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Effects of Reduced-Risk Insecticides on the Soybean Natural Enemy Community: 2005 Field Studies
W. J. Ohnesorg, M. E. O’Neal, and K. D. Johnson
Department of Entomology, Iowa State University, Ames, IA
Currently, soybean aphids are managed mainly by organophosphate and pyrethroid insecticides. These broad-spectrum insecticides kill soybean aphid and other insects whether benign or beneficial. Reduced-risk insecticides are thought to have limited non-target impacts by modes of action and/or chemistries that target specific habitats or species. This may conserve natural enemies that suppress soybean aphid populations. Our goal was to determine the impact of reduced-risk insecticides on both soybean aphids and on the natural enemy community in soybeans.
The following report describes the experiment conducted in 2005. This experiment was repeated in 2006. Our research methods and insecticide rates are described in the materials and methods»
Effects of insecticides on natural enemies
The abundance and diversity of the natural enemy community was measured with two methods: In-field identification and counts of natural enemies from five consecutive undisturbed plants (whole plant counts); and sweep-net samples (20 sweeps per replicate). Lady beetles (Coccinellidae) and pirate bugs (Anthocoridae) were the dominant natural enemies present (Table 1)
Table 1. Natural enemy community composition by sampling method for three dates combined (6/27, 7/26, and 8/16).
| Natural enemy |
Whole plant |
Sweep-net |
| Harmonia axyridis |
8.2% ab |
43.5% ab |
| Coccinella septempunctata |
0 |
0.5% a |
| Hippodamia convergens |
0 |
1.1% a |
| Hippodamia parenthesis |
0 |
0.3% a |
| Other Coccinellidae |
19% abd |
4.0% ab |
| Orius insidiosus |
40.5% ab |
12.2% a |
| Nabidae |
1.0% ab |
6.6% ab |
| Pentatomidae |
0.5% bc |
3.2% ab |
| Chrysopidae |
11.3% abc |
8.2% ab |
| Hemerobiidae |
0 |
0.3% a |
| Dolichopodidae |
0 |
7.4% a |
| Syrphidae |
9.7% ab |
4.5% a |
| Aphelinidae (mummy) |
4.6% d |
0 |
| Braconidae (mummy) |
2.1% d |
0 |
| Other parasitic Hymenoptera |
0 |
1.6% a |
| Spiders |
2.1% |
6.1% |
| Opiliones |
1.0% |
0.5% |
| Total natural enemies captured |
195 |
377 |
a=Adults; b=Immatures; c=Eggs; d=Pupae |
In 2005, we found no significant differences in natural enemy abundance across any treatments as sampled by sweep-net and direct visual observations. Additional sampling data from yellow sticky traps has yet to be analyzed. Figure 1 shows a comparison of natural enemies from untreated plots and those treated with low-risk insecticides.
Figure 1. Total natural enemies captured by sampling method and treatment. There were no significant differences across the 6 treatments at three sampling dates. |
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The three dates sampled were a) pre-foliar application 6/27/2005; b) pre-foliar application 7/26/05; and c) post-foliar application 8/16/2005. |
Effects of soybean insecticides on soybean aphids and soybean yield
The use of reduced-risk insecticides resulted in aphid kills and soybean yields that were greater than that of untreated soybeans, but less than that of soybeans treated with broad-spectrum insecticides. The highest aphid populations were in the untreated plots, followed by plots receiving seed treatments. Lowest populations were in plots treated with foliar insecticides (Figure 2).
Figure 2. Impact of various foliar (solid, colored lines) and seed-applied (dashed, colored lines) insecticides on soybean aphids during July and August. The highest aphid populations were in the untreated plots, followed by plots receiving seed treatments. Lowest populations were in plots treated with foliar insecticides. |
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Soybeans were planted on 22 May and foliar insecticides were applied on 2 August, 2005. |
Foliar-applied broad-spectrum organophosphate and pyrethroid insecticides resulted in the lowest aphid-day (AD) values, followed by the foliar-applied reduced risk insecticides Trimax and Fulfill. Treatments with the highest cumulative aphid-days were the nicotinoid seed treatments, which were statistically not different from the untreated (Figure 3).
Figure 3. Soybean exposure to aphids after application of various foliar insecticides. |
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Exposure based on accumulation of aphid days (AD) calculated by:
[(previous mean # aphids/plant + current mean # aphids/plant)/2] x time interval in days. The AD accumulated over the month-long sampling period are reported along with means labeled with a unique letter for statistical significance. |
Soybean yields were highest from the broad-spectrum insecticide plots, intermediate from the reduced-risk plots, and lowest from the untreated plots (Figure 4). Seed-treated soybeans provided limited aphid control and yield protection, but when the same active ingredient (imidicloprid) applied directly to seed (Gaucho) was applied to foliage (Trimax) there was a significant improvement in aphid control and yield. This improvement in yield protection, however, was still less than that provided by broad-spectrum insecticides.
Figure 4. Impact of reduced-risk and broad-spectrum insecticides on soybean yield (corrected for 13% moisture) with unique letters labeling the means for statistical significance. |
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