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Incorporating Granular Imidacloprid to Reduce Red-Headed Flea Beetle Injury
pest of container A nursery ornamentals since approximately 2007, the redheaded flea beetle (RHFB) (Systena frontalis) eats holes in the foliage of broadleaf container ornamental plants, reducing their marketability. In 2021, I conducted a nursery research trial comparing three rates of Mallet 0.5G insecticide containing imidacloprid, a systemic neonicotinoid insecticide, to test its effectiveness in reducing damage from this insect.
Incorporating Granular Imidacloprid
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to Reduce Red-Headed Flea Beetle Injury
By Danny Lauderdale
Area Specialized Agent, Nursery and Greenhouse North Carolina State University Extension danny_lauderdale@ncsu.edu
Materials and Methods
On April 1, 2021, 960 three-gallon pots were planted with Itea virginica ‘Henry’s Garnet’ plants rooted in six-cell packs, and three liners were potted into each three-gallon container. The four different substrate treatments tested included: 1. An untreated control (UTC) using the nursery cooperator’s standard potting mix 2. Addition of a low incorporated rate of Mallet 0.5G (3.4 pounds per cubic yard) 3. Addition of a medium incorporated rate of Mallet 0.5G (5 pounds per cubic yard) 4. Addition of a high incorporated rate of Mallet 0.5G (6.6 pounds per cubic yard)
Pots were arranged in a 16-block randomized complete block design with four replicated blocks of 60 pots per treatment. Pots were watered with overhead irrigation throughout the project, and no foliar insecticides were applied. Pots were spaced on July 9, 2021, and plants were pruned on July 15, 2021. Plants were monitored for RHFB damage every other week beginning in mid-May. Ratings of foliar injury and percentages of plants with damage were recorded once damage became easily noticeable.
Results and Discussion
Ratings began on Aug. 6, 2021, at 18 weeks after treatment (WAT). At 18 WAT and 20 WAT, all rates of Mallet 0.5G were providing statistically similar levels of control, with reduced percentages of plants with damage compared to the UTC. At 22 WAT, the medium and high rates of Mallet 0.5G resulted in lower percentages of plants damaged (17.75% and 15.25%, respectively) by RHFB, compared to the low rate of Mallet 0.5G (39.75%) and UTC (77.75%).
At 18 WAT, 20 WAT and 22 WAT, plants treated with all rates of Mallet 0.5G had lower foliar damage ratings compared to the UTC. The study concluded on Sept. 2, 2021, since plants were pruned again on Sept. 8, 2021.
Conclusions and Recommendations
Although overall RHFB pressure was low in the study, differences among the treated and UTC plants were obvious during the foliar damage rating period. By the end of the study, a significantly higher percentage of plants were damaged by RHFB when treated with the low rate of Mallet 0.5G, compared to plants treated with the medium and high rates. All Mallet 0.5G treatments resulted in lower foliar damage ratings compared to the UTC.
Percent of plants* damaged by RHFB with incorporation of Mallet 0.5G (potted on 4/1/2021)
Foliar damage rating means with the same letter within columns are not statistically different. Duncan’s Multiple Range Test, .05 level. (*Itea virginica ‘Henry’s Garnet’) **Foliar Damage Rating: 0=0%, 1=1-10%, 2=11-20%, 3=21-30%, 4=31-40%, 5=41-50%, 6=51-60%, 7=61-70%, 8=71-80%, 9=81-90%, 10=91-100%.
Percent means with the same letter within columns are not statistically different. Duncan’s Multiple Range Test, .05 level. (*Itea virginica ‘Henry’s Garnet’)
This study showed that Mallet 0.5G was effective when incorporated at potting and provided long-term reduction of damage. Based on the results of this study and other research and demonstrations I have conducted, granular imidacloprid insecticides are effective at reducing foliar damage from RHFB. You can expect to see very low levels of damage, even at medium and high rates, since RHFB has to feed on foliage to ingest the insecticide. I hypothesize that the pressure was low in this study due to the use of small sixpack rooted liners that were free of RHFB injury and population when rooted in 2020. The RHFB population in the study area slowly moved in from surrounding container production areas and remained low, since, due to sales demand, many of the surrounding susceptible plants were moved to other finishing areas. My
A greater percentage of UTC and low-rate treated plants had feeding injury by the end of the study, at 22 WAT, compared to plants treated with medium and high rates of Mallet 0.5G. Plants treated with medium or high rates of Mallet 0.5G had beautiful foliage with very limited RHFB feeding injury.
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recommendation is to use medium or high rates of Mallet 0.5G when incorporating it at potting to provide the highest and longest-lasting levels of RHFB foliar damage prevention.
Additional Information
In eastern North Carolina, treatments with imidacloprid targeting larvae are best prior to 250 growing degree days, based on 50 degrees Fahrenheit (GDD50), and I try for 200 GDD50 in established plants with known RHFB population or at potting for clean liners. Drench-applied imidacloprid is also effective with one drench application at spring potting (April 15) and has resulted in as little as no foliar damage for at least two months (June and July). Topdress applications of the other granular imidacloprid product labeled for nursery use (Marathon 1% Granular) have provided foliar damage reduction lasting more than three months. In a demonstration done with drench (Imidacloprid 2F) and granular (Marathon 1% Granular high rate) imidacloprid, plants treated at potting (June 2019) had no larvae in container substrate in April 2020 and had no foliar injury in June 2020. This was also true for imidacloprid (Imidacloprid 2F) liner drenches applied prior to potting.
Imidacloprid drench and granular insecticides are effective tools in reducing damage caused by RHFB. As with all pest-management tools, read and follow label instructions related to application rates; maximum allowable per acre per year; and all other crop, environmental and applicator safety precautions.
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