Phytoseiulus persimilis, a predatory mite that devours twospotted spider mites

Dr. Tom Bilbo & Dr. Jim Walgenbach

Fig. 1. Phytoseiulus persimilis adult beside eggs of the twospotted spider mite (Photo: Biobest Sustainable Crop Management)

• Background
• Distribution
• Description
• Prey
• Life Cycle in North Carolina
• Use in Biological Control
• Scouting
• Pesticide Toxicity to P. persimilis
• Additional Resources
• References

Classification: Acarina: Phytoseiidae, Phytoseiulus persimilis (Athias-Henriot)

Background

Phytoseiulus persimilis is a specialist predator of spider mites, such as the twospotted spider mite (TSSM, Tetranychus urticae Koch). It is a highly voracious and effective predator on a number of horticultural and ornamental crops and has become a key tool for managing spider mites in greenhouses for several decades (1). Use of P. persimilis in open-field crops is not nearly as common as in greenhouse production systems due in part to environmental and agronomic characteristics as well as a lack of research on how to effectively implement P. persimilis in these systems. P. persimilis has been used very successfully in field-grown strawberries in California and Florida for a number of years, but how this predator can be implemented in other horticultural crops, such as tomatoes, is unknown (2, 3). Throughout the last several years, the Walgenbach lab at NC State has been researching how to augmentatively release tomato-reared or commercially-purchased P. persimilis in staked field tomatoes to manage TSSM, as well as how to conserve P. persimilis populations that are currently established in North Carolina (4). Despite the reported limitations of P. persimilis on tomatoes due to defensive glandular trichomes (2), our research in experimental plots and commercial fields has demonstrated excellent, season-long TSSM control when P. persimilis is applied correctly.

Distribution

P. persimilis is native to Chile, but was accidentally introduced into Germany in 1959 (1), and has since been distributed globally as an important biological control agent in greenhouses (various crops) and field-grown strawberries. It has become established in the natural landscapes in many parts of the world, including Israel, southern California, Australia, New Zealand, South Africa, and in parts of the southeastern United States, including the Piedmont region of North Carolina (2, 4, 5).

Description

Appearance: Similar to other mites, the life cycle of P. persimilis includes the egg, three immature stages (larva, protonymph, deutonymph), and the adult. Adult mites are approximately 0.5 mm and females are slightly larger than adult males. While small, P. persimilis can be easily distinguished from TSSM and other mites by its bright red-orange and pear-shaped body and long legs, with the front legs projected forward. It also moves much faster than spider mites.

Development: Under hot, humid conditions, P. persimilis can develop from egg to adult in as little as 5 days (6). It is an effective predator of TSSM when temperatures are as low as 60°F and 60-90% relative humidity, with improved performance at higher temperatures and humidity. P. persimilis, like many other predatory mites, is sensitive to humidity, which can be a limitation to their success as biological control agents in field-grown crops. As humidity decreases below 60%, successful egg hatch declines steadily, although eggs can compensate for low humidity as long as it is punctuated by periods of higher humidity (70), which almost always occurs at night in NC. It is also important to note that respiring plants affect the microclimate within a crop, generally increasing relative humidity compared to ambient environmental conditions.

Fig. 2. Eggs of twospotted spider mite (top right) are smaller and more spherical than the more oval-shaped eggs of P. persimilis (bottom left). (Photo: University of Florida, https://mrec.ifas.ufl.edu/lso/spmite/b853a3.htm)

Fig. 3. The larval stage of P. persimilis has only six legs. (Photo: University of Florida, https://mrec.ifas.ufl.edu/lso/spmite/b853a3.htm)

Prey Specificity

P. persimilis is a highly specialized predator of spider mites, and sustained egg production requires the availability of spider mites as prey (8). This dependence on the availability of spider mite prey is a crucial consideration when deciding to release P. persimilis into a crop for management of TSSM. P. persimilis locates its prey using volatile chemicals emitted by plants being attacked by TSSM. In the absence of spider mites, P. persimilis will disperse from a field in search of food (other spider mites) (9).

Fig. 4. Twospotted spider mites heavily infesting tomato leaves (Photo: Tom Bilbo)

Annual Life Cycle in North Carolina

P. persimilis is not native to North Carolina, but in recent years has become established in some parts of the state, such as the Piedmont and Coastal Plain. It was likely established from escapees intentionally released in greenhouses or field-grown strawberries. Resident field populations have been detected each year in numerous tomato fields and have been observed to aid in the suppression of late-season TSSM (August-September) (4). However, their role in suppressing TSSM in tomato is limited by its arrival late in the season (July-September), a period when TSSM populations are typically at high levels. Ongoing research is studying the spatial relationship of P. persimilis infestations to the timing and distribution of populations across the landscape the following growing season. The goal is to devise strategies to allow P. persimilis to enter tomato fields earlier in the growing season and provide more effective early season suppression of TSSM.

Biological Control Strategies

Augmentative Biocontrol: Ongoing research in the Walgenbach lab is optimizing how to release P. persimilis in field tomatoes for effective, season-long suppression of TSSM, with the goal to reduce or eliminate the need to apply miticides. Currently, P. persimilis are recommended to be released shortly after the detection of TSSM, when populations are below 0.5 mites per leaflet. P. persimilis are released at a rate of approximately 20,000 per acre. This release rate does not need to be applied to an entire field, but only to a small section (1/10^th to 1/20^th) of a field. In a released section, the predatory mites will rapidly multiply and quickly disperse to other sections of a field. One experiment conducted in an 3-acre commercial staked tomato field (with no other major TSSM-host plants nearby) demonstrated excellent TSSM suppression by releasing 2,500 P. persimilis in a 0.14 acre section of the field. Importantly, the size of the field and orientation to other crops (TSSM-host plants) may influence how and where P. persimilis disperse, which will influence the timing and extent to which it suppresses TSSM. Ongoing research is determining how these factors need to be considered when P. persimilis are released to ensure reliable TSSM suppression.

Other factors important to the success of released P. persimilis include the environmental conditions and source of P. persimilis. Releases should be early in the morning or late in the afternoon to avoid high mid-day temperatures, and not immediately prior to a major rain event. The source of P. persimilis may also be important, whether they are bean-adapted (from commercial suppliers) or tomato-adapted (cultured in-house). While predators purchased from commercial suppliers have successfully established in tomato fields, ongoing research is examining the relative success of the two strains.

Conservation Biocontrol: Probably the most important factor affecting the success of released or naturally occurring P. persimilis is the type of pesticides used during production of the crop. See Table 1 for a list of frequently used pesticides and their likely compatibility with P. persimilis and other biological control agents in tomato fields.

Future recommendations will be updated with more specific guidelines on how to properly use P. persimilis for TSSM management in field tomatoes.

Fig. 5. Releasing P. persimilis into a commercial tomato field. P. persimilis were reared on tomato plants and whole plants were placed inside the staked plant canopy. (Photo: Tom Bilbo).

Fig. 6. Dispersion of P. persimilis throughout a 3 acre tomato field originating from a small release subsection. Week 0 indicates where ~2,500 P. persimilis were released into the field. P. persimilis were detected during random sampling 3 weeks after release. Four weeks after release, P. persimilis increase dramatically in the areas adjacent to the release subsection, and twospotted spider mite numbers rapidly decline. As time progresses, the highest density of P. persimilis moves across the field like a wave, nearly eliminating twospotted spider mites in their path.

Scouting

While it is possible to scout for P. persimilis without magnification, they are very small (~0.5 mm) and a low-power hand lens (4X – 10X) can make them easier to see without slowing down the scouting process. As P. persimilis is dependent on spider mites as a prey source, scouting should only occur where TSSM are present. The body is bright red-orange, globular- and pear-shaped, and can have a noticeable shine to it under sunlight. Immature stages are smaller than adults and body color is a less-pigmented, semi-transparent orange-red. Eggs, which do require magnification to see, are approximately 0.33 mm in diameter, oblong, and approximately twice the size of TSSM eggs. P. persimilis is also usually fast moving and when disturbed on a leaf will begin walking significantly faster than other similarly-sized insects and mites present. To scout for P. persimilis in the field, randomly sample 5-10 sites throughout the entire field, at each site inspect the underside of leaves (from the upper third of the plant) from 10 consecutive plants. On a leaflet, P. persimilis are often sheltered in the crevices and pockets created by a curled leaflet. After P. persimilis have been released into a field it typically takes 3-4 weeks for the population to have grown to sufficient abundance to be detected by random sampling.

It is always best to confirm the identity of P. persimilis using a 10X visor or hand lens, and collecting several mites in a container with alcohol for further inspection, if needed. In late-Summer/early-Fall, TSSM will increasingly take on an orange-red color due to the accumulation of keto-carotenoids ahead of winter, making it increasingly important to differentiate P. persimilis with magnification.

Fig. 7. P. persimilis (middle) and TSSM (top) on the underside of a tomato leaf, viewed under a 10X hand lens (Photo: Tom Bilbo)

Fig. 8. Twospotted spider mite viewed through a 10X hand lens (Photo: Tom Bilbo)

Pesticide Toxicity to P. persimilis and other predators

See Table 1 for toxicity of commonly used pesticides in fruiting vegetables to P. persimilis and Orius spp. (hemipteran predators).

Additional Resources

• Biological Control with Predators and Parasitoids
• Vegetable Pest Management
• Walgenbach Entomology Lab
• This project was funded in part by the USDA-National Institute of Food and Agriculture Organic Research and Extension Initiative (OREI), grant number 2018-51300-28425; and the USDA-NIFA Sustainable Agriculture Research & Education program (SARE), grant number LS17-284.

References

1. Van Lenteren, J-C. 2012 IOBC Internet Book of Biological Control. Available online.
2. McMurtry, JA and BA Croft. 1997. Life-styles of phytoseiid mites and their roles in biological control. Annual Review of Entomology 42: 291-321.
3. Rhodes, EA and OE Liburd. 2006. Evaluation of predatory mites and acramite for control of twospotted spider mites in strawberries in north central Florida. Journal of Economic Entomology 99: 1291-1298
4. Ditillo, JL, GG Kennedy, and JF Walgenbach. 2016. Effects of insecticides and fungicides commonly used in tomato production on Phytoseiulus persimilis (Acari: Phytoseiidae). Journal of Economic Entomology 109: 2298-2308.
5. McMurtry, JA, ER Oatman, PA Phillips, and CW Wood. 1978. Establishment of Phytoseiulus persimilis [Acari: Phytoseiidae] in southern California. Entomophaga 23: 175-179.
6. Stenseth, C. 1979. Effect of temperature and humidity on the development of Phytoseiulus persimilis and its ability to regulate populations of Tetranychus urticae [Acarina: Phytoseiidae, Tetranychidae]. Entomophaga 24: 311-317.
7. Le Hesran, S, T Groot, M Knapp, T Bukovinszky, T Forestier, and M Dicke. 2019. Phenotypic variation in egg survival in the predatory mite Phytoseiulus persimilis under dry conditions. Biological Control 130: 88-94.
8. Walzer, A and P Schausberger. 1999. Cannibalism and interspecific predation in the phytoseiid mites Phytoseiulus persimilis and Neoseiulus californicus: predation rates and effects on reproduction and juvenile development. BioControl. 43: 457–468.
9. De Boer, JG and M Dicke. 2004. The role of methyl salicylate in prey searching behavior of the predatory mite Phytoseiulus persimilis. Journal of Chemical Ecology 30: 255-271.