Spider mites are an important arthropod pest of strawberries in California. Plants are especially sensitive to damage early during the season. While the Oxnard area strawberries were hit by early and heavy infestations of spider mites, especially the twospotted spider mite, Tetranychus urticae, several growers in the Santa Maria area reported achieving good control by releasing predatory mites and timely applications of miticides. Predatory mites continue to play a major role in managing spider mites in strawberries and it is important to understand the impact of miticide applications on these beneficial arthropods.
Miticides vary in their impact on natural enemies and their safety to predatory mites is usually determined based on laboratory assays. This information is critical in making treatment decisions when predatory mites are also used for spider mite management. This article presents predatory mite (Phytoseiulus persimilis and Neoseiulus spp.) data from miticide evaluation studies conducted on commercial strawberry fields in 2011, 2012, and 2013 in Santa Maria.
2011: A small study was conducted to evaluate the efficacy of abamectin (Agri-Mek 0.15 EC, 16 fl oz/ac) and cyflumetofen (Nealta SC, 13.7 fl oz/ac) where treatments were applied twice using a backpack sprayer. A 100 gal/ac of spray volume was used. The second treatment was made 21 days after the first. Ten mid-tier leaflets from each plot were randomly collected and mites were counted using a mite brushing machine. Predatory mite numbers were available only from observations made 27 days after the second treatment. The average number of predatory mite adults was 2.5, 0.5, and 0.75/leaflet for untreated control, abamectin, and cyflumetofen, respectively.
2012: A small plot study was conducted using abamecting (Agri-Mek 0.15 EC, 16 fl oz/ac), bifenazate (Acramite 50WS, 1 lb/ac), entomopathogenic fungus, Beauveria bassiana (BotaniGard 22WP, 4 lb/ac), B. bassiana (BotaniGard 22WP, 4 lb/ac) + fenpyroximate (Fujimite 5EC, 2 pt/ac), cyflumetofen (Nealta SC, 13.7 fl oz/ac), fenpyroximate (Fujimite 5EC, 2 pt/ac), and spirotetramat (Movento, 5 fl oz/ac). Treatments were applied only once using a backpack sprayer (200 gal/ac for all and 150 gal/ac for B. bassiana treatments) and mite counts were made 0, 3, 7, 14, 21, and 28 days after treatment (DAT). Four days after the treatment, Microthiol Disperss was applied as a fungicidal treatment across the entire field and that could have had an impact on spider mite and predatory mite populations. Average number of predatory mite egg and mobile stages varied across different observation dates, but the differences were not statistically significant (Tukey's HSD P > 0.05).
Pre-treatment numbers and post-treatment average for eggs and mobile stages of predatory mites, 2012.
Percent change in eggs and mobile stages of predatory mites after treatment (average for post-treatment counts), 2012.
It was not clear why there were fewer predatory mite eggs and mobile stages in untreated control than some of the treatments throughout the observation period. When the percent change in post-treatment average was compared to the pre-treatment average, there was a 50-480% increase in predatory mite eggs and 7-280% increase in mobile stages in various treatments except for bifenazate where there was a 37% decrease in the number of eggs and no change in mobile stages.
2013: In a small plot trial, the efficacy of bifenazate (Acramite 50WS, 1 lb/ac), abamecting (Agri-Mek SC, 4.29 fl oz/ac), B. bassiana (BotaniGard ES, 1 qrt/ac) + bifenazate (Acramite 50WS, 0.75 lb/ac), Eco-Mite (rosemary and cotton seed oil, 1%), fenpyroximate (Fujimite 5EC, 2 pt/ac), fenpyroximate (Fujimite XLO, 2 pt/ac), Chromobacterium subtsugae strain PRAA4-1 (Grandevo, 2 lb/ac), Burkholderia spp. strain A396 (Venerate XC, 2 gal/ac), and cyflumetofen (Nealta SC, 13.7 fl oz/ac) was compared. Treatments were applied twice at weekly intervals using a backpack sprayer at 150 gal/ac rate. Mites were sampled 3 and 7 days after each application.
There was some variation in predatory mite populations in treated and untreated plots throughout the observation period. Significant differences were seen only on observations made 3 days after the first spray in eggs and 7 days after the first spray in mobile stages (Tukey's HSD P < 0.05).
Number of eggs and mobile stages of predatory mites on 3 and 7 days after first and second applications of various miticides, 2013.
Although the number of predatory mites or their eggs was not statistically different, average for four observation dates indicates their response to chemical, botanical, and microbial miticides.
These results may not correspond with those from laboratory studies conducted under controlled conditions, but they show the relative abundance of predatory mites in response to various miticides under field conditions./span>
Natural enemies play an important role in managing pest populations. Using predatory mites in strawberries against spider mites is a good example where growers take advantage of the potential of the natural enemies. Multiple species of predatory mites are commercially available and several California strawberry growers use biological control as a complementary tool to chemical control.
Predatory mites belong to four categories – Type I, Type II, Type III, and Type IV.
Type I: These predatory mites are specialists feeding exclusively on spider mites (family Tetranychidae) that produce considerable webbing. They require feeding on spider mites for their survival and reproduction. Type I predators are aggressive and voraciously feed on pest mites. Because of their dependence on spider mites, Type I specialists rapidly decline and cannibalize when pest mite populations decline.
Type II: These are also specialist predators, but they feed on spider mites and other species of mites. They also feed on pollen and in some cases on thrips and other species of predatory mites. Having more food choices for survival and reproduction, Type II specialists continue to be present in the absence of spider mites and are less likely to cannibalize.
Type III: These are generalist predators that feed on multiple species of mites that include spider mites, eriophyid mites, and tarsonemid mites and insects such as thrips and whiteflies. They also feed on pollen, honeydew, and plant juices. Type III generalists are also known to cannibalize and feed on other species of predatory mites in the absence of pest mites or other food sources.
Type IV: These mites primarily feed on pollen and can also feed on pest mites.
There are five species of predatory phytoseiid mites (Acari: Phytoseiidae) that are commercially available for spider mite control.
Phytoseiulus persimilis Athias-Henriot is a Type I specialist that exclusively feeds on spider mites. It is bright orange, teardrop-shaped and moves rapidly. It prefers cooler temperatures and is sensitive to hot and dry conditions. So, it is more effective during earlier parts of the production season before temperatures increase.
Egg and adult of Phytoseiulus persimilis. Egg is slightly oval and larger than twospotted spider mite eggs, which are round. Being able to identify predatory mites and their eggs is important during pest monitoring (Photo by Surendra Dara).
Neoseiulus fallacis (Garman) is a Type II specialist that primarily feeds on spider mites. It is translucent to peach or orange and appears to have a flatter body compared to spider mites or P. persimilis. It is also sensitive to hot and dry conditions.
Neoseiulus californicus (McGregor) is a Type II specialist that primarily feeds on spider mites, but also has Type III generalist characters. It appears similar to N. fallacis. It can withstand warmer conditions better than P. persimilis and N. fallacis. It can withstand cold temperatures for short periods and tolerates relative humidity range from 40-80%.
Galendromus occidentalis (Nesbitt) also known as western predatory mite is a Type II specialist that primarily feeds on spider mites. It prefers warm temperatures and tolerates dry conditions as low as below 30% relative humidity. It is sensitive to cooler temperatures.
Amblyseius andersoni (Chant) is a Type III generalist predator. It can tolerate high temperatures when relative humidity is high.
Among these predatory mites, P. persimilis, N. fallacis, and N. californicus are the most commonly used species in strawberries. Using the right species depending on the environmental conditions is important for the success of biological control. Timing insecticide and acaricide applications in a manner that is least disruptive to the predatory mites is essential. When chemical pesticides are necessary, softer materials should be selected.
In addition to the predatory mites, several species of natural enemies feed on spider mites. They include big-eyed bug (Geocoris spp.), black lady beetle (Stethorus sp.), black rove beetle (Oligota oviformis), brown lacewing (Hemerobius spp.), damsel bug (Nabis spp.), green lacewing (Chrysopa spp.), minute pirate bug (Orius tristicolor), predatory midge (Feltiella acarivora), and the predatory sixspotted thrips (Scolothrips sexmaculatus).
Top row: Big-eyed bug, black lady beetle, black rove beetle, brown (upper), and green lacewing (lower)
Bottom row: Damsel bug, predatory midge larva, minute pirate bug, and sixspotted thrips.
Photos by Jack Kelly Clark, UC IPM.
Croft, B. A., L. N. Monetti, and P. D. Pratt. 1998. Comparative life histories and predation types: are Neoseiulus californicus and N. fallacis (Acari: Phytoseiidae) similar type II selective predators of spider mites? Econ. Entomol. 27: 531-538.
Çakmak, I. A. Janssen, and M. W. Sabelis. 2006. Intraguild interactions between the predatory mites Neoseiulus californicus and Phytoseiulus persimilis. Exp. Appl. Acarol. 38: 33-46.
Rincon-Vitova. 2009. Catalog of beneficials. Rincon-Vitova Insectaries, Inc. (http://www.rinconvitova.com/CATALOG%202009%20screen.pdf)
Integrated pest management recommends using multiple approaches such as selection of resistant varieties, manipulation of habitat and modifying cultural practices in a manner that is not beneficial to the pest, and conserving natural enemies before using pesticides. In certain production systems that have well established agronomic practices with limited scope for modifying the habitat or cultural practices, pest management is primarily dependent on pesticide applications. There are several options for conventional strawberry production where pesticides from different classes (based on their mode of action) can be used against one or more pests. However, these options are limited in organic strawberries. As a result, some growers put a major emphasis on conserving natural enemies and timing the application of organic oils, pyrethrins, and other products in a manner that is least disruptive to natural enemies.
In a 2009 study, seasonal average for lygus bug (Lygus hesperus) numbers did not show a specific trend between organic and conventional strawberry fields. The average number of nymphal and adult stages of lygus bugs, from mid-May to the end of August, was 12.6 from 20 plants in an organic field. This average was 15.6 (between mid-May and mid-July) in a conventional field and 7.7 (between mid-May and mid-August) in another conventional field. Conventional fields received several pesticide applications while the organic field received only two during the study suggesting that pesticide applications did not really help to bring lygus populations below threshold levels (1 lygus bug nymph per 20 plants). As the natural enemy populations were not monitored during this study, the impact of pesticide applications on their numbers or their role in managing lygus populations was not clear.
To understand the lygus bug and their natural enemy interactions more, another study was conducted in 2013 on a commercial organic and a conventional strawberry field in Santa Maria. Observations were made at weekly intervals from June 24 to July 15 where 10 randomly selected plants from each of the four quadrants of an acre area were sampled using a beating tray. The number of young (1st-3rd instar) and mature nymphs (4th to 5th instar) and adults of lygus along with their natural enemies – big-eyed bugs (Geocoris spp.), minute pirate bugs (Orius spp.), green lacewings (unknown), damsel bugs (Nabis spp.), parasitic wasps (unknown), lady beetles (multiple species), and spiders (multiple species) was recorded.
Among various materials that were applied in the conventional field, abamectin (Epi-Mek 0.15 EC applied once), sodium tetraborohydrate decahydrate (Prev-Am, applied thrice), and sulfur (Microthiol Dispress applied once) were primarily used as acaricides. Novaluron (Rimon 0.83EC) was applied on May 22, about one month prior to the first sampling. Aadjuvant, pinene (polyterpenes) polymers (Miller Nu Film P) was also applied thrice between May 7 and June 22. In the organic field, neem oil (Trilogy) and alcohol ethoxylate (OROBOOST) were each applied thrice between May 1 and July 11.
A higher number of lygus bugs were seen in the organic field than in the conventional field throughout the observation period. While the older nymphs and adults stayed at or below the treatment threshold, younger nymphs were consistently above the threshold (2.5 to 4 per 20 plants) in the conventional field. In the organic field, they were consistently above the treatment thresholds during the observation period.
Occurrence of lygus bug nymphs and adults in a conventional and an organic strawberry field in Santa Maria from Jue 24 to July 15, 2013.
When the average number of natural enemies for the observation period was considered, the organic field had more big-eyed bugs than the conventional field. The organic field also had slightly higher numbers of minute pirate bug and lady beetles. Very low numbers of lacewings or parasitic wasps were present only in the organic field. In general, damsel bug numbers were also very low, but they were slightly higher in the conventional field. A higher number of spiders were seen in the conventional field compared to the organic field.
Although some key species of natural enemies were present at higher numbers in the organic field, they were not able to keep the lygus populations under control. On average, there were 10 natural enemies of all species per 20 plants in the conventional field and 12 in the organic field during the observation period. Applying novaluron at the right time appeared to control lygus bug nymphs and prevented population build up there after in the conventional field. While natural enemies generally play a significant role, this study suggests the importance of other options such as pesticides for managing lygus bugs.
Acknowledgements: Thanks to Dave Peck (Manzanita Berry Farms and Eraud Farms, Santa Maria) for his collaboration and Chris Martinez for technical assistance.
Western tarnished plant bug (Lygus hesperus), also known as lygus bug is a major pest of strawberries. Feeding damage results in misshapen fruit which is not marketable. Lygus bug is the target of several pesticide applications and keeping it under threshold levels continues to be a challenge for growers.
In 2009, one organic and two conventional strawberry fields in Santa Maria were monitored for two to three months between mid-May and the end of August at periodic intervals. On each observation date, 10 random plants were sampled using a beating tray from each of five different parts of an approximately 5 acre area. The number of young (1st to 3rd instar) and mature (4th to 5th instar) nymphs and adult stages of lygus bug were counted.
Two pyrethrin applications were made during the observation period in the organic field. Conventional field 1 received nine pesticide applications between early May to early July in the following sequence: fenpropathrin, naled+novaluron, naled, fenpropathrin, naled+novaluron, bifenthrin+novaluron, malathion+novaluron, malathion, and naled. In the Conventional field 2, six pesticide applications - fenpropathrin, malathion, bifenthrin+malathion, malathion, thiamethoxam+malathion, and bifenthrin – were made from mid-May to the end of July.
Regardless of multiple treatments, lygus bug numbers increased in all fields as the season progressed. In general, a higher number of younger nymphs was seen in all fields followed by adults and older nymphs. Lygus numbers remained above the treatment threshold (1 lygus bug nymph per 20 plants) on almost all observation dates in both organic and conventional fields. There was no particular trend in pest populations in response to the pesticide applications or the system of farming. Lygus bugs were above the treatment threshold in conventional fields with multiple pesticide applications as well as in the organic field with fewer spray applications and emphasis on natural enemy conservation. When the seasonal average for all life stages was considered, the highest number was found in conventional field 1 (15.6/20 plants), followed by the organic field (12.6), and conventional field 2 (7.7).
This study shows that lygus bug is a difficult pest to manage and necessitates the need for better management practices. Recent IPM studies (Dara 2013 and Dara 2014) with existing and new chemicals, botanical, and biopesticides showed promising results.
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./span>
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.
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.
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.