Insect resistance

The development of insect-resistant soybean varieties would reduce pesticide use in controlling insects, but little success has been achieved. The army worm (Spodoptera exigua), leaf beetles (Cerotoma trifurcata), soybean looper (Pseudoplusia includens), aphids (Aphis glycines) and grasshoppers (Melanoplus species) feed on soybean leaf, while the stink bug (Acrosternum hilare), corn earworm (Heliothis zea) and soybean stem borer (Dectes texanus texanus) feed on pods and stems (Boethel, 2004). The development of insect resistance in soybean is not a priority for most breeding programmes, probably because the magnitude of loss caused by insects is not as severe as that by disease. In addition, there are difficulties in combining high yield with insect resistance. Although breeding efforts have resulted in insect-resistant cultivars, the yield potential of these cultivars is generally lower than that of conventional cultivars under conditions of light insect pressure. For example, an extensive evaluation of germplasm from the US Department of Agriculture collection in the late 1960s identified three Japanese PIs resistant to a number of insects in soybean. These PIs (namely PI 171451, PI 227687 and PI 229358) were resistant to bean beetle (Epilachna varivestis), soybean looper (Pseudoplusia includens), velvet bean caterpillar (Anticarsia gemmatalis), cabbage looper (Trichoplusia ni) and corn earworm (Heliothis zea). However, because of the linkage drag, the resistance gene could not be transferred into a variety with high yielding ability (Boerma and Walker, 2005). Insect-resistance genes have been transferred into soybean through genetic transformation on an experimental basis (Mazier et al., 1997; Dang and Wei, 2007; Homrich et al., 2008). However, it is not as popular as herbicide tolerance in soybean and in other crops such as cotton (Gossypium species), probably because field resistance is not effective.

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