Use of multiple biological control agents for control of western flower thrips

Abstract

The western flower thrips (WFT), Frankliniella occidentalis (Pergande), is a serious widespread pest of vegetable and ornamental crops worldwide. Chemical control for Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae) on floriculture or vegetable crops can be difficult because this pest has developed resistance to many insecticides and also tends to hide within flowers, buds, and apical meristems. Predatory bugs, predatory mites, and entomopathogenic nematodes are commercially available in both the US and Israel for control of WFT. Predatory bugs, such as Orius species, can suppress high WFT densities but have limited ability to attack thrips within confined plant parts. Predatory mites can reach more confined habitats than predatory bugs, but kill primarily first-instar larvae of thrips. Entomopathogenic nematodes can directly kill or sterilize most thrips stages, but have limited mobility and are vulnerable to desiccation in certain parts of the crop canopy. However, simultaneous use of two or more agents may provide both effective and cost efficient control of WFT through complimentary predation and/or parasitism. The general goal of our project was to evaluate whether suppression of WFT could be enhanced by inundative or inoculative releases of Orius predators with either predatory mites or entomopathogenic nematodes. Whether pest suppression is best when single or multiple biological control agents are used, is an issue of importance to the practice of biological control. For our investigations in Texas, we used Orius insidiosus(Say), the predatory mite, Amblyseius degeneransBerlese, and the predatory mite, Amblyseius swirskii(Athias-Henriot). In Israel, the research focused on Orius laevigatus (Fieber) and the entomopathogenic nematode, Steinernema felpiae. Our specific objectives were to: (1) quantify the spatial distribution and population growth of WFT and WFT natural enemies on greenhouse roses (Texas) and peppers (Israel), (2) assess interspecific interactions among WFT natural enemies, (3) measure WFT population suppression resulting from single or multiple species releases. Revisions to our project after the first year were: (1) use of A. swirskiiin place of A. degeneransfor the majority of our predatory mite and Orius studies, (2) use of S. felpiaein place of Thripinema nicklewoodi for all of the nematode and Orius studies. We utilized laboratory experiments, greenhouse studies, field trials and mathematical modeling to achieve our objectives. In greenhouse trials, we found that concurrent releases of A.degeneranswith O. insidiosusdid not improve control of F. occidentalis on cut roses over releases of only O. insidiosus. Suppression of WFT by augmentative releases A. swirskiialone was superior to augmentative releases of O. insidiosusalone and similar to concurrent releases of both predator species on cut roses. In laboratory studies, we discovered that O. insidiosusis a generalist predator that ‘switches’ to the most abundant prey and will kill significant numbers of A. swirskiior A. degeneransif WFTbecome relatively less abundant. Our findings indicate that intraguild interactions between Orius and Amblyseius species could hinder suppression of thrips populations and combinations of these natural enemies may not enhance biological control on certain crops. Intraguild interactions between S. felpiaeand O. laevigatus were found to be more complex than those between O. insidiosusand predatory mites. In laboratory studies, we found that S. felpiaecould infect and kill either adult or immature O. laevigatus. Although adult O. laevigatus tended to avoid areas infested by S. felpiaein Petri dish arenas, they did not show preference between healthy WFT and WFT infected with S. felpiaein choice tests. In field cage trials, suppression of WFT on sweet-pepper was similar in treatments with only O. laevigatus or both O. laevigatus and S. felpiae. Distribution and numbers of O. laevigatus on pepper plants also did not differ between cages with or without S. felpiae. Low survivorship of S. felpiaeafter foliar applications to sweet-pepper may explain, in part, the absence of effects in the field trials. Finally, we were interested in how differential predation on different developmental stages of WFT (Orius feeding on WFT nymphs inhabiting foliage and flowers, nematodes that attack prepupae and pupae in the soil) affects community dynamics. To better understand these interactions, we constructed a model based on Lotka-Volterra predator-prey theory and our simulations showed that differential predation, where predators tend to concentrate on one WFT stage contribute to system stability and permanence while predators that tend to mix different WFT stages reduce system stability and permanence.