同时，研究者还测定了红花槭冠层内暗盔蚧的寄生率，以及哨兵猎物（sentinel prey）的捕食情况。分析结果显示，更高的树木物种丰富度与冠层覆盖度与更低的暗盔蚧种群密度显著相关。在物种多样性较高的生境中，单头天敌对应的蚧虫数量也更少。寄生性天敌（parasitoids）在多样性较高的生境中的红花槭上更为少见，但广食性捕食者的丰度不受树木多样性的影响。最后，树木物种丰富度与冠层覆盖度并未提升对蚧虫或哨兵猎物的生物防治效果。本研究结果表明，更高的树木多样性与更大的冠层覆盖度或可降低暗盔蚧的种群密度，但这一效应并不能完全通过天敌与生物防治的作用机制来解释。