Woody ornamental nurseries in the southeastern United States are diverse agroecosystems, where plants from dozens of species and genera may be grown in adjacent blocks or container yards. The diversity within the nursery results in an array of potential pest problems, depending on the time of year or plant species in question. In vegetable and row crop systems, key pests have been well characterized and management plans are available for most regions in the National integrated pest management (IPM) Database, where fewer resources are available for nursery growers (Southern IPM Center, 2018). The sheer diversity of plant material makes ornamental production more difficult to manage, given that pest complexes on most plants have not been studied at all, and where they have, severity of damage may differ between related species and cultivars. However, several major pest groups that attack a wide range of woody host plants, including broad mite, spider mites, eriophyid mites, thrips, and whiteflies (Southern Nursery IPM Working Group, 2014). In field and container nursery production in the southeastern United States, these pests are generally managed through cultural practices and rotations of conventional pesticides (North Carolina State University, 2017).
In recent years, augmentative biological control has become the core component of IPM programs in greenhouse ornamental production and other vegetable and fruit cropping systems (Buitenhuis et al., 2015; Pilkington et al., 2010). Most of the applied biological control research in ornamentals as a whole has been conducted on greenhouse floriculture and targeted specific pests and crops such as whitefly in poinsettia [Euphorbia pulcherrima (van Driesche and Lyon, 2003)] or spider mites and thrips on roses [Rosa sp. (Casey et al., 2007)]. The same cannot be said of woody ornamental nursery production, where relatively little applied research has been conducted on the use of biological control agents for pest management. Most of the work conducted to date in nursery production has been performed by biological control companies consulting with private nursery owners to address specific pests.
A large-scale evaluation of persimilis mite (Phytoseiulus persimilis) for control of two-spotted spider mite (Tetranychus urticae) on croton (Codiaeum variegatum) and areca palm (Chrysalidocarpus lutescens) was conducted in a Florida production facility (Cashion et al., 1994). Release of predatory mites on croton reduced the number of miticide applications by 87% to 92% compared with those made to control plants. No miticides were required on the areca palms treated with predatory mites over the 8-month trial period, whereas the control block required miticide applications at 10- to 14-d intervals. Subsequent releases were made every 2 weeks with one predator per plant. Pratt and Croft (2000) evaluated fallacis mite (Neoseiulus fallacis) and determined it was the best spider mite control agent in Pacific northwestern U.S. woody landscape plants such as freeman maple (Acer ×freemanii), apple (Malus sp.), spirea (Spirea bumalda), and rhododendron (Rhododendron sp.) because of its relatively wide host range and overwintering ability. Pratt et al. (2002) also compared control of spider mites by fallacis mites on 30 cultivars of landscape plants with different growth habits and found that canopy density was a strong factor for success, with predators performing better on shrubs and herbaceous perennials and less on conifers and shade trees. The authors did not observe differences in spider mite control between containerized and field-grown plants.
One predatory mite of particular interest for use in woody ornamental nursery production is the swirski mite. Introduced in 2005 to the market, swirski mite is currently released by commercial growers in more than 50 countries and can feed, develop, or do both on prey in assorted families of mites (Eriophyidae, Tarsonemidae, Tenuipalpidae, and Tetranychidae) and insects (Aleyrodidae, Aphididae, Diaspididae, Phoenicoccidae, Pseudococcidae, Thripidae, Noctuidae, Pyralidae, and Psyllidae) (Calvo et al., 2015). Swirski mite is an effective predator of broad mite (Onzo et al., 2012; Peña and Osborne, 1996; Tal et al., 2007; van Maanen et al., 2010) and can reproduce on various species of thrips, whitefly, spider mite (Calvo et al., 2015), insect eggs (Delisle et al., 2015), and asian citrus psyllid [Diaphorina citri (Juan-Blasco et al., 2012)]. Populations of swirski mite can be maintained on various species of pollen if no other insect food sources are available (Goleva and Zebitz, 2013). This flexibility in diet allows populations of swirski mite to establish and survive long periods in the absence of suitable prey (Goleva and Zebitz, 2013; Xiao et al., 2012).
In addition to its diet flexibility, swirski mite performs better than other commercially available mite predators at higher temperatures in the 30 to 32 °C range (Lee and Gillespie, 2011) and under simulated summer greenhouse conditions with widely fluctuating temperature and humidity levels (Hewitt et al., 2015). It can establish on mock orange [Murraya paniculata (Juan-Blasco et al., 2012)] and citrus [Citrus sp. (Palevsky et al., 2003)], suggesting it may also be an effective option in other ornamental trees and shrubs. Because some trees, such as flowering dogwood, prefer shaded conditions but are often grown in full sun (Burrows et al., 2015), there is a need for flexibility in a predator’s tolerance to different growing conditions, which can change from year-to-year or within the same season. For these reasons, swirski mite appears to have good potential as a tool for woody ornamental production in the southeastern United States.
Priorities for pest management research listed by the Southern Nursery IPM Working Group (2014) include eriophyid mites, spider mites, broad mite, and thrips—all pests that swirski mite will readily attack. Of particular interest are new methods for managing broad mite, which can be difficult to control with miticide applications alone. Based on the needs of the southeastern U.S. nursery industry and the dearth of efficacy data on the use of this predatory mite in woody ornamental production, this research was undertaken.
The goal of this study was to evaluate swirski mite in woody ornamental container production using a controlled-release sachet method of deployment. The sachet deployment method is marketed as a way to reduce the number of application intervals by allowing continued introduction of predators from the breeding colony kept within the sachet. However, detail is lacking on the appropriate use of sachets in woody ornamentals with regard to efficacy on small and large canopy trees, timing of first application, efficacy in shade vs. sun, etc. Deployment recommendations are based on greenhouse crops such as pepper (Capsicum annuum). Such recommendations may or may not correlate directly with the canopy size of individual trees in ornamental production. Often, the canopies of greenhouse or row crops are continuous, creating one large interconnected patch within which predators can disperse in search of prey (Casey and Parrella 2005). Woody plants can be arranged in such a way that each tree canopy is its own separate patch, making it more likely that predators will deplete their food sources and more difficult for the predators to disperse across the crop. Current sachet application recommendations also suggest that deploying sachets a few weeks before pest pressure is known to peak, giving the predators time to build up their populations.
Given the complexity of nursery production, it might be more appropriate to time the application of sachets with normal nursery routines such as transplant of liners into containers or deployment of overwintered material into container yards. In that way, the addition of sachets to the containers can be incorporated into current labor activities. In addition to the aforementioned concerns, there are also questions about the variability of production methods for certain plants, particularly those that might be grown in shade and sun. As microclimate could potentially alter the efficacy of both the sachets (Shimoda et al., 2017) and predators (Hewitt et al., 2015; Lee and Gillespie, 2011), it is important to know if this particularly release method will provide similar results under both conditions.
To assess the potential for the use of swirski sachets in outdoor container production of trees, we identified three questions of interest related to production practices. These questions address canopy size effects, timing of incorporation of sachets into the current production cycle, and differences of microclimate in trees grown under shade or full sun. The three experiments were conducted to answer the following questions: 1) Do the mites from the controlled-release sachets perform differently on small trees and large trees? 2) What is the optimal number of applications in the production cycle following transplant of rooted cuttings? 3) Is there a difference in the performance of sachets under shade cover or full sun?
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