Two-spotted spider mite (TSSM) is a serious pest of solanaceous and cucurbit crops throughout much of the Southern U.S. Control is achieved almost exclusively with expensive chemical acaricides, usually multiple applications per season, which has led to costly control measures and the development of acaricide-resistant populations. Naturally occurring populations of P. persimilis in Rowan County have been observed to do an excellent job of controlling TSSM on tomatoes in late August and September, provided growers recognize them as beneficials and avoid spraying harmful insecticides. Unfortunately, there is a considerable lag time between the buildup of TSSM and predators in tomato fields early in the season (July to mid-August), which is when mite populations and acaricide use are most intense. During this reporting period, we successfully conducted experiments related to objectives 1,2, 3 and 4. Important findings from the first year include documenting that the source of predatory mites (insectary purchased versus our own reared on tomato plants) had a significant impact on predator establishment in tomato fields. Also, the number of release sites of P. persimilis in tomato fields, ranging from 70 to about 280 per acre, did not affect their performance, probably because of the high mobility of P. persimilis. Where P. persimilis releases were successful, there was a three to four week interval between release of predators and an observed decline in TSSM. Use of the tomato banker plant system in commercial tomato fields to manage TSSM populations varied among fields, largely due to late releases of predators and use of incompatible insecticides by growers. Results of bean banker plant system were less conclusive due to cooperator use of pyrethroid insecticides in these fields. Finally, experiments to assess the compatibility of different insecticide programs with P. persimilis showed that P. persimilis populations were higher and TSSM populations declined more quickly in treatments using drip chemigation and soft insecticides compared with conventional broad spectrum products. These treatments also provided excellent insect control and resulted in the highest profitability. Studies conducted in 2018 will aid in modifying our proposed experimental designs and will improve the planning and execution for trials in 2018.
- Determine the distribution of resident populations of persimilis in different ecoregions of NC (mountains, piedmont and coastal plains), and overwintering sites in regions where they do successfully overwinter.
- Compare efficacy of different commercial sources and release rates of persimilis in controlling TSSM on tomatoes.
- Determine the level of TSSM suppression in tomatoes using a within field tomato-based banker plant system and a periphery bean-based banker plant system for releasing persimilis into commercial tomato fields.
- Compare different insecticide-based management programs for compatibility with persimilis, efficacy in managing key insect pests, and net profitability of tomato production.
Our specific objectives were to: 1) determine distribution and overwintering sites of P. persimilis in different ecoregions of NC; 2) compare the efficacy of different commercial sources and release rates of P. persimilis in controlling TSSM on tomatoes; 3) (a) Determine the level of TSSM suppression in tomatoes using a within field tomato-based banker plant system and (b) periphery bean-based banker plant system for releasing P. persimilis into commercial tomato fields; 4) Compare different insecticide-based management programs for compatibility with P. persimilis, efficacy in managing key insect pests, and net profitability of tomato production
Objective 1. Surveys for P. persimilis were conducted in commercial tomato and watermelon fields in mountains (Henderson and Buncombe Counties), Piedmont (Rowan), and coastal plains (Sampson, Gates Chowan) regions of NC. P. persimilis overwintering studies were established in the mountains and coastal plain regions in the fall of 2017. Chickweed flats (1020 trays, 11” W x 21.4” L x 2.4” D) surrounded by tomato plants infested with TSSM and P. persimilis in each habitat type at 2 sites; woods (pine and mixed hardwood), a weedy field margin adjacent to soybean, and in a small grain field at the Upper Coastal Plain Research Station in Rocky Mount and at MHCR&EC on 18 and 21-Oct., 2017, respectively. Secondly, we broadcast seeded four cover crops into 1 m2 plots at MHCR&EC on 29-Sept.; wheat and rye (19.4 grams per plot), pelletized vetch 9.5 grams per plot), fescue (9.7 grams of KY31), and one control plot consisting of bare soil. We then introduced a mite infested (TSSM + P. persimilis) bean or tomato plants into each plot on 20-Oct. We will evaluate the effect of habitat and cover crop on overwintering survival beginning early April 2018.
Objective 2. Field trials were conducted in the mountainous county of Henderson (River Loop and SNV) by establishing fourteen plots (9 rows x 50 ft) in four blocks, and each block was divided into sections by an unplanted, 3-ft alley at 100-ft intervals at each field site. TSSM were first detected on 10-July and 17-July at the River Loop and SNV locations, respectively. Subsequently, predatory mites (n = 1,000) were released into each plot (except the control plot) on 20-July at River Loop and on 3-Aug at SNV. This factorial study consisted of two P. persimilis sources and three release sites per plot. Predatory mite sources consisted of those purchased from Koppert Biologicals that were reared on bean (K-persimilis), and our laboratory colony reared on tomato (T-persimilis), and number of release sites consisted of 70, 140, and there were 1, 2 and 4 release sites per plot, which is was equivalent to 70, 140 and 280 release sites per acre. The release rate per acre remained constant at about 21,000 predators per acre. TSSM and P. persimilis were monitored weekly from the time of release until 5 and 18- Sept. in River Loop and SNV, respectively.
Objective 3a. A modified tomato banker plant system experiment was conducted by placing tomato plants infested with TSSM and P. persimilis approximately 0.5 acre sections of commercial tomato fields Rowan (Wetmore, Correll, Flemming, Patterson Farms) and Henderson Counties (Wilson and Station fields). Early season tomato banker plants were placed in each Rowan county farm site on 27-June, which was approximately 3 weeks after initial detection of mites in fields. This interval between TSSM detection and release of predators deviated from protocol (predators were to be released when TSSM were first detected), because TSSM were detected about 3 weeks earlier than anticipated, and consequently our P. persimilis colony was behind schedule. A single mite-infested tomato plant (TSSM + P. persimilis) was placed between tomato plants at a density of 70 release sites per acre, for a total of about 20,000 predators per acre on. Both TSSM and P. persimilis were sampled at weekly intervals until experiments terminated on 27-July. Late season tomato banker plant systems infested with TSSM and ~300 P. persimilis were placed at Wilson and Station fields on 29-Aug and 23-Aug, respectively, following the same protocol as above. Sampling occurred at weekly intervals and ceased at Wilson on 17-Oct., and on 19-Oct. at Station fields.
Objective 3b. For the bean banker plant system, a double row of beans, Phaseolus vulgaris ‘Blue Lake 274’, was planted in the drive rows in tomato fields at Correll Farms and two fields at North River Farms (Henderson County, NC). At Correll Farms, six 20-meter sections of drive rows were planted with a double row of beans on 22-June. On 6-July, TSSM were introduced to drive row beans by placing 8 TSSM-infested laboratory bean plants into each drive row section. Twenty days later (26-July) ~ 666 P. persimilis from each source (K- or T-persimilis) were introduced to 3 bean drive rows each. At North River Farms, two fields were used for trials; Mansion and Williamson. At Mansion field, six 25-meter sections of beans were planted into two drive rows on 7-July. Between 26-July and 3-August, bean plants in the drive rows were infested with TSSM by introducing 1-3 TSSM infested bean plants from our laboratory colony to each drive row. On 10-Aug. ~1000 mites from each source (T- or K-persimilis) were released into 3 drive rows each. At Williamson field, two 25-meter section of double-row bean were planted into drive rows on 24-Jul. Beans were infested with TSSM between 16-23 Aug; however, beans were mowed before the first release of P. persimilis and sampling was discontinued. At each field, TSSM and P. persimilis were sampled at weekly intervals from each tomato row in the sections adjacent to the bean drive rows. At each site, tomato rows adjacent to drive rows were sampled at weekly intervals, and sampling ceased on 24-Aug. at Correll and on 2-Oct. at Mansion fields.
Objective 4. Different insecticide-based management programs for compatibility with P. persimilis, efficacy in managing key insect pests, and net profitability of tomato production were evaluated at MHCR&EC in the summer of 2017. Tomatoes were seeded on 1-May and transplanted 1.5 ft apart on 9-June into black-plastic covered rows. Plots consisted of 3 25-ft rows of tomatoes planted on 5 ft centers. Each plot was separated by a 25 ft planting of soybean (‘Pioneer P38T42,’ DuPont Pioneer, Johnston, IA) to prevent P. persimilis dispersing between plots. Three insecticide programs (Table 1) were compared with a control (no treatment). Programs included a chemigation treatment where insecticides were injected into driplines (Admire Pro + Coragen 10 d after transplant, followed by Coragen + Venom at 4 wks) and systemically taken up by tomato plants, and 2 foliar treatments where insecticide were applied directly to leaves and fruit. Foliar treatments consist of a soft foliar treatment (a single foliar application of Beleaf 50SG, followed by Coragen, Radiant, and Venom, and a hard foliar treatment (a single foliar application of Dimethoate followed by Lannate, Sniper and Warrior). All foliar-applied insecticides were applied using a CO2 pressurized backpack sprayer. The first foliar treatments (2 June, 30 June, and 7 July) were applied using a 2-nozzle boom fitted with D4-25 nozzles 13.75 ft apart and calibrated to deliver 50 GPA. All remaining applications were applied using a 3-nozzle boom calibrated to deliver 80 GPA and with a nozzle spacing of 14.5 in. Flea beetle, thrips, and aphid populations were assessed June to Aug by either beating foliage on to a lamented piece of paper (8 x 10 in.) or by sampling flowers. TSSM-infested tomato plants from the lab were then introduced into each plot on 30-Jun. Experiment-wide spider mite populations averaged 0.4 mites/leaflet on 6-July and 12-July and 0.5 per leaflet on 17- July. On 20-July, ~185 P. persimilis were introduced into each plot by distributing mite-infested leaflets into each row. TSSM and P. persimilis populations were sampled weekly by examining 15 terminal leaflets in the upper tomato canopy. Vine-ripe tomatoes were harvested at weekly intervals between 10-Aug and 21-Sept from 12 row-feet of the center row. Tomatoes were sized, weighed, and graded for insect damage, defect, and marketability.
Table 1. Insecticides and dates of applications to tomatoes managed with different insecticide programs.
Objective 1. Rowan County in the central Piedmont was the only location where resident populations of P. persimilis were detected. Whether this distribution pattern was due to climatic conditions or production practices in these regions is unknown. P. persimilis overwintering experiments set up in the coastal plains and mountains in October 2017 should help to shed light on potential factors affecting distribution. These experiments will be assessed in mid-April of 2018.
Objective 2. Across all sampling dates, P. persimilis source and release site experiments indicated that TSSM populations were significantly greater in the K-persimilis plots compared to the T-persimilis plots and control plots (Fig. 1). Additionally, P. persimilis populations were greater in T-persimilis plots compared to the control and K-persimilis plots (Fig. 2). When cumulative mite days were analyzed by field and by release density per source (density of K-persimilis or T-persimilis in SNV or River Loop), there were no significant differences among release density (Table 2). These findings suggest that T-persimilis are better at both establishing and reducing TSSM population density compared to K-persimilis. Moreover, the host plant on which P. persimilis is reared strongly affects its efficacy on different plants from which it was reared.
Table 2. Cumulative mite days on tomatoes associated with field location and source
of predatory mites. Mills River, NC. 2017.
Objective 3a. Tomato banker plant system. At Wetmore Farm, initial results show that TSSM populations were greater in the release plot than in the control plot for all the but the final sample date. Between 29-Jun and 5-July, TSSM populations increased around 5.6X in the release plot, whereas they increased nearly 15X in the control plot (Fig. 3). Thereafter, TSSM populations in the release plot crashed, likely due to predatory mite populations. Endemic populations of predatory mites appeared to have invaded all plots at Correll and Flemming Farms, while at Patterson Farm, applications of Dimethoate made the day prior to, and applications of Steed made 2 days after predator releases, likely killed or negatively impacted the released mites. However, we determined that predatory mites are susceptible to residuals of pyrethriods and organophosphates, and additionally that release experiments should not be conducted where P. persimilis is already present. In addition to application of insecticides incompatible with P. persimilis, the delayed release of predators also likely contributed to the slower than expected response in these trials.
At the Station Field in the mountains, TSSM populations increased up to week 4; however, beginning 22-Sept. population density of TSSM was greater in the control plots than in release plots, and TSSM in release plots did not reach the economic injury level of 8 mites/leaflet (Fig. 4). This suggests that P. persimilis require a minimum of 3-4 weeks to effectively suppress TSSM populations, which is consistent with previous findings in small plot experiments.
Objective 3b. Bean banker plant system. At the Correll Farm site, TSSM populations in tomato rows adjacent to the T-persimilis drive rows averaged ~2.0 TSSM per leaflet, and this was lower than the average K-persimilis per leaflet (~ 4.6 mites per leaflet) 1-Aug (Fig. 5). However, an application of Steed insecticide (a premix of zeta cypermethrin and bifenthrin this is highly toxic to P. persimilis) on 28-July and again on 18-Aug killed most P. persimilis and suppressed TSSM. Field-wide, TSSM populations decreased to an average of ~0.2 TSSM per leaflet; P. persimilis populations decreased to an average of ~ 0.4 mite per leaflet. At the North River Farms test site, no P. persimilis were recovered at one site, and miscommunication with the grower at the other site beans resulted in beans being mowed before the first sample after P. persimilis release.
Objective 4. Compatibility of Insecticide Programs with P. persimilis. Secondary insect populations, including thrips, flea beetles and aphids did not differ among insecticide-treated plots. Also, there were no differences among insecticide treatments in direct pest damage (stink bugs, lepidopterans), all of which had significantly less damage than the control (Table 3). All three insecticide treatments generated significantly higher net profits than the control . Although the chemigation and soft insecticide program had higher net profitability than the broad spectrum treatment, these differences were not significant. Nonetheless, these results demonstrate that a chemigation-based or soft-chemistry foliar insecticide program was at worst equally effective as a broad spectrum program.
TSSM and P. persimilis populations in the chemigation, soft foliar, and control treatments did not differ from each other at any sampling date. However, TSSM populations did increase to higher densities in these treatments compared to “hard” insecticide treatment (Fig. 6), which was the result of pyrethroids with acaricide activity being applied to this treatment (i.e., Sniper and Warrior). However, P. persimilis populations did develop to significantly high densities in the chemigation, soft insecticide treatment and control compared to the conventional hard insecticide treatment (Fig. 7). P. persimilis peaked one week after TSSM peaked and were responsible for the dramatic decline in TSSM densities in the three former treatments from 17 to 23-Aug., indicating effective biological control of TSSM in the absence of acaricides.
Table 3. Efficacy of different insecticide programs on pest control, yield, and profitability of tomato production. Mills River,
The educational program is primarily targeting cooperative extension agents and commercial vegetable growers (both conventional and organic producers), as well as the pest management scientific community. Several approaches are being used to reach a diverse audience, including on-farm tests and demonstrations, field-days, a training workshop for extension agents and other interested people, presentations at commodity meetings, development of hardcopy and web-available extension publications, and a website dedicated to biological control of spider mites in vegetable crops. Information is also being delivered to the scientific community through presentations at scientific conferences and publications in scientific journals. During the first year of the project, the primary focus was on-farm tests (which occurred at six different locations), two presentations at commodity meetings (Winter Vegetable Conference in Asheville, NC, and Henderson County Fall Vegetable Meeting), and presentations at two field days. In addition, first year results were presented during a poster session at the Entomological Society of America Annual Meeting in Denver, CO, in November 2017. Current activities are now focused on developing extension publications to educate stakeholders on proper identification of Phytoseiulus persimilis in the field and strategies to preserve and enhance P. persimilis populations, primarily by avoiding harmful pesticides.
Educational & Outreach Activities
On-farm research and demonstrations were conducted with six tomato growers in the Piedmont and mountains of North Carolina. Meetings were held with growers before the growing season to explain the project and anticipated results. Predatory mites were released into their crops, and weekly visits were made to scout fields for spider mites and predatory mites, and to provide updates to growers. The objective was to educate participating growers so that they would become experts of spider mite biological control procedures.
Results of the first year of the project were presented at two separate commodity meetings; the Winter Vegetable Conference in Asheville, NC, on February 20-21, 2018, that included over 200 attendees, and a Henderson County vegetables grower meeting attended by 21 individuals on November 2, 2017. The session on biological control of spider mites include attended by 85 growers and 12 representatives from the crop protection industry. The Henderson County (Hendersonville, NC) meeting was attended by 17 growers, 2 crop protection industry representatives, and 2 extension educators.
Two field day events held in which research plots were on display and explained to visitors. The NC Tomato Growers Vegetable Field Day held at the Mountain Horticultural Crops Research Station (Mills River, NC) on 17 August, 2017, was attended by 275 people, of which 125 and 50 were estimated to be vegetable growers and crop protection industry representatives, respectively. A smaller on-farm field day attended by 8 growers was held in 10 October 2017 to discuss the use of chemigation for insect control and its safety to Phytoseiulus persimilis.
A draft copy of an extension publication on the compatibility of various insecticides with Phytoseiulus persimilis is being prepared and is expected to be completed in one month. Also, a new website focusing on biological control of twospotted spider mite on vegetables is underway, with a completion date of June anticipated.
It is too early in the project to expect anything more than elementary gains in knowledge or stakeholder behavior. A survey was conducted at the Winter Vegetable Conference in Asheville, NC, in February 2018, and several questions related to gaining baseline information on this project were proposed, including knowledge of insecticide effects on biocontrol agents and identification of predators of spider mites. Of the 50 respondents, only 2% of respondents indicated safety to biological control agents was a factor in choosing an insecticide to apply. However, approximately 70% correctly identified Phytoseiulus persimilis as an important predator of the twospotted spider mite. These results indicated that while many could identify P. persimilis, most were not aware of what insecticides were compatible with preserving the predator.