Disruption of weed biological control by an opportunistic mirid predator

Species cascades involving generalist predators, biological control herbivores, and invasive plants have great potential-to disrupt weed biological control efforts and the ability of herbivores to limit plant invasions. We found that Plagiognathus politis, a mirid bug typically reported as herbivorous and as an agricultural pest, was an avid opportunistic predator of Galerucella calmariensis, an herbivore introduced to North America for biological control of Lythrum salicaria (purple loosestrife). In field cages, adult and immature mirid bugs consumed large numbers of eggs and young larvae of G. calmariensis in addition to feeding on purple loosestrife leaves. We document a species cascade that benefited purple loosestrife; in mesocosms, mirid predation on G. calmariensis allowed purple loosestrife to maintain greater leaf, stem, and reproductive biomass than without predators. In field cages, however, top-down effects were weaker, probably because G. calmariensis had a temporal refuge from predation. An early-season window of low predation pressure (where beetle eggs hatched and eclosed earlier than mirids) allowed G. calmariensis to establish and persist within our field cages. Our experiments show that, although mirids initiated a clear species cascade in potted plant mesocosms, this was less evident in field cages, probably due to increased spatial and temporal complexity. However, since mirid predators consume large numbers of eggs and larvae, effectiveness of G. calmariensis as a biological control agent could be greatly suppressed when release sites are inhabited by P. politis. Interference of weed biological control by opportunistic generalist predators is particularly important to document because it is usually subtle and may remain unrecognized even if release efforts fail. However, despite this, herbivore establishment rates in weed control programs could be greatly improved through repeated large-scale releases that satiate potential predators, thus allowing herbivores to escape their limiting influence.