Diaprepes abbreviatus L. (Coleoptera: Curculionidae) is a polyphagous root weevil and pest of approximately 300 agricultural, ornamental, and wild plants (Simpson et al. 1996; Mannion et al. 2003). In citrus (Citrus spp. L.; Rutaceae), adults may damage trees by scoring young leaves; however, larval root feeding causes significant tree damage, leading to yield reductions and eventually tree death (Woodruff 1985; McCoy 1999; Knapp et al. 2000; Stuart et al. 2006). Adults are active from spring to autumn; an adult female lays an average of 11,000 eggs in her life span (Nigg et al. 2004), and neonates drop from foliage to the soil where they feed on roots and develop over several mo.

Florida, USA, is a major source of winter strawberries (Fragaria × ananassa Duchesne; Rosaceae) in the USA, with 4,000 planted ha and a production value of nearly $300 million USD (USDA/NASS 2018). Development of effective pest management practices will help enable Florida producers to be viable economically in an increasingly competitive market space (Suh et al. 2017). While the primary arthropod pests in Florida strawberry are twospotted spider mite (Tetranychus urticae Koch; Trombidiformes: Tetranychidae), flower thrips (Frankliniella occidentalis [Pergande]; Thysanoptera: Thripidae), and Frankliniella bispinosa Morgan; Thysanoptera: Thripidae), chilli thrips (Scirtothrips dorsalis Hood; Thysanoptera: Thripidae), and fall armyworm (Spodoptera frugiperda [J.E. Smith]; Lepidoptera: Noctuidae), secondary pests such as D. abbreviatus, occasionally require control (Caruso et al. 2019). Root feeding by larval D. abbreviatus causes foliar wilting often followed by plant death. Eggs are laid on young strawberry plants shortly after planting in early autumn by late-flying female D. abbreviatus, and damage typically is concentrated in small patches near field edges. However, plant damage was more widespread in a few fields adjacent to old, neglected, or recently uprooted citrus groves (J. Renkema, personal observation). Insecticides are available for weevils and grubs in Florida strawberry (Renkema et al. 2019), but they either are not labelled for application through drip irrigation to control root-feeding larvae or have long pre-harvest intervals not compatible with in-season harvest schedules. For insecticides that may be chemigated, growers typically are not able to restrict applications to areas of a field where damage occurs, and treating entire fields is not economical or sustainable. A tactic for control of D. abbreviatus in small areas of strawberry fields is needed.

Diaprepes abbreviatus has been managed successfully with the entomopathogenic nematode Steinernema riobrave Cabanillas, Raulston and Poinar (Rhabditida: Steinernematidae) in Florida citrus (Dolinski et al. 2012). Steinernema riobrave, an indeterminate forager, was first found in southern Texas, USA, and is acclimated to warm soils (Kaspi et al. 2010). Applications of commercially produced S. riobrave have caused larval mortality rates of D. abbreviatus in the field of 50% to greater than 90%, depending on application rates and other factors such as soil conditions (Duncan et al. 1996, 2003; Bullock et al. 1999). Over a yr, adult D. abbreviatus emergence was reduced by half by S. riobrave in one study (Duncan et al. 2007). In citrus, S. riobrave is applied to the soil using tractor-mounted sprayers or through drip irrigation (Duncan et al. 1999), but more targeted application methods used in other cropping systems for other entomopathogenic nematode species may be better suited to or adjusted for small applications in strawberry (Shapiro-Ilan & Dolinski 2015). Here we tested aqueous preparations of S. riobrave applied as a soil drench around the base of individual strawberry plants, but formulation of entomopathogenic nematodes may affect efficacy and ease-of-application, with pelletized baits showing promise in some scenarios (e.g., Hiltpold et al. 2012).

Our goal was to provide new knowledge on a potential management strategy for D. abbreviatus in Florida plasticulture strawberry production. Control methods that are effective, sustainable, reduce insecticide use, and are practical for treating only small, affected areas of strawberry fields are needed for mitigating losses due to D. abbreviatus root feeding. Therefore, the objectives of the research were to determine the efficacy of S. riobrave for control of late-instar D. abbreviatus in combination with determining the movement of larvae among strawberry plants.

Materials and Methods

RESEARCH PLOTS

An experiment was conducted during the 2017 to 2018 strawberry season in research plots at the Gulf Coast Research and Education Center, Balm, Florida, USA (27.7619444°N, 82.2272222°W). Bare-root strawberry transplants (cv. ‘Radiance') were set on 9 Oct 2017 (38 cm in-row spacing) in 2 rows in a raised, double-pressed, black plastic/ virtually impermeable film (Blockade^™, Berry Plastics, Evansville, Indiana, USA) mulched beds (1.2 m spacing) that were fumigated with Telone®C-35 (1,3-dicholoroprepene + chloropicrin) at bed formation (early Aug 2017). Plants were overhead irrigated for 7 to 10 d following transplanting and drip fertigated with 0.27 and 0.34 kg N per d from Nov to mid-Jan and mid-Jan through Apr, respectively. Herbicides Round-up® (glyphosate) and Chateau® (flumioxazin) were applied to row aisles before transplanting. Plants received regular applications of fungicides to control prominent strawberry diseases and early-season applications each yr of DiPel® DF (Bacillus thuringiensis, subsp. kurstaki Berliner; Bacillaceae) to control lepidopteran larvae.

Plots (3 m × 1 raised bed) were arranged in a randomized complete block design with 6 replications in a checkerboard pattern, so that the raised beds adjacent to each plot were unplanted and the total plot area was 12 m × 12 raised beds. In each plot, there were 13 contiguous plants, with 1 infested plant, and 4 peripheral, infested plants, separated from the 13 plants by unplanted space (Fig. 1).

Larval D. abbreviatus were from a laboratory colony maintained at the US Department of Agriculture, Agricultural Research Service facility in Fort Pierce, Florida, USA. The larval cohort used in both experiments were from eggs obtained 9 Sep 2017, with neonate larvae transferred to artificial diet in small cups on 15 Sep and older larvae transferred singly to diet cups on 17 Oct. Larvae were removed from diet and placed singly in empty cups, and the cups (n = 4 per plant) were overturned onto the soil around the base of the plants in the evening of 2 Nov. The following morning, the cups were removed and dead larvae that did not burrow into the soil were replaced with healthy larvae. On 14 Nov, dead leaves and ripe and ripening fruit were removed from plants, and ripe fruit was harvested for the duration of the experiment.

Fig. 1.

Strawberry plot layout for testing applications of Steinernema riobrave for controlling late-instar Diaprepes abbreviatus placed around the root zones of infested plants. Strawberries (38 cm in-row spacing) were planted into raised, earthen beds (1.2 m width) covered with black plastic mulch in research plots at the Gulf Coast Research and Education Center, Balm, Florida, USA.

Treatments were S. riobrave applied once, S. riobrave applied twice, imidacloprid applied once and water. On 16 Nov, S. riobrave (Nemasys®R), prepared in distilled water in the laboratory according to the manufacturer's instructions (25 IJ per cm²), imidacloprid (Admire®Pro, Bayer CropScience LP, Research Triangle Park, North Carolina, USA; 767 mL per ha) or water were applied as a drench (50 mL) to 100 cm² around the base of each plant in a plot using a hand-held CO₂-powered backpack sprayer (0.34 atm.) (R&D Sprayers, Opelousas, Louisiana, USA) and a single wand with the nozzle removed. The end of the wand was used to pull up the plastic around the base of the plant as the drench was being made. Steinernema riobrave were prepared and reapplied to plots on 22 Nov.

Table 1.

Healthy compared to wilting, dead, or missing strawberry plants due to Diaprepes abbreviatus root feeding in a commercial field near Dover, Florida, USA, treated 3 Jan 2017 with soil drenches of Steinernema riobrave nematodes (80,000 infective juveniles per plant) (n = 128 plants) or water (n = 96 plants) and evaluated 10 and 17 d after treatment.

Effects of drenches were evaluated on 16, 22, and 30 Nov, and 7 and 14 Dec by categorizing each of the 13 contiguous plants in all plots as either (1) healthy, (2) minor wilting, (3) major wilting, (4) mix of brown and green leaves, or (5) dead with all brown leaves. A randomly selected peripheral plant with a core of soil from its central root zone (11 cm diam × 30 cm depth) was removed with a golf-cup cutter from each plot on each of the first 4 evaluation dates. The soil was hand-sorted in the field for live and dead larvae. Dead or moribund larvae were placed in white traps (reference) in the laboratory to determine infection by S. riobrave. On 14 Dec, the root-zones of all 13 contiguous plants in each plot were removed and assessed for larvae in the same manner.

Results

RESEARCH PLOTS

The number of live larvae recovered from peripheral infested plants was similar among treatments pre-application (F = 0.70; df = 3,15; P = 0.57) and was not affected by treatment at 1 (F = 1.16; df = 3,15; P = 0.36), 2 (F = 2.41; df = 3,15; P = 0.13), or 3 wk (F = 1.00; df = 3,15; P = 0.42) post-application. Dead larvae infected with S. riobrave were recovered only from plants to which S. riobrave was applied (Fig. 2).

Fig. 2.

Mean live (± SEM) and dead Steinernema riobrave-infected or uninfected, late-instar Diaprepes abbreviatus from soil in the root-zone of a randomly selected peripheral strawberry plant (n = 6) (see Fig. 1) after 1 or 2 applications of infective juvenile Steinernema riobrave (25 per cm² in 50 mL water), 1 application of imidacloprid (Admire^® Pro, 767 mL per ha), or water (control) were made 16 and 22 Nov 2017 around the base of each plant. Plants were in raised, plasticulture earthen beds in research plots at the Gulf Coast Research and Education Center, Balm, Florida, USA.

The number of live larvae recovered from centrally infested plants was similar among treatments (F = 1.28; df = 3,15; P = 0.32) and similar for the distal plants (F = 1.43; df = 3,15; P = 0.27). For the proximal plants, larval recovery varied by treatment (F = 6.07; df = 3,15; P = 0.007) with fewer larvae found in plots treated once or twice with S. riobrave than in control plots (Table 2).

Categorization of strawberry plant health was not affected by treatments in central, infested plants at pre-application, (P = 0.80), at 1 wk after application (P = 0.31), 2 wk (P = 0.60), 3 wk (P = 0.55), or 4 wk (P = 0.55) (Fig. 3A), or in proximal plants at pre-application (P = 0.90), 1 wk (P = 0.31), 2 wk (P = 0.58), 3 wk (P = 0.75), or 4 wk (P = 0.90) (Fig. 3B). For distal plants, plant health categorization did not differ among treatments at pre-application (P = 0.99), 1 wk (P = 0.26), or 2 wk (P = 0.57), but it did differ at 3 wk (P = 0.01) and 4 wk (P = 0.01) (Fig. 3C).

Discussion

Based on our results, aqueous applications of S. riobrave provided a modest level of control of D. abbreviatus in plasticulture strawberry production in Florida. In the grower field experiment, there were 17% more healthy and 7% fewer dead plants by 17 d after an application of S. riobrave compared to an application of water. In the research plot experiment, no live D. abbreviatus were recovered from plants proximal to the D. abbreviatus-infested plants when S. riobrave was applied to all plants, and almost all dead larvae that were recovered were infected with S. riobrave. In proximal and distal plants, there were low rates of plant wilting in all treatment plots, up to about 10% in control plots. Wilting was reduced by 1 or 2 applications of S. riobrave in distal plants, but a similar effect was not clear in proximal plants. Applications of S. riobrave may not be able to rescue a wilting plant with significant root feeding by D. abbreviatus, but preventative applications to visually healthy plants minimized further plant wilting and loss.

Table 2.

Mean (± 95% CI) numbers of late-instar Diaprepes abbreviatus recovered from soil in the root-zone of strawberry plants when 1 or 2 applications of infective juvenile Steinernema riobrave (25 per cm² in 50 mL water), 1 application of imidacloprid (Admire® Pro at 767 mL per ha), or water (UTC: untreated control) were made 16 and 22 Nov 2017 around the base of each plant. Plants were in raised, plasticulture earthen beds in research plots at the Gulf Coast Research and Education Center, Balm, Florida, USA. See Figure 1 for location of central, proximal, and distal plants in plots. Means followed by the same letter are not significantly different (Tukey's HSD test, P < 0.05).

There were no significant differences in the efficacy of 1 or 2 applications of S. riobrave and 1 application of imidacloprid for control of D. abbreviatus. In citrus, the efficacy of imidacloprid and S. riobrave on D. abbreviatus also was similar (Bender et al. 2014). Admire^® Pro (42.8% imidacloprid) is labelled for soil and chemigation application in strawberry for aphids and whiteflies. However, the pre-harvest interval for Admire^® Pro is 14 d, limiting use in Florida strawberry to the early season before plants produce ripe fruit, because the harvest interval typically is 3 or 4 d once fruit begins to ripen. Similar efficacy between 1 and 2 applications of S. riobrave may be due to most larval infection occurring after the first application, and to high S. riobrave survival rates in soil in irrigated strawberry beds protected by black plastic mulch. Entomopathogenic nematodes are susceptible to desiccation and have reduced virulence and viability in dry soil conditions (Patel et al. 1997). Less than optimal soil temperatures may have reduced the efficacy of S. riobrave, because average hourly Jan nighttime soil temperatures under black plastic in strawberry root zones were less than 21 °C (Deschamps et al. 2019), and applications in citrus are recommended when soil temperatures are consistently above 21 °C (Duncan & Mannion 2021).

In plants infested with D. abbreviatus (central plants), there were similar numbers of dead and wilted plants in plots with 2 applications of S. riobrave as in control plots (mean plant ratings of about 4) by the last sample date, but more healthy plants on average in plots with 1 application of S. riobrave. However, there was already 50% plant death in plots with 2 applications of S. riobrave before the first application (mean plant rating of about 2.5), which was more than in other treatments. Conversely, plots with 1 application of S. riobrave had more dead and wilting proximal plants than other treatments, meaning D. abbreviatus larvae may have moved away from central plants to proximal plants more quickly in plots with 1 application of S. riobrave than other in other plots. From this experiment, the value of a second application of S. riobrave is not clear, but a second application at a longer interval, greater than a wk may be warranted, if wilting progresses to greater levels on plants adjacent to the initially infested and highly wilted plant.

In addition to determining efficacy of S. riobrave in plasticulture strawberry, we designed our research plot experiment to assess larval movement of D. abbreviatus. In untreated control plots, larval recovery after 7 wk (2 Nov to 14 Dec) was similar between central and proximal plants, which were about 40 cm apart. Movement likely was motivated by crowding at the central plant and a resulting search for new food resources, because most of the central plants were severely wilted or dead by the end of the experiment. Vertical movement of D. abbreviatus neonates has been studied (Quintela & McCoy 1998), and larvae typically remain in about 1 m radius of citrus tree trunks (Bates et al. 2015), but little is known about horizontal movement of late-instar D. abbreviatus in soil. A couple D. abbreviatus larvae were recovered from distal plants, suggesting movement of almost 1 m in 7 wk is possible but not common.

Fig. 3.

Progression of damage to strawberry plants (n = 13 per plot) when a central plant in each plot was infested with 4 late-instar Diaprepes abbreviatus. All plants in a plot received 1 or 2 soil applications of infective juvenile Steinernema riobrave (25 per cm² in 50 mL water), 1 application of imidacloprid (Admire^® Pro, 767 mL per ha), or water (control) on 16 and 22 Nov 2017, and plants were categorized weekly from pre-application (16 Nov) up to 4 wk post-application (14 Dec) using a rating scale of (1) healthy, (2) minor wilting, (3) major wilting, (4) mix of brown and green leaves, or (5) dead with all brown leaves. Strawberry plants were in raised, plasticulture earthen beds in research plots at the Gulf Coast Research and Education Center, Balm, Florida, USA.

Overall, larval D. abbreviatus recovery rates were low at an average of 0.7 out of 4 larvae from central plants in control plots, and no larvae were found in peripheral plants from control plots by 3 wk after treatment applications. We searched only for larvae in plant root zones and missed any larvae moving between plants. In addition, some larvae likely died, because uninfected dead larvae were found in the peripheral untreated control plants. Diaprepes abbreviatus late-instars are capable of moving from 1 strawberry plant to another but caused a relatively low level of injury to plants to which they moved.

In the research plot experiment, S. riobrave were applied to all plants in a plot 2 wk after placing D. abbreviatus on 1 central plant. No live D. abbreviatus were recovered from proximal plants treated with S. riobrave even though minimal plant wilting occurred. It is likely that uninfected larvae moved from the central to proximal plants, began feeding on plant roots, were infected by S. riobrave, but ceased feeding and died before significant plant damage was caused. Since we allowed D. abbreviatus 2 wk to begin feeding and moving before applying S. riobrave, an earlier application to central plants only as soon as wilting was noticed, may have produced similar or even better results. This possibility needs to be tested in a future experiment. From a practical perspective, it will be more economical and thus feasible to selectively apply S. riobrave in large strawberry fields to wilting plants only and not to large numbers of plants adjacent to them.

There is increasing interest in managing D. abbreviatus through entomopathogenic nematode conservation, as abundant and suppressive entomopathogenic nematode communities were found in some Florida citrus orchards (Duncan et al. 2013). Conventional strawberry fields are fumigated annually and will require annual S. riobrave augmentation, but entomopathogenic nematode communities should be evaluated in organic strawberry production where cover crops and physical methods are used in place of chemical fumigation, and may result in conserved and suppressive entomopathogenic nematode communities. Because it appears that D. abbreviatus-damaged plants are localized to certain fields or in field areas, yearly monitoring of larval patches and plant wilting will determine whether or not they are seasonally stable and, if so, then S. riobrave applications can be consistently applied to these fields or field areas. More knowledge about D. abbreviatus flight and oviposition periods in strawberry is needed to guide S. riobrave application timing decisions, and persistence of S. riobrave in plasticulture earthen beds should be determined to guide timing of repeat applications. Currently, S. riobrave applications may not be cost-effective as they are only sold in large quantities for use in citrus; localized applications would require a change in marketing and production of S. riobrave. In conclusion, S. riobrave successfully infected and killed late-instar D. abbreviatus in strawberry and was as effective as imidacloprid in reducing plant wilting. With further research to determine optimal application strategies, S. riobrave can be successfully developed into safe and relatively easy-to-use biological control method to reduce impacts of D. abbreviatus in strawberry.