Why are phytoseiid predatory mites not effectively controlling Echinothrips americanus?

The poinsettia thrips, Echinothrips americanus Morgan (Thysanoptera: Thripidae), is a key pest of various ornamental and vegetable greenhouse crops. As current biological control alternatives lack efficiency, applying chemicals remains the dominant control strategy, thereby heavily disturbing the biocontrol-based integrated management of other pests. For a range of other thrips pests, phytoseiid predatory mites have shown to be effective biocontrol agents, being able to overcome the thrips' physical and chemical defense armory. Here, we investigated potential underlying causes for the lack of phytoseiid efficacy in controlling E. americanus. First, we assessed the nutritional value of E. americanus for the predatory mite Amblydromalus limonicus (Garman and McGregor) (Acari: Phytoseiidae) when its physical or chemical defenses were eliminated by freezing the thrips. The phytoseiid could complete its immature development when frozen thrips instars were offered, but not when these were offered alive. Subsequently, we tested whether adult female A. limonicus had a higher predation rate on first instar E. americanus when they had been given experience with either live or frozen E. americanus during their immature development (i.e., conditioning). Conditioning significantly increased the predation capacity of the phytoseiid. Finally, we tested the control potential of conditioned A. limonicus versus naïve ones when exposed to E. americanus on sweet pepper plants. In contrast to the laboratory trials, at the plant level, conditioning did not yield better control. Possible factors explaining insufficient control of E. americanus by phytoseiids are discussed.