The red-headed flea beetle, Systena frontalis, is an insect pest of container-grown nursery production systems in the Midwest, Northeast and Southeast regions of the United States. Redheaded flea beetle adults feed on the leaves of a wide-range of ornamental plants grown in nursery production systems including, but not limited to: Cornus spp., Hydrangea spp., Itea spp. and Weigela spp. Redheaded flea beetle adults cause damage when feeding on the leaves of ornamental plants, reducing aesthetic quality and salability, which results in an economic loss. For example, redheaded flea beetle adult feeding damage caused losses of $483,871 or 11% of overall sale of plant material in 2020 at Loma Vista Nursery in Ottawa, Kansas.
Redheaded flea beetle adults are five millimeters in length, shiny black, with a red head (Figure 1). They have enlarged hind femora that allow them to jump like a flea, hence the common name. Adults feed on the upper and lower leaf surfaces, which results in necrotic leaf spotting and/or holes in leaves (Figures 2 and 3). The extent of feeding damage can vary depending on the plant species fed upon. Redheaded flea beetle adults are difficult to manage with foliar applied insecticides because their jumping behavior and propensity to fall onto the growing medium surface when disturbed allows them to escape exposure from insecticide sprays.
Foliar applications of insecticides are intended to protect leaves from feeding damage as opposed to directly killing redheaded flea beetle adults. However, foliar applications of insecticides do not provide adequate protection of leaves from redheaded flea beetle adult feeding damage. Furthermore, insecticide spray applications need to be conducted on a weekly basis, which is too labor intensive and not cost efficient for large nursery operations.
spp., caused by adult redheaded flea beetles
Systemic insecticides, when applied to the growing medium, are absorbed by plant roots and then translocated through the vascular system (i.e. xylem and phloem) into aboveground plant parts (e.g. leaves) where insects feed. Insects are killed when they ingest a lethal concentration of the systemic insecticide active ingredient during feeding. One of the primary benefits of systemic insecticides is that plants are protected for extended periods of time (e.g. eight to 10 weeks depending on the plant and systemic insecticide used) from feeding by insect pests, which can reduce foliar insecticide spray applications. Systemic insecticides are used primarily to protect plants from phloem and/or xylem-feeding insect pests such as aphids, whiteflies and leafhoppers. In addition, some species of leaf-feeding beetles may be susceptible to systemic insecticides. However, there is no information available on the effects of systemic insecticides on redheaded flea beetle adults. Therefore, the objective of our study was to determine if the systemic insecticides thiamethoxam, dinotefuran, acephate, cyantraniliprole and imidacloprid protect plants from feeding by field-collected populations of redheaded flea beetle adults under greenhouse conditions.
Materials and methods
The following study involved two experiments conducted under greenhouse conditions at Kansas State University (Manhattan, Kansas) from 2021 to 2022. The active ingredient, trade name, rate used and manufacturer information for the two greenhouse experiments are listed in Table 1.
Greenhouse experiment 2021
Redheaded flea beetle adults were collected on Sept. 15, 2021, from container-grown Hydrangea paniculata ‘Limelight Prime’ plants at Loma Vista Nursery. Adults were transported to the Horticultural Entomology and Plant Protection Laboratory in the Department of Entomology at Kansas State University (Manhattan, KS), aspirated into 33-milliliter vials, and starved for 24 hours.
Thirty-five Itea virginica ‘Little Henry’ plants were obtained from Loma Vista Nursery on July 2, 2021. The plants were started from cuttings using existing stock plants. The cuttings were transplanted on June 10, 2021 into 4.5 liter (1.1 gallon) containers with a growing medium consisting of 90% pine bark and 10% sand. The plants were fertilized with 18:2:7 (N:P:K) Nursery Polyon Controlled-Release Fertilizer (Harrell’s, LLC; Lakeland, Florida) with each 4.5 liter container receiving 15 grams of fertilizer.
There were six systemic insecticide treatments including thiamethoxam (Flagship), dinotefuran (Safari), cyantraniliprole (Mainspring), imidacloprid (Marathon II), acephate (Acephate), and cyfluthrin + imidacloprid (Discus) (Table 1). On Sept. 9, 2021, the systemic insecticide treatments were prepared in 946 milliliter (32 fluid ounce) plastic spray bottles and 300 milliliters of each treatment solution was applied as a drench to the growing medium of each plant using a 600-milliliter glass beaker. For the cyfluthrin + imidacloprid (Discus) treatment, one tablet was inserted 5 millimeters into the growing medium of each 4.5-liter container. Three-hundred milliliters of tap water were applied to remove the active ingredients from the tablet so they could be absorbed by the plant roots in the growing medium. Plants were grown in a greenhouse at 18 to 41°C (64 to 106°F), 31 to 100% relative humidity, and under natural light. There were seven treatments, including a water control, with five replications per treatment.
The experiment was set up as a completely randomized design using 35 clear plastic observation cages [45.7 x 45.7 by 60.9 centimeter (length x width x height)] arranged in rows on top of two wire-mesh greenhouse benches (4.3 x 1.1 meter). Each cage had a lid with a hole and two holes (12.7 centimeter diameter) on opposing sides covered with insect exclusion screening (0.15 x 0.15 millimeter: Green-Tek; Janesville, Wisconsin) to allow for ventilation and prevent redheaded flea beetle adults from escaping.
The treated Itea plants [17.8 ± 0.6 centimeters (mean ± SEM) in height] were placed randomly among the 35 plastic observation cages seven days after application of the treatments. Plants were watered as needed with 200 to 300 milliliters (6 to 10 fluid ounces) of tap water to minimize leachate. On Sept. 16, 2021, after the 24-hour starvation period, eight field-collected redheaded flea beetle adults were released into each plastic observation cage with an Itea plant. The temperature and relative humidity in the cages were recorded over the duration of the experiment using a data logger (Traceable Thermo-Hygro: Fisher Scientific; Hampton, N.H.). The temperature and relative humidity in the plastic observation cages during the course of the experiment were 19 to 34°C (66 to 93°F) and 68 to 100% relative humidity, under natural day light conditions.
On Sept. 30, 2021, 15 days after releasing the redheaded flea beetle adults into the plastic observation cages with the Itea plants, a damage ranking scale from 0 to 10 was used to quantify redheaded flea beetle adult leaf feeding where 0=0% leaf damage, 1=<1 to 10% leaf damage, 2=11 to 20% leaf damage, 3=21 to 30% leaf damage, 4=31 to 40% leaf damage, 5=41 to 50% leaf damage, 6=51 to 60% leaf damage, 7=61 to 70% leaf damage, 8=71 to 80% leaf damage, 9=81 to 90% leaf damage, and 10=91 to 100% leaf damage. In addition, for each treatment, the number of leaves fed upon was recorded and then the leaves were removed and placed into a 3.8 liter (1 gallon) plastic storage bag. The storage bags containing the leaves were transported from the greenhouse to the laboratory where they were frozen for 48 hours, then placed into a plant press, and dried for two weeks. After drying, the amount of leaf area fed upon was assessed by placing a transparent section of graph paper, containing 4 millimeter2 squares, over each leaf.
Ranked damage data associated with redheaded flea beetle adult feeding, the number of leaves fed upon, and leaf area fed upon (millimeter2) by the adults were normalized using a Log10+1 transformation procedure. Data were analyzed with treatment as the main effect. Treatment means were separated using Tukey’s honestly significant difference test when the analysis of variance indicated a significant main effect. Parametric statistics were used to analyze the ranked data (i.e. categorical/ordinal data). All data presented are non-transformed.
Greenhouse experiment 2022
The procedures for the 2022 greenhouse experiment were similar to the 2021 greenhouse experiment described above. Redheaded flea beetle adults were collected on Sept. 16, 2022 from container-grown Hydrangea paniculata ‘Little Lime Punch’ plants at Loma Vista Nursery and starved for 24 hours. Thirty-five Itea virginica ‘Little Henry’ plants were obtained from Loma Vista Nursery on July 11, 2022. The plants were started from cuttings using existing stock plants. The cuttings were transplanted on May 2, 2022 into 4.5 liter containers.
The treatments were the same as in 2021 (Table 1). On Sept. 9, 2022, the insecticide treatments were applied as a drench to the growing medium, except for the cyfluthrin + imidacloprid (Discus) treatment (see greenhouse experiment 2021). Plants were grown in a greenhouse at 22 to 46°C (71 to 114°F), 40 to 91% relative humidity, and under natural light. There were seven treatments, including a water control, with five replications per treatment.
The treated Itea plants (23.2 ± 0.6 centimeters in height) were placed randomly among the 35 plastic observation cages seven days after application of the treatments. On Sept. 17, 2022, after the 24-hour starvation period, eight field-collected redheaded flea beetle adults were released into each plastic observation cage with an Itea plant. The temperature and relative humidity in the plastic observation cages during the course of the experiment were 22 to 37°C (71 to 98°F) and 0 to 100% relative humidity, under natural daylight conditions.
On Oct. 2, 2022, 15 days after releasing the redheaded flea beetle adults into the plastic observation cages with Itea plants, the damage ranking scale described in the 2021 greenhouse experiment was used to quantify redheaded flea beetle adult leaf feeding. Data analysis was identical to the 2021 greenhouse experiment described previously.
Results
Greenhouse experiment 2021 Redheaded flea beetle adult feeding damage rankings (F=13.1; df=24, 6; P<0.0001), the number of leaves fed upon (F=10.12; df=24, 6; P<0.0001), and leaf area (millimeter2) fed upon (F=10.05; df=24, 6; P<0.0001) were significantly different across the treatments. Overall, Itea plants associated with the thiamethoxam (Flagship) at 0.49 grams/946 milliliters, dinotefuran (Safari) at 1.27 grams/946 milliliters, and acephate (Acephate) at 0.85 grams/946 milliliters treatments had the least amount of leaf area fed upon (<50 millimeters2) by redheaded flea beetle adults (Table 2).
Greenhouse experiment 2022 Redheaded flea beetle adult feeding damage rankings (F=7.6; df=24, 6; P=0.0001), number of leaves fed upon (F=15.08; df=24, 6; P<0.0001), and leaf area (millimeter2) fed upon (F=20.11; df=24, 6; P<0.0001) were significantly different across the treatments. Overall, Itea plants associated with the thiamethoxam (Flagship) at 0.49 grams/946 milliliters, dinotefuran (Safari) at 1.27 grams/946 milliliters, and acephate (Acephate) at 0.85 grams/946 milliliters treatments had the least amount of leaf area fed upon (<32 millimeters2) by redheaded flea beetle adults (Table 3).
Discussion
The results from our study demonstrate that the systemic insecticides, thiamethoxam (Flagship), dinotefuran (Safari), and acephate (Acephate), provide protection from redheaded flea beetle adult feeding based on the number of leaves and leaf area (millimeters2) fed upon by redheaded flea beetle adults. The information associated with our study is important to nursery producers because we found that the three systemic insecticides reduced redheaded flea beetle adult feeding damage, which will allow for salability of nursery-grown crops that are susceptible to redheaded flea beetle adult feeding including: Hydrangea spp., Itea spp., Weigela spp. and Cornus spp.
Thiamethoxam (Flaghship) and dinotefuran (Safari) are neonicotinoid insecticides with similar physical and molecular properties, and mode of action, which involves modulation of the nicotinic acetylcholine receptor. However, the systemic insecticides differ in water solubility. For example, the water solubility of thiamethoxam (Flagship) and dinotefuran (Safari) is 4,100 and 39,830 parts per million (4.1 and 39.0 grams/liter at 20°C), respectively. Furthermore, the systemic insecticides are converted into metabolites that, in general, are more toxic to insects than the original active ingredients. For instance, thiamethoxam (Flagship) is converted into clothianidin and dinotefuran (Safari) is converted into UF [1-methyl-3-(tetrahydro-3-furylmethyl) urea] and DN [1-methyl-3-(tetrahydro-3-furylmethyl) guanidium dihydrogen].
Acephate (Acephate) is an organophosphate systemic insecticide that is converted into the metabolite, methamidophos, in plant leaves. Methamidiphos is more toxic to chewing and sucking insect pests than acephate. The water solubility of acephate (Acephate) is 790,000 parts per million (79 grams/liter at 20°C), which means the systemic insecticide moves quickly throughout the plant. Systemic insecticides that have a high water solubility are rapidly translocated throughout plants and reach plant tissues (e.g. leaves and stems) fed upon by insect pests.
Redheaded flea beetles can overwinter in the containers of plants grown under nursery conditions. Consequently, the application of systemic insecticides may kill redheaded flea beetle adults before they emerge from the growing medium, which would reduce the number of adults feeding on plant leaves. In addition, the systemic insecticides will likely kill the larval stages of the redheaded flea beetle, thus reducing the number of adults emerging from the growing medium.
In conclusion, our study demonstrates that the systemic insecticides, thiamethoxam (Flagship), dinotefuran (Safari) and acephate (Acephate), protected plants from redheaded flea beetle adult feeding. Therefore, applications of any one of the three systemic insecticides may contribute to less plant damage from redheaded flea beetle adults during the growing season. Future studies will assess the length of residual activity of the systemic insecticides in protecting plants from redheaded flea beetle adult feeding and determine the number of applications that are needed during the growing season.
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