Box 7: NCEs, Trap Crops, and Push-Pull Systems
Enemy-induced dispersal can create large-scale shifts in spatiotemporal pest distribution, a phenomenon that may be put to use to improve pest management programs. For example, enemies that induce stable, predictable spatiotemporal pest patterns may allow for more precisely targeted pesticide applications. Another potential route is to use enemy-induced dispersal in tandem with trap-cropping or push-pull systems. Trap cropping is the use of highly attractive “trap” plants to lure pests out of the main crop, whereas push-pull systems add a repellant “push” intercrop to the “pull” trap crop (Cook et al. 2007). Enemies may be utilized as a second “push”, driving pests out of the main crop and into the trap crop. This effect was studied by Lee et al. (2011) who demonstrated an increased level of whitefly dispersal from poinsettia into the cucumber trap crop when natural enemies were present in poinsettia. Whiteflies preferred settling in cucumber over poinsettia, but once settled in poinsettia, they did not tend to move to cucumber. Of the three natural enemies tested, only one increased whitefly dispersal into cucumber, demonstrating the importance of the specific pest and enemy pairing in this scenario.
Predictable and stable movement of pests from the main crop into the trap crop may be more likely with certain combinations of enemy, pest, and plant traits. Ideally, enemies would primarily occupy the main crop, making it more dangerous than the trap crop and inducing pest dispersal into the trap crop. This could occur when enemies are habitat specialists with a strong preference for the main crop, due to plant chemical cues (Reddy 2002), oviposition site preferences (Coll 1996; Lundgren & Fergen 2006), or omnivorous needs (Coll 1996; Kopta et al. 2012). It could also occur if enemies are relatively immobile and can be released solely into the trap crop, which could be possible with inundative or inoculative biological control. Reduction of natural enemy dispersal has been a goal in other contexts, such as releasing wingless ladybirds to prevent them from leaving the focal field (Lommen et al. 2008), and it is possible that similar efforts could work at a within-field scale as well.
Complications may arise if enemies do not primarily occupy the main crop, instead preferring the trap crop, the spaces between crops, or matching pest abundance. If the enemy prefers the trap crop, it may have the opposite effect as intended, reducing pest preference for the trap crop and increasing abundance in the main crop. However, if enemies prefer the trap crop, but pests still disperse into it, the trap crop may still be effective, and enemies may then have strong effects on the pests that establish there. If enemies, perhaps ground-dwelling predators, prefer spaces between crops, then they may increase the risk of dispersal in any direction, reducing effectiveness of the trap crop. Finally, if enemies track pest distribution, they may induce dispersal both into and out of the trap crop. This could have a range of effects, depending on the timing of dispersal, cost of dispersal, and amount of trap crop. For example, if enemies track pests, forcing them to move back and forth between trap and main crops, but dispersal is very costly, the repeated dispersal may have high fitness costs for the pest. In this case, the lack of unidirectional movement into the trap crop may be more than made up for.
Just as multiple enemies may have additive, synergistic, or disruptive effects on pests, so too might natural enemies and trap cropping techniques. Pest management outcomes may be optimized with a careful consideration of pest, enemy, and crop combinations, necessitating more research on this topic beyond the promising existing studies.