Biological Control of Two-spotted Spider Mite on Vegetables

Results and Discussion

Objective 1. Predator distribution and overwintering survey.  Resident populations of P. persimilis were observed in about half of tomato fields surveyed in the piedmont county of Rowan; 7 of 14 and 5 of 11 fields in 2017 and 2018, respectively.  No resident populations were detected during summer sampling in any of the mountain or coastal plain counties in either year.  However, overwintering studies demonstrated that P. persimilis did successfully overwinter at the Upper Coastal Plain Research Station in Rocky Mount, indicating that the predator is likely capable of becoming established.  While the climate in the coastal plain is considered ideal for year-around survival of P. persimilis, its absence is likely due to the lack of areas with concentrated and consistent production of crops that support high populations of TSSM, such as that in Rowan County.  P. persimilis can only survive on TSSM, and when TSSM decline to very low numbers the predator goes extinct.  Rowan County has a long history of commercial tomato production, a crop that is highly susceptible to TSSM because of high fungicide.  The high fungicide use is critical because it prevents fungal epizootics from suppressing TSSM populations – fungal epizootics following rain decimate TSSM populations except on those crops frequently sprayed with fungicides.  The coastal plain region is dominated by row crops (corn, cotton, soybean) that are infrequently sprayed with fungicides and therefore allow for frequent fungal epizootics that quickly suppress TSSM populations.  This is further supported with observations by Rebecca Schmidt-Jerries (personal communication) who detected resident P. persimilis populations in an area of the coastal plain region of South Carolina that has a history of concentrated tomato and strawberry production.  The failure to detect resident populations of P. persimilis in the mountains of western NC, where there are high concentrations of tomato production, was due to winter temperatures being too low for survival of P. persimilis.

Objective 2. Bean vs. tomato-adapted predator releases in commercial fields.  Studies to compare the effect of host crop on which P. persimilis were produced – TSSM infested tomato (T strain) versus bean (B strain) – showed that host plant did indeed have an effect on how well the predator performed on tomato. When season cumulative mite days were calculated for all treatments, there were no significant differences in either TSSM or P. persimilis populations, although trends showed that TSSM populations were lower and P. persimilis higher in plots where tomato-adapted predators were released (Table 1).  When populations were averaged across all sample dates, TSSM populations were significantly lower and P. persimilis higher in the treatment where tomato-adapted persimilis were released compared to either bean-adapted predators or the control.  These findings suggest that P. persimilis reared on tomato were better suited for establishing and reducing TSSM populations compared to those reared on beans.

Objective 3a. Tomato banker plant system.  During the first year of tomato banker releases in commercial fields in 2017, numerous problems were encountered related to grower application of insecticides toxic to P. persimilis and endemic populations of P. persimilis masking the impact of released predators.  One late-season release in Henderson County did demonstrate the potential for releases.  Predators were released on 27 August in a field with a growing TSSM population of about 1 mite per leaflet.  TSSM populations continued to increase for about 3 weeks when P. persimilis were first detected, after which TSSM populations stabilized at about 7 to 10 mites per leaflet while they increased to 30 mites/leaflet in the non-released section of the field (Fig. 2).  These findings of predators becoming established 3 to 4 weeks after release is consistent with previous small plot studies. 

In 2018, P. persimilis were released in four commercial tomato fields in Rowan County, with dates of release indicated by the arrows in Fig. 3.  Releases were conducted in sections of fields ranging from 0.25 to 1 acre.  Wetmore 1 was the only location with two releases of predators, and Correll was the only location where an acaricide application was made – Nealta on 6 July.  The goal of studies this year was to release predators at TSSM densities of ≤0.5 mites per leaflet – as soon as possible after initial detection of TSSM in fields.  In three fields (Wetmore 1, Fleming and Correll) there was a lag time of 3 to 4 wks between release of P. persimilis and detection in sampling.  The Wetmore 2 location was a later planting and there was an apparent 7 wk interval between release and detection, which suggests that the predator population observed in this field was an endemic population.  It is noteworthy that releases helped maintain TSSM populations below the economic injury level of 8 mites/leaflet in all fields.  These results indicate that early season releases of P. persimilis within a single tomato row at a rate of ~21,000 an acre when TSSM average ~1 per leaflet can successfully reduce their populations after 4 – 5 weeks.

As a potential system to reduce the cost of biological control with P. persimilis, in 2019 a single release (inoculation) of predators occurred on one edge of a tomato field, spread across a 2.5 acre field was monitored. Subsequent monitoring showed that released predators spread across the entire field over a six-wk period (Fig. 4). Additional work will be necessary to determine the influence of TSSM population density on the rate of spread, and the identification of spatial release points to more efficiently achieve dispersal of the predator across a field.

Objective 3b. Bean banker plant system. These experiments were conducted in a commercial tomato field in Rowan County in 2017 and at the Mountain Horticultural Crops Research Station in Mills River in 2018 and 2019. In 2017 in the commercial field at the Correll Farm, an endemic population of P. persimilis invaded the field and masked the effect of the banker plant study. In 2018, record rainfall at the MHCREC (37 inches of rain between May and August) prevented TSSM populations from becoming established. In 2019, the banker bean system successfully established P. persimilis and reduced TSSM in adjacent tomato plots. P. persimilis were released in TSSM-infested bean plants where populations increased and dispersed to nearby tomatoes where they became established. Analysis of cumulative mite days (CMD) indicated that P. persimilis populations were significantly higher in tomato rows one (near) and three (mid) away from a central banker bean system (Fig. 5). TSSM populations were significantly reduced in adjacent (near) tomatoes only. The effects of TSSM population suppression through P. persimilis predation in this experiment were likely reduced by significant damage incurred to the banker beans from weekly spray equipment and an unsupervised tomato harvesting crew. Despite this significant damage to the banker beans, it is noteworthy that P. persimilis still established and dispersed to suppress TSSM.

Objective 4. Insecticide program efficacy, profitability, and predator compatibility. In 2017 and 2018 there were no differences among insecticide programs in secondary pests, including thrips and flea beetles, but potato aphid populations were higher in the control than insecticide treatments. Also, there were no differences among insecticide programs in either total or marketable yields (Table 3), nor in net profits generated by any insecticide program, all of which were significantly higher than the non-treated control. Higher profits in 2017 vs. 2018 were due to a combination of higher yields and price of tomatoes in 2017. High rainfall in 2018 was responsible for reduced yields. Insect damage was relatively high in the control in both years, reaching 25.6 and 33.9% of fruit in 2017 and 2018, respectively. The cause of insect damage in each year was due to three groups of pests; lepidopterans, thrips and stink bugs. Damage was split evenly among the three groups of pests in 2017, with 8.5, 6.0, and 8.5% damage caused by lepidopterans, thrips, and stink bugs respectively. In 2018 lepidopterans were responsible for 91.5% of all insect-damaged fruit. High populations of cabbage looper were a primary cause of this damage in 2018. Soil application of Coragen (for lepidopteran control in the chemigation treatment) is known to be less effective against this insect than other lepidopterans, and was the cause of higher insect damage in the chemigation treatment in 2018.

TSSM and P. persimilis populations in the chemigation, soft foliar, and control treatments did not differ from each other on any sampling date in 2017 (Fig 6). However, TSSM populations did increase to higher densities in these treatments compared to the “hard” insecticide treatment, which was the result of pyrethroids with acaricide activity being applied to this treatment (i.e., Brigade and Warrior). Typically there is a resurgence of TSSM following suppression with pyrethroid insecticides, but that was not observed in this study. However, P. persimilis populations did develop to significantly higher densities in the chemigation treatment, soft insecticide treatment, and control compared to the conventional hard insecticide treatment . 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. Heavy rains in 2018 resulted in extremely low TSSM densities; highest density occurred in the hard foliar treatment at 2.5 mites/leaflet on the last sample date, and populations never exceeded 1 per leaf in any of the other treatments.