Tomatoes (Solanum lycopersicum) are grown in hoophouses (Fig. 1) throughout the U.S. with one of the major pests being the twospotted spider mite (Tetranychus urticae) (Fig. 2). The twospotted spider mite feeds on the chlorophyll content of plant cells causing damage often referred to as “stippling” or “speckling” (Fig. 3). Twospotted spider mites prefer warm, dry conditions for development and reproduction. For instance, development time from egg to adult can be completed in seven to 10 days at temperatures >80ºF. The reasons why twospotted spider mite populations are difficult to deal with when using miticides include short generation time, high female reproductive capacity (fecundity), feeding habitat (leaf undersides) and propensity to develop resistance to miticides.
We have been working with several greenhouse producers including Dan Kuhn (Depot Market, Courtland, Kan.) in developing biological control programs designed to regulate populations of the twospotted spider mite in tomatoes grown in hoophouses. Prior to implementing the biological control program, the primary method of dealing with twospotted spider mite populations involved routinely applying miticides.
However, once populations of the twospotted spider mite reached outbreak proportions (Fig. 4), it was too late to apply any miticides. As such, in order to maintain populations below damaging levels, weekly miticide applications are required. However, this level of selection pressure may result in twospotted spider mite populations developing resistance to subsequently used miticides. Another issue is that bumblebees (e.g. Bombus terrestris) are used to pollinate tomato crops (Fig. 5), so it is important to avoid directly and indirectly harming bumble bees with miticides.
Choosing a natural enemy
We decided to implement a biological control program that entailed regular releases of a commercially available natural enemy for the twospotted spider mite. The natural enemy, or biological control agent, that was released into the hoophouses was the predatory mite, Phytoseiulus persimilis. Phytoseiulus persimilis is a Type I predatory mite, which means that the predatory mite only feeds on twospotted spider mite.
Here are the general biological characteristics of Phytoseiulus persimilis:
- Most effective predatory mite for regulation of twospotted spider mite populations.
- Specialist predatory mite that feeds on all life stages (eggs, larvae, nymphs, and adults) of the twospotted spider mite.
- Bright orange to red in color, pear-shaped with long legs, and larger and more active than twospotted spider mites.
- Both adults and nymphs actively search for twospotted spider mites
- Require a temperature around 68ºF and relative humidity near 75 percent.
- Plant leaves that are touching allow the predatory mite to disperse throughout the crop.
Releases were initiated before twospotted spider mites were detected in the tomato crop. The predatory mite was shipped in plastic containers (Fig. 6), with one container used for each hoophouse. It is important to understand that biological control is a preventative strategy — not reactive.
Bi-weekly releases were conducted throughout the growing season until the crop had produced over 50 percent fruit. In the end of the first year, the tomato plants did not exhibit any symptoms of twospotted spider mite damage (Fig. 7), and we were not able to detect any twospotted spider mites during our scouting efforts.
In order to be successful in developing and implementing a sound biological control program, you have to establish a routine scouting program by monitoring for twospotted spider mites once per week throughout the growing season using the beat method, which involves shaking leaves over a white sheet of paper (Fig. 8) and assessing the presence of both the twospotted spider mites and predatory mites. The scouting program helps to determine population trends and detect localized infestations (“hot spots”) of twospotted spider mite within the hoophouses, thus targeting specific locations within the hoophouse where to release the predatory mite.
In the first year of the study, we did not release any predatory mites in hoophouses with a cucumber (Cucumis sativus) crop. What happened? Well, as shown in Fig. 9, the cucumbers were heavily infested with twospotted spider mites, which substantially impacted yield. We are in the second year of the study with tomatoes, as well as making releases of other natural enemies including the aphid parasitoid, Aphidius ervi, on pepper (Capsicum annuum), and cucumber and strawberry (Fragaria ananassa) crops.
So, what are the benefits of implementing a biological control program against the twospotted spider mite in hoophouses with tomatoes or other crops?
The benefits include:
- Less expensive than standard miticide programs (both directly and indirectly)
- Avoid stimulating problems with other insect or mite pests (“secondary pest outbreak”)
- Minimal yield losses
- less harmful to workers (no residues)
- Less impact on non-target organisms including pollinators (e.g. bumble bees
- No issues associated with miticide resistance.