Environmental context and herbivore traits mediate the strength of associational effects in a meta-analysis of crop diversity

1. Crop diversification offers a promising solution to meet expanding
global food demands while maintaining ecosystem services. Diversification
strategies that employ mixed planting to reduce pest damage (e.g.,
intercropping), termed "associational effects” (AE) in the ecological
literature, can decrease (associational resistance) or increase
(associational susceptibility) herbivore abundance on a focal plant. While
application of AE to agroecosystems typically reduces pest abundance, the
range of outcomes varies widely. 2. We conducted a meta-analysis using 272
estimates of insect herbivore abundance on crops neighbored by a
conspecific or heterospecific from 44 studies undertaken on six
continents. We focus on four agricultural crops well-represented from
sites across the globe to test hypotheses related to understanding how
herbivore traits (diet breadth, feeding guild, origin), plant traits (crop
type, phylogenetic distance to neighbor), and environmental context
(climate, experimental design) contribute to variation in the outcomes of
associational effects. 3. Overall, bicultures provided a strong reduction
of insect abundance on the focal crop. Climate and interactions between
herbivore traits, particularly diet breadth and origin, and plant traits
or environmental context mediated the strength of associational effects.
4. Bicultures provided the strongest reductions in insect abundance at low
latitudes and this effect decreased at higher latitudes, but only for
insects with certain traits. Abundance of generalist herbivores and
globally distributed pests tended to be most strongly negatively affected
by bicultures, under certain contexts, whereas specialist herbivores and
native pests were less affected by neighbors. 5. Synthesis and
application. This meta-analysis highlights that variability in AE is
determined in part by the interactive effects of herbivore traits and
environmental context and provides guidance for incorporating beneficial
ecological interactions into integrated pest management strategies.