S1039: Biology, impact, and management of soybean insect pests in soybean production systems.(S-1010)
Statement of Issues and JustificationSoybean is a key crop in the U.S., which supplies 35% of the world's production. In 2006, more acres of soybean were harvested in the U.S. (74.6 million) than acres of any other crop, including corn (70.6 million). Soybeans are produced in 31 states and 3 provinces in the eastern, central, and southern U.S and Canada, from Quebec to Florida, and Delaware to Nebraska. Given this large acreage and wide distribution, it is not surprising that soybean continues to suffer from insect pests that impact plant growth, grain quality, and yield.
Whether it is a result of accumulated years in soybean production, changes in cropping practices, or global climate change, the distribution and impact of native and established pests is increasing in soybean. The populations of soil pests such as slugs, grubs, and millipedes, and foliar and pod feeders such as bean leaf beetle and stink bugs, are increasing in many regions. The distribution of other insects, such as Dectes stem borer and pyrethroid-resistant Lepidoptera (such as corn earworm), appears to be growing. Producers are encountering insect problems that they have never seen or managed. From a research standpoint, changes in pest distribution and status require greater understanding of pest biology and movement, as well as the development or modification of scouting and control methods. Although pesticides are often the easy first tools used to deal with new or growing insect problems, long-term sustainable solutions must include host plant resistance and biological control.
As in many other production systems, invasive species in particular are also an increasing issue in soybean production. The introduction of a single species can significantly and negatively impact the profitability of soybean production, and increase risks to human health and the environment. In the northern U.S. and eastern Canada, the discovery of the soybean aphid (SBA) in 2000 fundamentally changed soybean insect management. SBA feeding impacts all components of yield; under heavy aphid pressure, yield differences between treated and untreated research plots may reach 50%. On a landscape level, SBA outbreaks often correspond to potyvirus outbreaks in dry beans, snap beans, vine crops, and potato, broadening the impact of this invasive insect. From an economic standpoint, SBA infestation increases the cost of soybean production. In 1999, the year prior to discovery of SBA, the National Agricultural Statistics Service estimated that less than 1% of the soybean acres in Illinois, Indiana, Michigan, Minnesota, and Ohio were treated with insecticide (NASS 2000). In 2005, an outbreak year, estimated insecticide use in these same states ranged from 9% (IL) to 42% (MI) (NASS 2006); in 2006, when outbreaks were more sporadic, 56% of the acreage in Minnesota was treated (NASS 2007). At least 20 different foliar and seed-applied insecticides are now used for aphid control on millions of acres, adding $10-$20 per acre to production costs (Song et al. 2006). Although scouting methods and an economic threshold (ET) for aphid control were developed under the previous multi-state project, S-1010, these management practices are only applicable to soybeans in the R1 to R5 growth stages planted in 30-inch rows. The thresholds must be modified to account for plant maturity group and age, differences in planting systems (for example, varying row widths), and populations of biological control agents. Developing a dynamic threshold, however, requires a great deal more knowledge of aphid interactions with host plants, natural enemies, and the environment. By developing common research protocols implemented simultaneously in multiple states, we can rapidly modify the existing ET for SBA under different production practices, e.g., maturity groups, row spacing, planting date, etc.
Specialty soybean production is a growing, profitable market for producers, including production of seed, non-GMO, identity preserved (IP) and USDA-certified organic soybeans. IP soybeans are sought for traits such as high protein, high isoflavone, or improved flavor or texture in the case of tofu (bean curd) and edamame (vegetable soybean). University and private seed companies are breeding soybeans to deliver lower linolenic acid composition than conventional soybeans. Low lin soybeans produce oil that does not need to be hydrogenated, thus reducing the amount of trans fat in foods made with these IP soybeans. Since USDA implemented national organic standards in 2002, the U.S. organic industry has grown 20-percent or more annually (USDA 2007). By volume, soybeans make up the largest segment of organic legumes in the U.S. In 2005, 122,217 acres of certified organic soybeans were produced in the U.S., with Minnesota, Iowa, Michigan, Ohio and Wisconsin among the top five acres produced (USDA 2007). Canadian producers planted organic soybeans on 19,922 acres in 2005 (Macey 2006). Production practices and quality requirements of specialty beans differ from those of conventionally-produced roundup-ready beans, thus insect management practices (scouting, thresholds, and control measurements) are also likely to differ.
Chemical use has also changed dramatically in soybean production since the inception of the S1010 project. As mentioned previously, the introduction of soybean aphid resulted in an increase in pesticide use on soybeans in the Midwest. In most soybean production areas, the introduction of soybean rust will change soybean disease management; this will influence the entire production system, including insect management. For example, fungicide applications may actually increase insect pressure by reducing the impact of naturally occurring entomopathogenic fungi that control insects and mites. Since most fungicides cannot cure plant disease, they must be applied prophylactically. Preventative applications of fungicide made prior to infection may encourage growers to make unnecessary tank-mixes with insecticides. Industry recommendations to customers now routinely include tank mixes of insecticides and fungicides for plant health benefits, even in the absence of pest pressure. The recent registration of seed-applied insecticides also encourages growers to use products as insurance treatments rather than basing applications on scouting in an integrated pest management context. Besides violating the principles of IPM, prophylactic use of insecticide adds to the cost of production and increases the potential for insecticide resistance. From a research perspective, the efficacy and economic viability of insurance treatments need to be addressed in a concerted way. To move beyond chemical use, particularly to control bean leaf beetle, soybean aphid, stink bug, and other chronic pests, alternatives are critically needed. These alternatives include both biological control and host plant resistance, preventative tactics which form the foundation of IPM, and which must be integrated with therapeutic tactics such as insecticide use.
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