Topdressing Granular Imidacloprid to Reduce Red-Headed Flea Beetle Injury

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Leaf damage

RHFB feeding injury on Itea virginica

The red-headed flea beetle (RHFB), Systena frontalis, is a major pest of many container-grown ornamentals that eats holes in leaves making plants unmarketable. Foliar spray programs to prevent damage require repeated applications, are time-consuming, and are costly. Since growers have been seeking alternatives to repeat application spray programs, I completed a nursery research trail during 2021 to compare three different rates of Marathon 1% Granular Insecticide (Marathon 1G) which contains the active ingredient imidacloprid, a systemic neonicotinoid insecticide.

Materials and Methods

On March 29, 2021, 1152 Itea virginica ‘Little Henry’ liners grown in 4″ containers


Topdressing with Marathon 1G

were potted to 3-gallon containers using the nursery cooperators standard potting mix. Nursery employees placed plants into 16 blocks of 72 plants each. Plants were spaced approximately five inches apart in blocks and blocks were 10 feet apart in a triangular spacing. On March 30, 2021, I treated 12 plant blocks with one of 3 different rates of Marathon 1G insecticide (4 blocks with each rate). Treatment topdress rates were Low (4 tsp. per 3-gallon container), Medium (5.5 tsp. per 3-gallon container), and High (7 tsp. per 3-gallon container). 4 plant blocks were left untreated. Treatments to blocks were made to create a randomized complete block design. All plants were watered with overhead irrigation as needed which was determined by nursery personnel. Plants were all pruned once on June 9, 2021, to remove 4-6 inches of growth to

Plants in pots

Randomized treatment blocks.

promote branching and uniformity. Plants were monitored for RHFB injury based on a rating scale and percent of plants damaged. Rating began on June 8, 2021, once damage was easily noticeable and was recorded every other week through September 14, 2021.

Results and Discussion



At all treatment dates, all treatment rates resulted in a significantly lower percent of plants damaged than the untreated control. At 10, 12, 14, 16, 18,

Untreated plant

Example of untreated plant damage on Sept. 14, 2021. 4.2153 rating

20, and 24 WAT (Weeks After Treatment) there were no differences between the 3 treatment rates. At 22 WAT there were slight differences in percent of plants damaged by RHFB between low, medium, and high treatment rates. At 22 WAT the percent of plants damaged even at the low rate remained low at 6.20% compared to 2.78% for the medium rate, and 0.70% for the high rate. At the final treatment date when 100% of the untreated plants had damage, only 6.63% of plants treated with the low rate had any damage, 2.78% of plants treated with the medium rate

Treated plant

Example of low rate treated plant damage on September 14, 2021. 0.0698 rating

had any damage, and 1.40% of

plants treated with the high rate had any damage.

Treated plant

Example of medium rate treated plant damage on September 14, 2021. 0.0278 rating

Surprisingly, early in the study at the 10 WAT foliar damage rating, the high rate had some similarity to the untreated control but also had some similarity to the low and medium rates. All mean ratings were below 1 at that point (a 1 rating is 1 to 10% of plant leaves with injury). Following the first data collection at 10 WAT plants were pruned on 6/9/21. After that, the 12 WAT foliar damage ratings and all other ratings through 24 WAT showed a significant difference between all rates of Marathon 1G when compared to the untreated control. There was no difference from 12 WAT to 24 WAT between any of the 3 treatment rates of Marathon 1G and the mean damage rating remained low (below 1) through the entire trial for all treatment rates.

RHFB pressure was high in plant blocks bordering the research area. The highest pressure was on the south side of the study area and secondarily from the north side of the research area. Plant injury in the untreated research blocks increased the quickest and the highest foliar damage ratings were seen in the

Treated plant

Example of high rate treated plant damage on September 14, 2021. 0.0140 rating

most southern block first and then the most northern block next. The plants in the two central untreated blocks had foliar damage ratings and percent of plants damaged climb as the WAT increased but they tended to be lower than the southern and northern block ratings and percentages. By the end of the study, all untreated control plants had foliar damage while few treated plants at any rate had very low levels of damage.

Conclusions and Recommendations

At the end of the trial all rates of Marathon 1G resulted in plants with a significantly lower foliar damage rating and percent of plants damaged when compared to the untreated control. This study showed that Marathon 1G was effective at all rates in preventing damage from RHFB that would make plants unmarketable. Based on what I have seen in other studies, low rates of granular imidacloprid whether topdressed or incoporated will prevent significant plant injury at least through the middle of a crop production cycle and possibly longer as was true in this study. Medium and high rates of granular imidacloprid have always provided long-term control of RHFB and prevented significant damage.

Topdressing granular imidacloprid products for RHFB control may be useful to growers with concern over worker protection standards and personal protective equipment required for the entire potting and handling crew if imidacloprid products are incorporated in the potting mix. However, the only additional piece of personal protective equipment needed with incorporating granular imidacloprid products over granular bifenthrin products that many growers use for fire ant quarantine requirements is protective eyewear.

Additional Information

Based on my prior research and this, imidacloprid products are very effective at controlling RHFB larvae when applied to nursery substrate prior to egg hatch. It is important if potting up larger liners, (for example quart or 1 gallons) that had a RHFB infestation the previous year, to make applications for larvae in order to mostly eliminate first-generation adults in the crop and prevent early heavy injury to plants and subsequent population explosion. In eastern NC, treatments with imidacloprid targeting larvae are best made prior to 250 growing degree days based on 50 degrees Farenheit (GDD) and I try to shoot for 200 GDD. Drench applied liquid formulations of imidacloprid are effective also and seem to result in no foliar damage for at least 2 months (June and July) with one drench application at spring potting (April 15). Incorporation of the other granular imidacloprid product labeled for nursery use (Mallet 0.5G) at medium and high rates at potting (April 1) has provided foliar damage prevention in studies and demonstrations lasting 3 months (June, July, August). In a demonstration done with drench (Imidacloprid 2F) and granular (Marathon 1G high rate) applications of imidacloprid, plants treated during their potting year (June 2019) had no larvae in their container substrate the following spring (April 2020) and had no foliar injury in June 2020. This was also true for imidacloprid (Imidacloprid 2F) liner drenches applied prior to potting and drenches of the same applied after potting. Imidacloprid liquid (drench) and granular insecticides are effective treatments to help reduce damage caused by RHFB. As with all pest management tools, read and follow label instructions related to the application rates, maximum allowable per acre per year, and all other crop, environmental, and applicator safety precautions.

Learn more about RHFB research and demonstrations