Landscape-scale pest suppression is mediated by timing of predation

Background/Question/Methods

There is increasing evidence that biological control of agricultural pests is affected by the landscape context, although the mechanisms behind this pattern have received little attention.  Population ecology theory predicts that one key mechanism mediating successful pest suppression is early predator arrival to agricultural fields.  Alternatively, complex landscapes may provide abundant and diverse natural enemy assemblages that compensate for delayed predator arrival.  Since higher levels of landscape predators do not always cascade down to increase pest suppression, we hypothesized that this mismatch is due to the timing of predator arrival to agricultural fields.  We disentangle the mechanisms driving landscape context effects on pest suppression by manipulating the timing of predation in agroecosystems located across a gradient of landscape complexity in a subtropical horticultural region in Australia.  Predator impacts were manipulated in fallow fields using exclusion cages on potted plants with sentinel aphid populations  in a factorial design: 1) early predation (only during week 1), 2) late predation (only during week 2), 3) continuous predation (during both weeks), and 4) predator exclusion control.  Landscape context was quantified by mapping all habitats in a 2 km radius from focal fields, and natural enemy abundance in focal fields was monitored with sticky traps.

Results/Conclusions

We found stronger pest suppression when predators were allowed immediate access to sentinel aphid populations than when delayed for a week, although continuous predation was required to keep aphid populations low.  Contrary to previous reports from temperate agricultural landscapes, natural and semi-natural vegetation had neutral or negative effects on pest suppression, whereas landscapes with higher proportion of alfalfa within a 1.5 km radius showed increased levels of aphid suppression.  In addition, higher proportion of alfalfa at 0.5 km radius was correlated with increased natural enemy abundance in the focal fields studied. When landscapes were classified into two groups according to their proportion of alfalfa, we showed that early predation on low-alfalfa landscapes (3% of landscape) resulted in similar levels of pest suppression as late predation in high-alfalfa landscapes (11% of landscape), suggesting that early arrival of predators can compensate for low levels of predator sources in the landscape.  Our results suggest that timing of predator arrival to agricultural fields has as much importance as landscape predator abundance to mediate pest control in agroecosystems. Therefore, smaller sources of natural enemies located nearby field crops maybe as effective to suppress pest populations as larger sources located more distantly.