Increased planting of aphid-resistant soybean could mean fewer acres sprayed

Farm Forum

Soybean aphids have begun showing up in Midwest soybean fields this summer. In the next couple of weeks, aphid infestations will likely increase and may reach treatment thresholds in many fields. That will trigger spraying of soybean fields with insecticides to prevent the aphids from robbing crops of yield. Soybean aphid outbreaks have led to tens of millions of acres being sprayed in the Midwest in some years.

The soybean aphid is a small, light green insect that pierces the leaves and stems of soybean plants. Its populations can build rapidly, doubling within just a few days when temperatures are in the upper 70s and low 80s. Soybean aphids suck sugary plant sap and rob the soybean plant of energy, causing as much as 50 percent yield loss.

Soybean aphid was first detected in soybean fields of 10 states in 2000. It has since spread to threaten soybeans in at least 13 states and three Canadian provinces.

In the past few years, some farmers have planted aphid-resistant soybean varieties to thwart aphid populations and prevent insecticide sprays in their fields. Aphid-resistant soybean varieties have been commercially available since 2010. Overall, the proportion of soybean acreage planted to aphid-resistant varieties remains small, though much greater on organic farms.

Scientists have identified five sets of major genes, known as Rag genes, that are responsible for aphid resistance in soybean. Each gene or set of genes is distinct, conferring its own aphid resistance.

The first commercially available, aphid-resistant soybean varieties have carried either the Rag1 gene alone or a combination of the Rag1 and Rag2 genes. The combination of two or more resistance genes is known as a pyramid.

Rag1 has two forms of resistance. The first, known as antibiosis, slows aphid development and reproduction, and it reduces their survival. The second, called antixenosis, makes aphids uneasy about settling on a plant. Rag2 carries only the antibiosis trait.

Why has the amount of soybean acreage planted to aphid-resistant varieties remained relatively low so far? There are many reasons, according to Louis Hesler, a research entomologist with the USDA Agricultural Research Service. He has worked to identify and develop aphid-resistant soybeans for the last decade at USDA’s North Central Agricultural Research Laboratory in Brookings, S.D.

Hesler says that aphid-resistant varieties are still relatively new in soybean and may take time for wider adoption. He also says that soybean aphid does not reach economically serious levels every year, perhaps decreasing incentive to plant aphid-resistant varieties each year.

Finally, Hesler says, planting a resistant variety of soybean does not mean that all soybean aphids will be eliminated from the field. Many growers may simply want to plant a resistant variety and then not have to worry about soybean aphid in that field. However, growers are still encouraged to scout fields for soybean aphids, in case their populations rise to treatment levels.

Initial tests showed that Rag1 and Rag2 genes were effective in preserving soybean yield and precluding the need for insecticide sprays. Moreover, no yield penalty has been found for varieties with Rag1, Rag2, or the two-gene pyramid.

Entomologists like Hesler are monitoring aphid levels on plants with resistance genes. That’s especially important since particular kinds of soybean aphids known as biotypes have been found. Biotypes are genetic strains of soybean aphid that can survive on soybean plants carrying one or more of the aphid-resistance genes.

Researchers know of four soybean aphid biotypes in North America. Biotype 1 aphids do poorly on all aphid-resistant soybeans. Biotype 2 survives relatively well on soybean plants with resistance based solely on Rag1, whereas biotype 3 survives well on Rag2 plants.

A fourth biotype was recently discovered that colonizes soybean plants with either Rag1 or Rag2 alone, and on ones with the Rag1/Rag2 pyramid. The ability of this fourth biotype to colonize plants with both Rag1 and Rag2 may complicate use of aphid-resistant lines, as the pyramid has been especially effective in limiting field populations of soybean aphid so far.

The prevalence of resistant-breaking biotypes across northern soybean production areas is unknown. So, for now, the caveat to scout soybean fields for soybean aphid still applies, even if an aphid-resistant variety has been planted.

In the meantime, researchers like Hesler are working on at least two fronts to increase acreage planted to aphid-resistant soybeans and reduce the number of soybean fields sprayed for soybean aphid. First, Hesler and other researchers continue to search for and identify new soybean lines resistant to soybean aphid. The new lines may yield new resistance genes that could be used alone or together in pyramids to combat the spectrum of soybean aphid biotypes.

Second, researchers are determining the extent of various aphid biotypes across the northern soybean production zone. If individual resistant-breaking biotypes are restricted to relatively small geographic areas, it may then be possible to recommend planting aphid-resistant soybean varieties that are tailored against the particular biotype in those areas.

“The key is to stay one or even two steps ahead of the soybean aphid, in terms of management tactics,” Hesler says. “You always try to anticipate how aphid pest populations might be able to get around the current management approach and be ready with an effective alternative tactic.”