Western tarnished plant bug (Lygus hesperus), also known as lygus bug, is a major pest of strawberries in California.  Ideal environmental conditions and the diversity and abundance of crop and weed hosts that support the pest contributed to its increasing populations.  Twospotted spider mite (Tetranychus urticae), greenhouse whitefly (Trialeurodes vaporariorum), western flower thrips (Frankliniella occidentalis), and some lepidopterans are other major pests in California strawberries. 

According to the Pesticide Use Report, there was an increase in the use of six pesticides from 2011 to 2012 and four of them were applied for managing the lygus bug in California strawberries (CDPR, 2013).  An increase of 15% in abamectin, 62% in acetamiprid, 19% in bifenthrin, and 29% in novaluron was reported during this period while the harvested strawberry acreage increased from 38,000 to 38,500 during this period.  The primary target of these four insecticides was lygus bug.  There was a 6% decline in fenpropathrin and 11% decline in malathion, which are mainly used for controlling whiteflies.

Strawberry acreage and pesticides (pounds of active ingredient) used from 2008 to 2012 in California.

About a quarter million pounds of pesticides were applied for arthropod pest management in strawberries in 2012 and about 80% of them were chemical pesticides.  The remaining 20% are biopesticides and botanicals, among which 19% were Bacillus thiringiensis-based pesticides used against lepidopteran pests.  That leaves a very small proportion (0.53%) for pytherins, azadirachtin, and entomopathogenic fungus, Beauveria bassiana.  There was a 36% reduction in the use of azadirachtin in 2012.

Various chemical, botanical, and biological pesticides used in 2012 in California strawberries (above) and their proportion (below)

My recent studies demonstrated the potential of azadirachtin, B. bassiana, rosemary and cotton seed oil mix, and some bacterial-based insecticides in managing lygus bug, and spider mites on strawberries (Dara 2013, 2014, and unpublished).  A balanced approach where chemical pesticides are rotated and combined with other alternatives can improve pest management efficacy and promote environmental safety.

References

CDPR.  2013.  Summary of pesticide use report data 2012.  California Department of Pesticide Regulation, Sacramento, CA.  http://www.cdpr.ca.gov/docs/pur/pur12rep/comrpt12.pdf

Dara, S.  2013.  Field trials for managing aphids on broccoli and western flower thrips on lettuce.  April issue of CAPCA's Adviser magazine, pp. 29-32.

Dara, S.  2014.  New strawberry IPM studies with chemical, botanical, and microbial solutions.  February issue of CAPCA Adviser magazine, pp 34-37.

Strawberry field with spider mite infestation.  Plants in the lower part of the photo are stunted due to twospotted spider mite damage (Photo by Surendra Dara)

Multiple species of spider mites infest strawberries in California.  Twospotted spider mite (Tetranychus urticae) is a predominant and commonly occurring species in all strawberry growing regions on the Central Coast.  Strawberry spider mite (Tetranychus turkestani) can also be found along with twospotted spider mite populations especially in the Santa Maria area during warmer parts of the production season.  Another species of spider mite, Lewis mite (Eotetranychus lewisii) appears to occur in the Oxnard area in fields near caneberries.  Lewis mites have been known to occur in the Santa Maria area at very low numbers.  Carmine spider mite (Tetranychus cinnabarinus) may also be present at low numbers in some areas.

Damage: Spider mites are usually present on the under surface of the leaf and form webbing at high densities.  They scrape the leaf surface and feed on the plant juices.  Damage appears as yellow mottling on the upper leaf surface initially.  As the feeding continues, damage leads to scarring and eventual drying of the tissue.  Uncontrolled populations can cause stunted plant growth, yield reduction, or even death of the plant.

In the Santa Maria area, spider mites are reported to occur as early as in December soon after planting in some years.  Aggressive management practices including the timely release of predatory mites can keep spider mite populations under control.  Warm, dry, and dusty conditions can favor spider mite population buildup.

Yellowing (above), drying of leaves (below) from spider mite damage.  Webbing is extensive at high population densities. (Photos by Surendra Dara)

Biology: For twospotted spider mites, eggs are round and almost translucent when deposited.  Larvae emerge from eggs and have three pairs of legs.  Larvae pass through protonymph and deutonymph stages before becoming adults.  Except for the larvae, remaining mobile stages have four pairs of legs.  Two dark spots on the either side of their body give these mites their name.  Adult males are small and wedge-shaped.  Adult females are oval and larger than males. Twospotted spider mites complete their life cycle (from egg to adult stage) in about seven days at 86^oF.

Life stages of the twospotted spider mite.  Eggs (round and translucent), larva (three pairs of legs), protonymph, deutonymph, adult male (wedge-shaped), and adult female.  (Photo by Surendra Dara)

Read more about spider mites and their damage in my previous articles at http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=5771  and http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=11600. 

Monitoring: Regularly monitor fall-planted strawberry crop during the first four months for spider mite infestations.  Collect 10 mid-tier leaflets/acre from a small field (5 leaflets/acre if the field is large) and count the number of mites using a hand lens or using the mite brushing machine.  Economic threshold is 5 mites/mid-tier leaflet.  For summer-planted strawberry crop, the threshold is 10 mites/mid-tier leaflet during the first few months.  After harvesting commences, thresholds are higher and range between 15-20 mites/mid-tier leaflet.

Management options

• Thorough and regular monitoring is important to make treatment decisions.
• Promote vigorous plant growth through adequate chilling, water and nutrition management.
• Avoid excessive nitrogen as it may increase mite infestations.
• Use low fences, drive slowly and water roadways to prevent dusty conditions.
• Avoid practices that disrupt natural enemy populations and use miticides that are safe to natural enemies. 
• Alternate chemicals with different modes of action to reduce the risk of resistance development where strawberries are continuously grown. 
• If you suspect mite resistance to a particular chemical, do a simple test before spraying.  Prepare a small quantity of the chemical to be sprayed at a concentration equal to the field application rate.  Dip a mite-infested leaf in the liquid, let it dry and then keep it in a container or a bag.  For comparison, dip another mite-infested leaf in plain water and keep it separately.  Observe the leaves one and two days after the treatment.  Depending on how many mites are dead on treated leaf compared to the control (water treatment), the effectiveness of the chemical can be determined.
• Conserve or release natural enemies like predatory mites.  Phytoseiulus persimilis is a specialist predator and is effective early in the season.  Neoseiulus californicus, which tolerates warmer temperatures and a wide range of humidities, is a generalist predator and a predominant species later in the season.  Studies indicate that P. persimilis is not an option for controlling Lewis mite.  There are several other natural enemies like minute pirate bug (Orius tristicolor), a small, black lady beetle (Stethorus spp.), a small, black rove beetle (Oligota oviformis), big-eyed bugs (Geocoris spp.), brown lacewings (Hemerobius spp.), green lacewings (Chrysopa spp.), six-spotted thrips (Scolothrips sexmaculatus), damsel bugs (Nabis spp.), a cecidomyiid fly maggot (Feltiella acarivora), and a predaceous midge.

 Predatory mite (Neoseiulus sp.) and its larger and oblong egg (top right corner) along with small round spider mite eggs (bottom).  (Photo by Surendra Dara)

Female twospotted spider mite and its eggs along with adult predatory mite, Phytoseiulus persimilis. (Photo by Jack Kelly Clark)

Bagrada bug adults on grass (Photos by Bryan Jones, San Francisco)

Invasive Bagrada bug continues to spread to various counties in California.  Compared to previous years, reports of Bagrada bug infestations came in quite late in 2013.  They also seemed to survive cold winters.  How they survive cold temperatures when there are no cultivated or weed hosts is not clear, but they can hide under vegetation, in the crop debris, or the top layer of soil.

Bryan Jones of San Francisco just reported seeing a Bagrada bug on his backyard grass.  This is probably the first report of its spread to San Francisco.  It was also reported from Fresno, Monterey, San Benito, and San Bernardino Counties, suspected to be present in Santa Cruz County in 2013, and appears to be in Kern County for a couple of years or more. Out side California, it is present in La Paz, Maricopa, Pinal, and Yuma Counties in Arizona, Luna, Santa Fe, Socorro, and Valencia Counties in New Mexico, and some parts of Utah and Texas.  Unlike other invasive pests such as the brown marmorated stink bug or the Asian citrus psyllid which moved to the west coast from eastern parts of the US, Bagrada bug is moving to the east.

Bagrada bugs on Brussels sprouts (above) and radish in Bakersfield (Photo by Surendra Dara)

Regular monitoring, timely treatments during the early stages of plant development, crop rotation with non-host plants are some of the available management options.  Using sweet alyssum or other hosts as trap crops does not seem to be an effective strategy accordingly to some research.

Bagrada bug Pest Alert can be downloaded from: http://www.ipm.ucdavis.edu/PDF/pestalert/bagradabug.pdf

Fungi such as Beauveria bassiana, Isaria fumosorosea, Metarhizium brunneum are pathogenic to mites and insects and are primarily used for pest management.  Some of these are known to endophytically colonize plants and offer protection against arthropod pests feeding on those plants.  Some studies have indicated that entomopathogenic fungi can also provide protection against plant pathogens.

In an effort to explore the endophytic potential of entomopathogenic fungi for strawberry pest management, studies were conducted in 2010 using commercial and California isolates of B. bassiana and M. brunneum where B. bassiana successfully colonized strawberry plants and persisted for up to 9 weeks in various plant tissues.  Observations during this study suggested that endophytic entomopathogenic fungi could be aiding in plant health probably through mycorrhizal activity by improving water and nutrient absorption.  Based on these observations, a small study was conducted to evaluate the impact of B. bassiana on strawberry plant health in comparison with a commercial product intended to promote plant growth, health, and yield.

This study was conducted in collaboration with Los Angeles County Pitchess Detention Center where inmates assisted in taking care of the plants and collecting data.  As a part of the MERIT (Maximising Education Reaching Individual Transformation) Masters program, some inmates were selected to participate in this project.  Since this was the first project in such collaboration, a simple experiment was designed for easy execution and data collection.

Treatments included i) Untreated control, ii) HYTA – which contains soil-based microorganisms that enhance nitrogen absorption, solubilize nutrients, and build soil organic matter, and iii) B. bassiana (Mycotrol –O).  Transplants of the strawberry variety Monterey were treated by applying HYTA or B. bassiana to the root system in transplant treys.  Planting was done 48 hours after treatment in 20X5X2' raised beds.  Plants were regularly watered and fertilized with fish emulsion.  Plant health was monitored for about three months starting from 4 weeks after planting.  Plants were periodically observed and their health was rated on a scale of 0-5 where 0= dead, 1=weak, 2 and 3=moderate, 4=good, and 5=very good.  

Strawberry plants were grown in 20X5X2' wooden beds.  (Photo by Adrianne Ferree)

Except for a few aphids on some plants, the trial did not have any pest infestations.  Plants treated with B. bassiana were rated higher than untreated control or those treated with HYTA throughout the observation period although differences were not statistically significant on all observations dates.  While the plant health rating ranged from 2.3 to 3.0 for untreated control, it was between 2.6 and 3.5 for those treated with HYTA and 2.9 to 4.1 for those treated with B. bassiana.  The average seasonal health rating was 2.8, 3.2, and 3.6 for untreated, HYTA, and B. bassiana, respectively.  Fruit yield could not be monitored due to some technical difficulties. 

Strawberry plant health from treating with HYTA and B. bassiana compared to untreated control

Beneficial microorganisms establish symbiotic relationship with plants and serve as extended root system improving the absorption of nutrients and moisture.  By colonizing the roots and other plant tissues, they can also provide protection against plant pathogens.  Several commercial formulations of fungus and bacteria based beneficial organisms are currently available for use on various crops.  This preliminary study demonstrates that B. bassiana promotes plant health and can play an important role in crop production in addition to the primary role of pest management.   Additional studies can further explore the potential of entomopathogenic fungi in promoting plant growth and improving yield as well as providing protection against pests and diseases.

Adrianne Ferree, Assistant Director, Jail Enterprises Unit collaborated in this project with the support of Chief Alex Yim.  One of the two inmates who participated in this project took an online Plant Science course and used this study as his class project.  He intends to pursue agronomy after his release.  It is a great experience for me to be involved in the Maximizing Education Reaching Individual Transformation (MERIT) Masters program and introduce agricultural research to the inmates.

Acknowledgements: Thanks to Mike Fahner, Cedar Point Nursery for providing transplants and Joe Coelho, DB Specialty Farms for providing drip tapes and plastic mulch. 

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References

Bisutti, I. L., S. Steen, and D. Stephan.  2013.  Does Metarhizium anisopliae influence strawberries in presence of pest and disease?  XLVI Annual meetings of the Society for Invertebrate Pathology, August 11-15, Pittsburgh, PA.

Dara, S.K. and S. R. Dara.  2010.  Endophytic colonization of entomopathogenic fungi in strawberry plants.  XLIII Annual meetings of the Society for Invertebrate Pathology, July 11-15, 2010, Trabzon, Turkey.

Dara, S. K., S. R. Dara,. and S. S. Dara.  2013.  Exploring the potential of Beauveria bassiana and azadirachtin for managing strawberry pests.  Proceedings of the II International Strawberry Congress, September 4-6, 2013, Antwerp, Belgium.

Miller T.C., W. D. Gubler, F. F. Laemmlen, S. Geng, and D. M. Rizzo. 2004.  Potential for using Lecanicillium lecanii for suppression of strawberry powdery mildew, Biocon. Sci. Technol., 14: 215-220.

Ownley, B.H., R.M. Pereira, W. E. Klingeman, N.B. Quigley, and B. M. Leckie. 2004. Beauveria bassiana, a dual purpose biocontrol organism, with activity against insect pests and plant pathogens. Lartey, R.T., Caesar, A.J., editors. Research Signpost, Kerala, India. Emeging Concepts in Plant Health Management. 2004. p. 255-269.