Micro-sprinklers in strawberries. Photo by Surendra Dara

Strawberry is an important commercial crop in California primarily grown on the Central Coast in Watsonville, Santa Maria, and Oxnard production areas.  Strawberry crop requires 24-29” of irrigation water for a typical production season based on fall plantings.  Irrigation is primarily administered through drip tapes installed under plastic mulch during bed preparation.  In addition to the drip irrigation throughout the crop life, supplemental irrigation through overhead aluminum sprinklers is administered during the first few weeks after transplanting.  Overhead irrigation is practiced to leach out salts from the root zone and to support the establishment of new transplants.  Strawberries are sensitive to salinity and this supplemental irrigation is believed to reduce or prevent salt injury.  In the Oxnard area, overhead aluminum sprinkler irrigation is considered very important to prevent dry conditions which could result from Santa Ana winds.  However, overhead aluminum sprinkler irrigation requires a significant amount of water and can be an inefficient system.  Evaporation, limited surface area for water penetration due to plastic mulch on the beds, and potential run off are some of the disadvantages associated with this overhead sprinkler system.

               Water is an important resource for growing plants and it has become scarce due to epic drought conditions in California.  Conserving water through improved irrigation practices is a critical area for maintaining acreage of a lucrative commodity such as strawberry.  Micro-sprinklers, which are commonly used in orchard systems could offer an efficient alternative to conventional aluminum sprinklers.  Micro-sprinklers, established on strawberry beds, can deliver water in a more targeted manner with minimum or no run off.  They could also help modify the microclimate in the strawberry canopy and create humid conditions that discourage spider mite pest populations and promote predatory mites which are sensitive to dry conditions.

               A study was conducted at Manzanita Berry Farms in Santa Maria during 2014-2015 production season to evaluate the potential of micro-sprinklers in strawberry production.  Objectives of this study included i) conservation of irrigation resources without affecting strawberry plant growth and fruit yield, ii) impact on pest and predatory mite populations, and iii) impact on powdery mildew and botrytis fruit rot.

Experimental design

A block of strawberry (variety BG-6.3024 planted on 6 November, 2014) was divided into two parts with beds aligned from south to north direction.  The west half of the block was assigned for micro-sprinklers and the east half for the grower standard with aluminum sprinklers.  Each block had about 60 beds (about 306-365' long) and aluminum sprinklers were established in furrows every 40' (7-8 beds in between) while micro-sprinklers were established on every third bed.  Micro-sprinklers were placed 16' apart (on every fourth bed) and had a 15' spacing within a bed.  Within each treatment section six 20' long plots were marked to measure plant, pest, and disease parameters. 

Installing micro-sprinkler system (Field crew at Manzanita Berry Farms, Santa Maria)

Micro-sprinkler (left) and grower standard with aluminum sprinklers (right) sections of the field

Data collection and results

Irrigation – Conventional sprinkler irrigation was made 14 times from 6 to 29 November, 2015 at a rate of 125 gallons per minute while micro-sprinkler irrigation was made 1-3 day interval at a rate of 40 gallons per minute using 35 PSI pressure.  During this period, aluminum sprinklers delivered 120,000 gallons of water over 16 hours of total irrigation while micro-sprinklers delivered 81,600 over 34 hours of total irrigation.   This translates to 32% of water saving in just 3 weeks and could be more in situations where overhead irrigation is administered for extended periods.  Micro-sprinkler irrigation was continued for 15 min twice a week for the rest of the production period.  Distribution uniformity could not be measured in grower standard treatment in this study, but it is believed to be between 50-60% at 70 PSI based on other studies.  Distribution uniformity for the micro-sprinklers was 74% at 35 PSI when measured on 16 January, 2015.  When electrical conductivity (EC) was measured on January 1 and February 1, 2015, it varied between 0.47 and 0.49 dS/m in grower standard treatment and was at 0.54 dS/m in micro-sprinkler treatment.  Although EC in micro-sprinkler plots was significantly higher (P < 0.0007) than in grower standard plots, it was within the safe limit of 0.7 dS/m.

Cumulative volume of water delivered in micro-sprinkler and grower standard sections of the field.  There was a saving of 38,400 gallons per acre in just about three weeks. 

Yield – Total and marketable berry yield data were collected 2-3 times a week between 7 February and 12 June, 2015 for a total of 34 sampling dates.  There was no significant difference in total or marketable berries (P > 0.05) when the seasonal averages for grower standard and micro-sprinkler plots were compared.   During the observation period, 44,322 gr (97.7 lb) and 43,452 gr (95.8 lb) of marketable berries/plot were produced in grower standard and micro-sprinkler treatments, respectively.  

Plots were covered with netting for exclusive harvest data collection.

Marketable berry yields per plot in micro-sprinkler and grower standard sections from February to June, 2015

Total strawberry yields (marketable and unmarketable) per plot during the study period.

Plant canopy and health– Growth was recorded by measuring the width of the plant canopy across and along the bed from 20 random plants per plot on the 6^th of each month from January to March, 2015.  Plant health was monitored at the same time by on a scale of 0 to 5 where 0 = dead, 1 = weak, 2 = moderate-low, 3 = moderate-high, 4 = good, and 5 = very good.  Plants in micro-sprinkler treatment had significantly smaller canopy in January (P = 0.004) and February (P =0.0006), but caught up with the grower standard by March (P = 0.14).  Plant health rating during this period also followed a similar trend, but the differences were significant only in February (P = 0.02).

Size of the plant canopy and plant health condition from January to March, 2015.

Both micro-sprinkler and grower standard plants look equally healthy and productive (Photo taken on 26 May, 2015)

Twospotted spider mite and predatory mite – One mid-tier leaflet was sampled from each of the 10 random plants within each plot and the number of eggs, nymphs, adult pest and predatory mites were counted using a mite brushing machine.  Sampling was made once a month from February to April, 2015, but due to sparse numbers and uneven distribution useful data could not be obtained.

Powdery mildew– One trifoliate leaf from 20 random plants within each plot were collected and checked under microscope for mycelial growth and powdery mildew severity was rated on a 0 to 4 scale where 0 = absent, 1 = 1-25%, 2 = 26-50%, 3 = 51-75%, and 4 = 76-100% of leaf area with infection.  Sampling was made on 15 April and 16 and 24 June, 2015.  Powdery mildew severity was significantly less in micro-sprinkler treatment on 15 April (P = 0.009) and June 24 (P = 0.01).

Severity of powdery mildew on three observation dates.

Botrytis fruit rot – Berries harvested from each plot were kept at room temperature in plastic clamshell boxes and disease severity was measured 3 and 5 days after harvest using the 0 to 4 scale used for powdery mildew.  Observations were made on 26 March, 13 April, 22 May, and 16 June, 2015.  In general, botrytis fruit was less severe in micro-sprinkler treatment, but significant difference were seen 3 days after harvest for samples collected on 22 May and 16 June (P = 0.02).  

Severity of botrytis fruit rot when observations were taken 3 and 5 days after harvest.

Conclusions

Micro-sprinkler system contributed to a significant reduction in overhead irrigation water without affecting the marketable berry yield.  With less pressure required to deliver water through micro-sprinklers, they could also contribute to energy savings.  EC value of below 0.7 dS/m suggests that micro-sprinklers were as effective as aluminum sprinklers in leaching out salts. Due to the lack of sufficient mite infestations, the benefit of micro-sprinklers in spider mite management could not be determined.  Data also suggest that powdery mildew and botrytis fruit rot could be reduced by micro-sprinklers, but additional studies are required to confirm these preliminary observations.  An initial estimate by the vendor suggests that equipment and handling costs of the micro-sprinklers are more or less similar to those of the aluminum sprinklers.

Chris Martinez and rest of the field crew, Manzanita Berry Farms, Santa Maria after transplanting

Acknowledgements: Thanks to Dave Peck, Manzanita Berry Farms for his collaboration, Chris Martinez for his field assistance, Manzanita field crew for help with planting, irrigation, and yield data collection, Danilu Ramirez, Fritz Light, and Tamas Zold for their technical assistance, and RDO Water and Netafim for partial funding of the study.

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Reference:
Dara. S. K. 2012.  Salt injury in strawberries. UCCE eNewsletter, Strawberries and Vegetables.  http://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=6820

Western tarnished plant bug (Lygus hesperus), which is generally referred to as lygus bug, is a major pest of strawberries on the California Central Coast.  Lygus bug feeding on developing berries causes fruit deformity.  Deformed or ‘cat-faced' berries are not desirable for fresh market and lygus bug damage results in significant yield losses.  Lygus bugs typically move into strawberry fields in early to mid-spring and thrive in fall-planted and summer-planted fields during the following months through multiple generations.  Degree-day calculations and timing of treatments is difficult for lygus bug management in strawberries due to multiple sources (wild and cultivated hosts) and continuous movement of populations among different hosts.  Conventional growers typically rely on pesticide applications and use of bug vacuums is gaining popularity in the recent years.  Lygus bug management continues to be a challenge with these tools and emphasizes the need for IPM strategies that use several control options.

Studies conducted in 2012, 2013, and 2014 in commercial Santa Maria strawberry fields showed that non-chemical alternatives such as azadirachtin, entomopathogenic fungi, and bacteria-based pesticides can play an important role in managing lygus bug and other insect pests.  Such botanical and microbial alternatives were also critical in managing twospotted spider mites.  IPM approach beyond rotating chemicals among different modes of action groups is necessary for obtaining effective control, maintaining environmental sustainability, and reducing the risk of pesticide resistance.

An IPM study was conducted in 2015 at Sundance Berry Farms in the Santa Maria area using almost all available IPM tools.  The following groups of options were used in different combinations and rotations and evaluated for their efficacy against lygus bug, western flower thrips, and greenhouse whitefly.

Chemical pesticides: Pyrethrins (formulations proprietary and Brigade, IRAC mode of action group 3A – sodium channel modulators), neonicotinoids [(formulation Assail 70 WP, IRAC group 4A), sulfoximines (formulation Sequoia, IRAC group 4C), and butenolides (formulation Sivanto, IRAC group 4D) – all of them are nocotinic acetylcholine receptor competitive modulators], flonicamid (formulation Beleaf 50 SG, IRAC group 9C – modulators of chordotonal organs), and benzoylureas (formulation Rimon 0.83 EC, IRAC group 15 – inhibitors of chitin biosynthesis).

Botanical pesticide: Azadirachtin (formulations cold pressed neem, Neemix, AzaGuard, and Debug Turbo), which is an insecticide, insect growth regulator, antifeedant, and a repellent.

Entomopathogenic fungi: Beauveria bassiana (formulation proprietary), Isaria fumosorosea (Pfr-97), and Metarhizium brunneum (Met 52 EC)

Mechanical: Vacuuming twice a week at one pass each time at 2 mph.

The study included 12 treatments that included an untreated control, Assail 70 WP alone and vacuuming alone as grower standards.  Treatments were administered on 26 August, 2 and 9 September, 2015 using a tractor-mounted sprayer.  A spray volume of 100 gpa was used for pesticide treatments.  Each treatment had six 75' long (4 row) beds and four replications distributed in a randomized complete block design.  Before the first treatment and 6 days after each treatment, 20 random plants from the middle two beds in each plant were sampled for insect pests and beneficial arthropods.  Number of young and old nymphs, and adult lygus bugs, thrips, adult whiteflies, big-eyed bugs, minute pirate bugs, lace wings, damsel bugs, ladybeetles, parasitic wasps, predatory thrips, predatory midge larvae, and spiders were counted from each sample plant.  Data were subjected to ANOVA and significant means were separated using Tukey's HSD test.

Pre-treatment and post-treatment (average of three counts) numbers of lygus bug nymphs and adults in different treatments.

Percent change in lygus bug (all life stages) and various natural enemy (all species combined) populations after three spray applications compared to pre-treatment counts

Ranking of the treatments based on percent change in lygus populations by the end of three spray applications

Percent change in western flower thrips and adult greenhouse whitefly populations after three spray applications compared to pre-treatment counts

Lygus bug: Lygus bug populations were very high during the study period (treatment threshold 1 nymph/20 plants) and control was difficult, in general.  Sequoia/Sivanto/Belaf rotation provided the highest control where there was a 29% reduction in all life stages compared to pre-treatment numbers.  Sivanto/Sivanto/Vacuum treatment was the only other treatment that provided a 12% control.  B. bassiana+pyrethrum/Vacuum/Rimon+Brigade treatment prevented the population buildup and lygus numbers increased in all other treatments.  The popular practice of vacuuming was ranked 6^th.  Having two passes instead of one pass might increase the efficacy of vacuuming, but results emphasize that multiple tools need to be considered for managing lygus bugs in strawberries.

Natural enemies: Percent change post-treatment indicated that natural enemy populations were relatively higher in Pfr-97+Neemix/Pfr-97+Neemix/Vacuum followed by Sequoia/Sivanto/Beleaf, and Sequoia/Sequoia/Vacuum and B. bassiana+neem/B. bassiana+pyrethrum+neem/B. bassiana+pyrethrum.

Western flower thrips: Post-treatment counts showed that thrips populations were reduced only in Rimon+Brigade/Met52+Debug Turbo/Met52+AzaGuard and B. bassiana+neem/B. bassiana+pyrethrum+neem/B. bassiana+pyrethrum treatments.

Greenhouse whitefly: Adult whiteflies occurred at very low numbers during the study and population reduction from post-treatment counts was seen only in Vacuum/Sivanto+Debug Trubo/Rimon+Brigade, Sivanto/Sivanto/Vacuum, and Rimon+Brigade/Met52+Debug Turbo/Met52+AzaGuard treatments.

This study demonstrates the efficacy of various chemical and non-chemical tools in various combinations against lygus bug, western flower thrips, and greenhouse whitefly and growers can make appropriate treatment decisions based on these results.

Acknowledgements: Thanks to Dave Murray for collaborations with this study, Ted Ponce for coordination, Sundance Berry Farms crew, Chris Martinez, Fritz Light, Tamas Zold, and Kristin Nicole Stegeman for their technical assistance, and industry partners for the supply of materials and/or financial support.

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Lygus bug nymphal and adult stages

Lygus bug or western tarnished plant bug (Lygus hesperus) is a major pest in California strawberries and causes significant yield losses by contributing to the fruit deformity.  Lygus bug is a hemipteran insect and has piercing and sucking mouthparts.  They prefer plant parts rich in proteins and lipids.  Developing berries and achenes offer as a good source of nutrition in strawberries and hence they are normally seen in the inflorescence.  When lygus bug inserts its mouth parts and sucks the plant juices, the tissue at the site of feeding does not grow normally resulting in fruit deformity as berries develop.  Deformed berries are not marketable for fresh market and growers adopt various control strategies to manage lygus bugs and limit damage.  Chemical pesticides are the popular choice for managing lygus bugs and the use of bug vacuums is also increasing in the recent years. 

While the treatment threshold is one lygus nymph/20 plants, infestations are generally very high above the threshold requiring aggressive management practices.  Although treatment decisions are typically made based on lygus sampling, it is not uncommon (based on personal communication with some growers and PCAs) for fruit deformity to influence treatment decisions.  In light of this scenario, it is important to determine the role of lygus bug in deformed strawberries among other causes such as poor pollination, genetic factors, and environmental conditions such as cold temperatures.

Literature suggests that fruit deformity due to lygus and other causes can be determined by the size of achenes (Zalom et al., 2014).  Achenes in the deformed and normal areas of the fruit are more or less of the uniform size if the deformity is due to lygus bug.  Achenes are of different sizes if the deformity is due to factors other than lygus damage.

A study was conducted in September, 2015 to evaluate the role of lygus bug and other factors in strawberry fruit deformity.  Deformed berries were collected from 18 conventional and 10 organic strawberry fields.  Conventional fields were sampled nine times and organic fields were sampled 5 times.  On each sampling date a field block was divided into four quadrants and at least 100 deformed berries were collected from each quadrant.  Each berry was examined categorized as lygus- and non-lygus-related based on the size of the achenes and shape of the berry.  Data were subjected arcsine transformation and statistical analysis and significant means were separated using Tukey's HSD test.

In general, lygus bug damage was significantly higher (P = 0.0002) in organic fields than in conventional fields. When the causes for the deformity were compared, the proportion of deformed berries due to lygus bug damage was significantly higher (P < 0.00001) than those due to other causes in both conventional and organic fields.  It is, however, important to note that 41% of the deformity in conventional fields and 33% in organic fields was due to factors other than lygus bug.  These results are important in understanding the role of various lygus bug and other factors in causing fruit deformity and making appropriate treatment decisions.  Sampling the fields for lygus bugs is always the right way to make a treatment decision rather than counting on deformed berries.

Information on lygus bug biology, sampling, and management can be found at the following resources:

Lygus bug biology and damage video: www.youtube.com/watch?v=7sZu9nirOPQ

Lygus bug monitoring and treatment threshold video: https://www.youtube.com/watch?v=Z7o7fafxuhE

Lygus bug management video: https://www.youtube.com/watch?v=5-4qcvceqmU

UC IPM Pest Management Guidelines: http://www.ipm.ucdavis.edu/PMG/r734300111.html

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Reference

Zalom, F. G., M. P. Bolda, S. K. Dara, and S. Joseph (Insects and Mites). 2014. UC IPM Pest Management Guidelines: Strawberry.  University of California Statewide Integrated Pest Management Program. Oakland: UC ANR Publication 3468.  June, 2014.

Strawberry is an important commodity in California with a crop value of $2 billion (NASS, 2013).  Lygus bug or western tarnished plant bug (Lygus hesperus), twospotted spider mite (Tetranychus urticae), greenhouse whitefly (Trialeurodes vaporariorum), and western flower thrips (Frankliniella occidentalis) are considered as important arthropod pests of strawberries which can cause significant yield losses.  According to the Pesticide Use Report of California Department of Pesticide Regulation (2014), more than 200,000 lb of chemical insecticide and miticide active ingredients were used in strawberries in 2012. Among the 50,000 lb of biorational active ingredients that were additionally applied, 97% were Bacillus thuringiensis products used against lepidopteran pests.  Apart from the release of various species of predatory mites against twospotted spider mites, pest management in strawberries is mainly dependent on chemical pesticides and IPM is generally limited to the rotation of pesticides in different modes of action groups.

In an effort to develop an effective IPM program with a particular emphasis on lygus bug management, research has been conducted for the past few years in Santa Maria to evaluate the role of various non-chemical alternatives.  Field studies in 2013 showed that botanical and microbial pesticides can be effectively used in combination and rotation with chemical pesticides (Dara, 2014).  Additional studies were conducted in 2014 to evaluate the efficacy of various combinations and rotations of new and existing chemical pesticides along with botanical, earth-based, and microbial pesticides.

Methodology
A large scale field study was conducted during June and July, 2014 in a conventional strawberry field of variety Del Rey at Goodwin Berry Farms, Santa Maria.  Chemical pesticides included those from IRAC mode of action groups 3A (sodium channel modulators) 4A (neonicotinoids), 4C (sulfoximines), 6 (chloride channel activators), 9C (selective homopteran feeding blockers), and 15 (inhibitors of chitin biosynthesis).  Additionally, diatomaceous earth, azadirachtin, and two entomopathogenic fungi, Beauveria bassiana and Metarhizium brunneum (formerly known as M. anisopliae) were also used.  Diatomaceous earth is a powder form of fossilized remains of diatoms and contains silicon dioxide as an active ingredient.  Silicon dioxide interferes with the integrity of the cuticle by absorbing the waxy layer and causes mortality due to desiccation.  Both B. bassiana and M. brunneum are soilborne entomopathogenic fungi which cause infection when a conidiospore comes in contact with an insect or a mite.  Azadirachtin, a secondary metabolite present in neem seed, is a limonoid compound which interferes with the synthesis of various proteins and thus affects molting, mating, sexual communication, and reproductive ability.  It also has insecticidal properties and acts as an antifeedant and repellent.  Using these alternatives can help pest management which is sometimes difficult to achieve with chemical pesticides alone.

Treatments included an untreated control, a wettable powder formulation of acetamiprid as the grower standard, and other materials in different combinations and rotations (Tables 1 and 2).  Each plot had seven 75' long and 64” wide beds and treatments were replicated four times in a randomized complete block design.  Treatments were administered late afternoon or in the evening using a tractor-mounted sprayer except for diatomaceous earth dust which was applied by a backpack dust applicator.  Three applications were made at 7-8 day intervals and observations were made once before the first application and 5-6 days after each application.  On each observation date, 20 plants were randomly sampled from the middle three beds of each plot by gently beating the plant to dislodge insects into a container.  The number of aphids, lygus bugs (young and mature nymphs and adults), thrips, whitefly adults, and various species of natural enemies were counted from each plant.  Natural enemy complex included bigeyed bug (Geocoris spp.), minute pirate bug (Orius spp.), lacewing (Chrysoperla spp. and Chrysopa spp.), damsel bug (Nabis spp.), lady beetle (multiple species), parasitoids (multiple species), and spiders (multiple species). Data were analyzed using statistical procedures.

Table 1. List of treatments used in this study and their application rates per acre – Active ingredients

*3A Sodium channel modulators 4A Neonecotinoids, 4C Sulfoxamines, 6 Chloride channel activators, 9C Selective homopteran feeding blockers, and 15 Inhibitors of chitin biosynthesis.

Table 2. List of treatments used in this study and their application rates per acre – Trade names

*3A Sodium channel modulators 4A Neonecotinoids, 4C Sulfoxamines, 6 Chloride channel activators, 9C Selective homopteran feeding blockers, and 15 Inhibitors of chitin biosynthesis.

Results and Discussion

Actual numbers of various pests and natural enemies are presented in Table 3 and percent change post-treatment compared to pre-treatment is presented in different figures.

Table 3.  Pest and natural enemy populations from various treatments before and after treatment per 20 sample plants.  Post-treatment counts include averages for three spray applications.  Refer to Tables 1 and 2 for the list of treatments.

Aphid: Negligible number of aphids was seen only in few treatments and data are not presented.

Lygus bug: Lygus numbers increased in all treatments after treatment and there were no statistical differences (P > 0.05).  However, when the percent change, compared to pre-treatment counts, was considered, some treatments appeared to be more effective than others in preventing population buildup.  The high rate of Sequoia (treatment 8) limited the increase to 14% followed by the rotation of Diafil high rate-BotaniGard low rate+Assail 70 WP-Met 52+Molt-X (treatment 12) indicating the potential of non-chemical alternatives for lygus bug management (Table 4).  When BotaniGard+Molt-X combination was applied twice after Rimon+Brigade combination (treatment 4), it appeared to be the fourth best rotation limiting the population build up to 54%.  Untreated control and Assail 70 WP had the highest lygus numbers with 383% and 1083% increase, respectively.

Percent change in all stages of lygus bugs after three spray applications.

Treatments ranked according to their efficacy as expressed by the percent change/control of lygus bugs after three spray applications.

Thrips:There was a general reduction in thrips numbers post-treatment.  There was a 48% reduction in their post-treatment numbers in untreated control while it varied from 35% treatment 2 to 68% in treatment 12.

Percent change in western flower thrips populations after three spray applications.

Whitefly adult:Most of the treatments reduced whitefly populations except for one treatment where there was a 20% increase (treatment 11 – Diafil low rate followed by Sequoia low rate+Molt-X, and Met 52) compared to a 13% in untreated control.  There was a 7 to 78% reduction in whitefly populations in all other treatments.

Percent change in greenhouse whitefly adult populations after three spray applications.

Natural enemy complex:The number of big-eyed bug, parasitoids, and spiders significantly varied among various treatments post-treatment (P < 0.05, data not shown).  When the percent change was considered for the entire natural enemy complex, there was a reduction in all treatments with 41% reduction in untreated control and 53-86% reduction in various treatments.

Diafil application left a white deposit on strawberry plants for several days making the berries unmarketable.  It may not be practical for managing lygus bug, which usually appears after fruit production starts.

These results support last year's data in demonstrating the potential of non-chemical alternatives such as microbial and botanical pesticides.  These tools are essential for sustainable pest management and can make a significant reduction in chemical pesticide use without compromising the control efficacy.

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References

California Department of Pesticide Regulation.  2014.  Summary of pesticide use report data 2012: Indexed by commodity.

Dara, S. K.  2014.  New strawberry IPM studies with chemical, botanical, and microbial solutions.  CAPCA Adviser 17: 35-37.

National Agricultural Statistics Service (NASS)  2013.  California agricultural statistics: 2012 crop year.