Effect of Insecticide-Coated Seeds on Protaphorura fimata (Collembola: Poduromorpha: Onychiuridae) Feeding Damage

The subterranean springtail, Protaphorura fimata Gisin (Collembola: Poduromorpha: Onychiuridae), is a serious pest of lettuce (Lactuca sativa L. [Asteraceae]) in the Salinas Valley of California (Joseph et al. 2015, J. Econ. Entomol. 108: 228–236). Protaphorura fimata feeds on developing seeds, which results in severe stunting, seedling mortality (Joseph et al. 2015; Joseph 2017a, Entomol. Exp. Appl. 162: 69–76), and even partial to complete stand loss of lettuce (Joseph et al. 2015). In the Salinas Valley, lettuce alone is valued at approximately US$1.4 billion and broccoli at US$423 million (Monterey Co. Crop Report 2015). Although lettuce is grown year round in the Salinas Valley, P. fimata–related stand losses have been mostly reported from January through May. Protaphorura fimata individuals are active in cooler temperatures (∼15°C) and cause feeding injury to slowly germinating lettuce seeds (Joseph 2017a).

A recent study showed that most of the pyrethroid, neonicotinoid, and organophosphate insecticides registered on lettuce in California are effective against P. fimata (Joseph 2017b, J. Entomol. Sci. 52: 68–81). However, there are environmental issues with large-scale use of these pesticides; for example, high concentrations of organophosphate (chlorpyrifos and diazinon) residues were detected in the water bodies posing risks to nontarget organisms and public health through contaminated water (Hunt et al. 2003, Environ. Monit. Assess. 82: 83–112). These organophosphate insecticides are now strictly regulated (California Environmental Protection Agency, Central Coast Water Board, 2013, http://www.waterboards.ca.gov/rwqcb3/water_issues/programs/ag_waivers/ag_order.shtml). Use of pyrethroid insecticides in California's Salinas Valley is under stringent scrutiny (Central Coast Regional Water Quality Control Board, 2016, http://www.waterboards.ca.gov/centralcoast/water_issues/programs/tmdl/docs/salinas/sed_tox/index.shtml) because pyrethroid insecticide residues were transported into surface waters in suspended sediments, and their levels in water were found toxic to nontarget organisms (Anderson et al. 2003a, Environ. Pollut. 124: 523–532; Anderson et al. 2003b, Environ. Toxicol. Chem. 22: 2375–2384; Anderson et al. 2006, Environ. Pollut. 141: 402–408; Ng and Weston 2009, UC Water Resources Center Technical Completion Report, Project No.WR1018). A similar water quality issue has been developing with neonicotinoid insecticides, particularly imidacloprid in the Salinas Valley. These existing and emerging environmental issues warrant more research to develop integrated pest management strategies that reduce insecticide use for P. fimata control.

At-plant application of insecticide along the seed line is critical for P. fimata control as P. fimata attacks the germinating stages of lettuce (Joseph et al. 2015). Because the Central Coast region of California receives intermittent rain from January to May, the wet conditions in the lettuce fields can restrict farm machinery traveling over the fields, thus preventing insecticide spray applications. Seed pelleting technology with seed treatment applied with the coating results in a precise placement of insecticide dose around the seeds and can benefit the environment by decreasing the risk of potential off-field movement of insecticide through run-off water or sediments. Also, this insecticide delivery method will substitute for plant insecticide spray for P. fimata control. Moreover, the incidence, distribution pattern, and population size of P. fimata within the field can be nonuniform and unpredictable, which poses a challenge to successful efficacy studies against P. fimata in the field. Therefore, this study was conducted in the laboratory. A previous study showed that clothianidin and thiamethoxam were effective against subterranean springtail when coated on sugar beet (Beta vulgaris L.) seeds (Boetel et al. 2008, Sugarbeet Research and Education Board of Minnesota and North Dakota, http://www.sbreb.org/research/ento/ento08/subterranean.pdf). The objective of the present study was to determine efficacy of insecticides against P. fimata when coated on the lettuce seeds.

The assays were conducted at the University of California Cooperative Extension Entomology Laboratory in Salinas, CA. Protaphorura fimata specimens were field-collected in 2014 and these insects were used to build laboratory colonies. The colonies were maintained in sealed 473.1-ml clear plastic containers (product 9061, Frontier Agricultural Sciences, Newark, DE) and were fed with fish food flakes (Wardley, The Hartz Mountain Corp., Secaucus, NJ) at biweekly intervals. A 2-cm layer of Clear Lake Clay soil was placed in the bottom of the containers and covered with moist paper towels. The P. fimata containers were sprayed with tap water at biweekly intervals to keep the soil moist. The containers were held at 20°C and ∼ 45% relative humidity in complete darkness in the laboratory cabinet. These containers with established P. fimata populations were used for the assay and were not fed for 2 weeks prior to the assays.

Head lettuce seeds ‘Regency' (Snow Seeds Co., Salinas, CA) were first pelleted by Incotec (Salinas, CA) and then sent to Cornell University for an overcoating treatment with clothianidin, thiamethoxam, or spinosad (Table 1). Each seed treatment formulation was suspended in DISCO L520 seed treatment binder (Incotec Salinas, CA). Thiram was applied to all treatments at a labeled rate, and an additional treatment with no thiram (fungicide) was included to determine if thiram affected springtail feeding. All seed treatments were applied with a rotary pan seed treater (Model R-6, Universal Coating Systems, Independence, OR) (Taylor 2003, pp. 1291–1298, Encyclopedia of Applied Plant Sciences, Elsevier Academic Press, Amsterdam, The Netherlands). The liquid seed treatment suspensions were quickly absorbed into the seeds during coating, and then allowed to air-dry overnight. The treatments were packaged and returned to the senior author's lab for entomological investigations.

Table 1 Insecticides coated on lettuce seeds for evaluation against P. fimata in the laboratory assay.

View Fullscreen

Three pelleted seeds of each insecticide treatment were placed on the soil surface of the each container. After placement, the soil was watered to moisten the seeds and, thereafter, food and water were not provided. All containers were covered using a clear plastic wrap (Glad Cling Plastic Wrap; Glad Products, Oakland, CA) and sealed using Parafilm (Bemis Company, Oshkosh, WI) around the edge of the containers. The insecticide-coated seeds were exposed to P. fimata for 7 d after which the seeds were evaluated for number of seeds germinated, number of plants with feeding injury, number of feeding injury sites, and fresh weight (Fig. 1). This assay was conducted in three sets with five container replicates at a given time for a total of 15 replications in a completely randomized design.

View Figure

• Download as Powerpoint
• Download Figure

The number of germinated seeds and injured seedlings were determined per container (replicate). Data on feeding injury sites as well as fresh weight were averaged per container. All the data were log-transformed (ln[x +1]) to establish homogeneity of variance and were subjected to analysis of variance using the general linear model (PROC GLM) procedure in SAS (SAS Version 9.4. SAS Institute Inc., Cary, NC). The means were separated using the Tukey's honestly significant difference method (α = 0.05).

Number of seeds germinated was significantly greater for clothianidin- and thiamethoxam-coated seeds than for nontreated seeds without thiram (F = 4.8; df = 4, 51; P = 0.004; Fig. 2A). There was no significant difference in seed germination between nontreated seeds with and without thiram. Number of discrete injury sites were significantly lower in clothianidin-, thiamethoxam-, and spinosad-coated seeds than in nontreated seeds (F = 37.7; df = 4, 47; P < 0.001; Fig. 2B). Feeding injury to seedlings was significantly lower in clothianidin- and thiamethoxam-coated seeds than in thiram nontreated seeds or thiram-treated seeds (F = 14.7; df = 4, 48; P < 0.001; Fig. 2C). The fresh weight was significantly greater in the clothianidin treatment than in nontreated seedlings with or without thiram (F = 4.0; df = 4, 47; P = 0.007; Fig. 2D).

View Figure

• Download as Powerpoint
• Download Figure

Results show that lettuce coated with clothianidin, thiamethoxam, and spinosad have the potential to reduce P. fimata feeding injury. Clothianidin (NipSit, Valent USA, Walnut Creek, CA) in particular, is registered on head lettuce and can be used for P. fimata control. This information is definitely promising for P. fimata control in the Salinas Valley fields, especially from January through May when the rain and wet field conditions make insecticide spray application using tractor-mounted sprayer difficult to impossible.