Control of Soybean Aphid (Aphis glycines) with Natural Insecticides
Test Conducted By:
Plant Research Services
Dr. Patrick J. McGinnity
Objective:
To evaluate the efficacy of four natural or naturally derived insecticides for the control of soybean aphid (Aphis glycines) in field grown soybeans.
Methodology:
Soybeans were selected in a southeastern Iowa location that exhibited a serious infestation of adult soybean aphids. The soybeans were planted on 30 cm centers. Plots were 6 rows wide and 7.3 meters long. The center 2 rows were treated and the outer rows were left as buffers between the plots.
A pressurized handheld sprayer was used to apply the products to the plots. Nozzles were configured to drop down into the canopy and apply the product directionally onto the soybean plants. The products were applied using hollow cone nozzles at 45 psi of pressure. Two volumes of carrier were evaluated, 40 and 60 gallons of spray solution per acre.
Treatments were replicated six times and evaluations were measured at 24, 48 and 96 hours after treatment. The products evaluated are listed in the table below:
| Active Ingredient |
Formulation |
Brand name |
Application Rate |
| Neem based Fatty Acids |
25% potassium salts
of fatty acid |
K-Neem |
1 gal/A |
| K Salt of Fatty Acids |
49% potassium salts
of fatty acid |
M-Pede / Safer |
.5 gal/A |
| d-limonene |
5% d-limonene |
Orange Guard |
1 gal/A |
| Capsaicin |
5% capsaicin |
Unknown |
.5 gal/A |
Treatments were applied in full sunlight early in the afternoon. Outside temperature was 23 C and there was a 5 mph wind out of the SW. There was no dew on plant surface and soil moisture was adequate. The soybeans were in early pod fill and the relative infestation of soybean aphids was determined by randomly selecting 20 plants from the test plot area and counting the number of live adult aphids. The average number of aphids per plant was found to average 450 aphids per plant with a range of 250 to 1100.
Control measurements were determined by visually evaluating the number of necrotic aphids on 6 randomly selected plants for each plot and taking the average and transforming it to control index.
Results and Discussions:
The efficacy of the treatments was evident within 24 hours and increased slightly at 48 hours with very little increase in activity following the 48 hours after treatment (See Table 1). There was a significant interaction between volume and efficacy. As spray volume was increased to 60 gallons per acre, the efficacy of the treatment increased as well. This held true for all of the products tested, except for the capsaicin. Since the other products rely on contact with the insect pest for control, anything that can increase the coverage of the target pest should enhance the activity of the pesticide.
Table 1. The efficacy of four natural insecticides on the soybean aphid (Aphis glycines) in a field environment.
| Treatment |
Rate |
Spray Volume |
24 hr. + |
Control Index *
48 hr. + |
96 hr. + |
| K-Neem |
1 gal/A
1 gal/A |
40 gal/A
60 gal/A |
8.2b
9.6a |
8.6a
9.4a |
8.4b
9.2a |
| M-Pede |
5 gal/A
5 gal/A |
40 gal/A
60 gal/A |
7.0c
8.9a |
8.0b
9.0a |
7.8c
8.8a |
| d-limonene |
1 gal/A
1 gal/A |
40 gal/A
60 gal/A |
6.8c
7.9b |
7.2c
7.8b |
7.5c
7.8c |
| Capsaicin |
5 gal/A
5 gal/A |
40 gal/A
60 gal/A |
3.0d
3.2d |
2.8d
3.3d |
2.9d
3.1d |
* Ratings are an index between 1-10 where 1 is no activity and 10 is complete control or kill of the pest.
+ Numbers followed by the same letter are not significantly different from each other.
The fatty acid based products performed very similar to each other in terms of insect mortality. The neem based fatty acid trended to higher control numbers than the other fatty acid products tested. The best control overall was obtained when the fatty acid products were applied at the higher spray volumes. Insects treated with the fatty acids slowed down noticeably and appeared to stop feeding. The aphids became necrotic turning a dark color within 24 hours and stopping all movement.
The control for the d-limonene was significantly less than the fatty acid based products but significantly better than the capsaicin based product. The capsaicin product had little effect on controlling the pest directly, however the manufacturer claims that the mode of action is mostly as a repellent and not a direct toxicant.
There was no visible phytotoxicity from any of the fatty acid based products regardless of the spray volume. The d-limonene caused some leaf burn to the soybean plants however the damage was mostly cosmetic and isolated to the leaf margins. The capsaicin did not cause any phytotoxicity but was very irritating to exposed skin.
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