Landscape-scale mass-action of spiders explains early-season immigration rates in crops

Abstract

Context Early-season immigration into arable fields by natural enemies is key for effective biocontrol, but little is known about the mechanisms underlying immigration processes. Objectives Here we test the mass action hypothesis for ballooning spiders, stating that local immigration rates are positively related to the amount of spiders in the surrounding landscape. Methods Immigration rates of spiders were assessed by sticky traps in remnant vegetation, in arable land 25-125 m from remnant vegetation, and in arable land further than 400 m from remnant vegetation. The experiment was conducted at 18 locations across two landscapes and repeated three times in a 2-week period in 2007 and 2008. Spider densities in crop and noncrop habitats were assessed by beat sheet sampling and used to calculate spider loads in landscape sectors around the experimental locations at five spatial scales. Results Regression analysis indicated that immigration rates were influenced by meteorological variables and landscape context at 2 km and possibly beyond. Regression models that included spider load at relevant spatial scales received more statistical support from the data than models with the proportion of remnant vegetation and crops. Regression analysis further indicated that wheat and-to a lesser extentremnant vegetation are important habitats for the recruitment of ballooning spiders. Conclusions Our study provides support for the mass action hypothesis by showing that a combination of land-use variables with habitat specific spider densities allows the generation of functional cover types with greatly improved explanatory power.Electronic supplementary material The online version of this article

Landscape context does not constrain biological control of Phenacoccus manihoti in intensified cassava systems of southern Vietnam

“…One hint of A. lopezi's responsiveness to landscape complexity though can be found in its density-dependent response to P. manihoti during the early season. In early season, parasitism rates are likely more reflective of landscape-level influxes and less obscured by numerical increases from in-field reproduction (see Schelhorn et al, 2015b, Bianchi et al, 2017. Similar to Hammond & Neuenschwander (1990), positive density dependence was found at low host densities and parasitism rates declined rapidly at higher P. manihoti densities.…”

“…Phenacoccus manihoti is an efficient colonist that draws upon two main strategies: 1) airborne dispersal by crawlers, and 2) passive colonization with infested planting material (Nwanze et al, 1979). For airborne movement, immigration rates are directly tied to the amount of potential colonizers in the surrounding landscape (e.g., Bianchi et al, 2017). In small-field landscapes, crop harvest extended over longer periods into the early dry season, and possibly allowed for a sustained movement of airborne P. manihoti nymphs through a so-called 'green bridge' effect.…”

Do diverse landscapes provide for effective natural pest control in subtropical rice?

“…One case concerns the positive association between the abundance of sheet weaving spiders (Linyphiidae) in middle rice and the proportion of early rice in the surrounding landscape, at four spatial levels. Ecologically, this might be interpreted as a carry‐over effect between rice crops, and the ballooning dispersal behaviour of these spiders may explain why this association extends over a wide spatial range (100–1,000 m radius; Bianchi, Walters, Cunningham, Hemerik, & Schellhorn, ). However, this positive association was rather weak (with coefficient values of about 0.2) and did not translate in a negative association between pest abundance or yield loss and the proportion of early rice.…”

Variation in parasitoid attraction to herbivore-infested plants and alternative host plant cover mediate tritrophic interactions at the landscape scale

“…It is also important to consider which natural enemies are preferred to be attracted to the field as biological control agents. While generalist natural enemies, such as spiders, may be abundant in a variety of crop and semi-natural habitats (Bianchi et al 2017), and can switch prey when the pest species is temporarily not available, specialist natural enemies may require more specific habitat types, linked to their specific host plant species.…”