Aphid hide-and-seek: why within-plant location of pests matters in a biological control program for multiple aphid species

During the application of biological control in any crop, a multi-species pest complex can complicate matters. Greenhouse floriculture growers in the Northeastern U.S., Canada, and the UK currently have to contend with both green peach aphid (Myzus persicae Sulzer) and foxglove aphid (Aulacorthum solani Kaltenbach).  Previous short-term studies with the predator Aphidoletes aphidimyza Rondani (Diptera: Cecidomyiidae) indicate that this natural enemy prefers to oviposit in aphid patches on new plant growth, leading to differential attack rates for the two aphid species (A. solani preferring to feed on lower leaves).  Here, we used longer-term trials to determine how these oviposition decisions affect control outcomes, and to see if crop growth stage had an effect.  When both M. persicae and A. solani were present, control of M. persicae by A. aphidimyza was consistent at all plant stages, with 74 - 99% control achieved.  In contrast, control of A. solani was inconsistent in the presence of M. persicae (14 - 80% control).  Highest control rates of A. solani were seen when plants were budding, likely due to a higher proportion of aphids moving from lower leaves to meristems/plant buds.   Further trials revealed that control of A. solani was significantly greater in the absence of M. pericae than in its presence.  This study not only illustrates how variation in within-plant distribution of pests can affect the level of biological control, but also that apparent competition can be an important consideration within pest complexes, even when innundative releases of a natural enemy are used.