James E. Board and David J. Boethel

Insects that eat soybean leaves, such as the soybean looper, velvetbean caterpillar and green cloverworm, usually attack in late August and September. With the exception of varieties planted in April, most soybeans planted in Louisiana are in the mid to late seed-filling period at this time. Economic thresholds have been developed to indicate to farmers when insecticide application is required to avoid yield loss, but these thresholds do not reflect the soybean’s increased tolerance to defoliation as it progresses through the seed-filling period and, therefore, may be overly conservative.

For example, total defoliation of the plant at mid seed filling reduces yield by about 40 percent, whereas total defoliation at late seed filling reduces it only by 20 percent. Because determination of economic thresholds for each specific stage of seed filling is difficult, an alternative approach based on light interception was investigated. Light interception by a crop is the percentage of the sun’s light intercepted by the canopy. It can easily and rapidly be measured with an instrument called a light bar or approximated with an experienced eye. Light interception is a good candidate for use as an insecticide-application criterion, because it is based on the mechanism of how defoliation during seed filling reduces yield.

Many researchers have recognized that insect defoliation reduces yield primarily through reduced photosynthetic activity caused by decreased light interception. Previous studies have demonstrated the importance of maintaining 95 percent light interception (of whatever the control level is) during seed filling for optimum yields. Our objectives in the following studies were to determine specific light interception criteria for insecticide application at the mid and late seed filling stages and to determine if these criteria are applicable across a broad range of soybean varieties.


Defoliation Study

Centennial soybeans were planted two years (1993 and 1994) at an optimal date and subjected to partial and total defoliations at the mid and late seed filling periods. Treatments at both stages were no defoliation (control), 33 percent defoliation, 66 percent defoliation and 100 percent defoliation. Actual defoliation levels differed somewhat from these desired levels and are reported in Table 1. Because defoliation treatments affected yield similarly in both years, data were averaged across years. Results confirmed the greater tolerance to defoliation at late compared with mid seed filling.

At mid seed filling, a small drop in light interception from 98 percent to 92 percent (caused by a 41 percent reduction in leaf area) was sufficient to significantly reduce yield by 4 bushels per acre. Yield losses became higher as defoliation increased and light interception declined. Thus, optimum yield at mid seed filling depended on keeping light interception at 95 percent of the control level. In contrast, at late seed filling a decrease in light interception from 90 percent (because of leaf loss, the control level of light interception declines as seed filling progresses) to 74 percent, created by a 61 percent reduction in leaf area, did not significantly reduce yield. Significant yield losses (large enough to justify insecticide application) at this time occurred only with total defoliation. Based on this result, avoiding yield loss at late seed filling depended on maintaining light interception at 82 percent of the control level.


Tests on 10 Varieties

Ten soybean varieties representing Maturity Groups IV, V, VI and VII were tested in 1997 and 1998 to determine if the criteria for insecticide application described above were applicable across a range of different varieties. These varieties were given a partial defoliation treatment at the midpoint of seed filling sufficient to reduce light interception to just below 95 percent of the control level and a total defoliation treatment at late seed filling. Yields for these treatments were compared to a control to determine if yield losses were similar to those for Centennial in the previous experiments. In 1997, partial defoliation at mid seed filling for all varieties, except DP3478 and Hutcheson, resulted in small yield losses (Table 2). In five cases, yield losses were large enough to be statistically significant. When averaged across all varieties, there was a 5 bushel per acre yield loss, a reduction similar to the 4 bushel per acre loss with Centennial. Yield losses in 1998 were similar, although a little larger, compared to 1997.

Yields for all varieties were reduced by partial defoliation at mid seed filling (Table 2). Significant reductions occurred for six of the varieties. Averaged across all varieties, there was a 6 bushel per acre yield loss. Results from both years supported the criterion that insecticide application at the midpoint of seed filling is required whenever light interception falls to 95 percent of the control level. Results from the 1997-98 studies confirmed insecticide application at late seed filling is justified to prevent total defoliation. In 1997, yield reductions caused by total defoliation were significant for all varieties except Hutcheson. In 1998, total defoliation resulted in significant yield losses for all varieties. The yield reductions, averaged across varieties, were 11 bushels per acre in 1997 and 12 bushels per acre in 1998, figures similar to the 10 bushels per acre yield loss by Centennial in the previous study. Based on these results, insecticide application at late seed filling is justified when light interception falls to 82 percent of the control level.

Light Interception Criteria

Use of light interception criteria for insecticide application by farmers and consultants depends on proper identification of the mid and late seed filling periods, as well as determination of light interception. Developmental periods for soybean varieties within a maturity group tend to be similar because of strong genetic control. For May plantings, approximate times for mid and late seed filling for the different maturity groups are listed below.

Mid Seed Filling Late Seed Filling Early Maturity Group IV: Early August Mid August Normal Maturity Group IV: Mid August Late August Maturity Group V: Mid to Late August Early September Maturity Group VI: Late August to Early September Mid September

Light bars can be purchased commercially for about $1,500 to $2,000. Recordings should be taken between 11 a.m. and 3 p.m. while the sun is not blocked by clouds. More research is needed to determine the optimum number of samples required per field. However, the technique offers an alternative to the traditional method of sampling and assessing the insect population and the defoliation level to determine whether an insecticide application is warranted. With training and experience, private pest management consultants, extension agents and farmers may be able to accurately assess light interception levels by visual observations. This would be advantageous because it would avoid the expense of the light bar and reduce sampling time. Also, determinations could be made at any time of the day, as long as the field could be entered.