Soybean Aphid Numbers Increase in Some Northern Missouri Counties

Published: August 29, 2008

Soybean aphids continue to cause problems in several northwestern and northeastern counties with new problems developing in north central counties during the past two weeks. Soybean most at risk, are double crop or late planted beans in the early reproductive stages of growth. The economic threshold is calculated at 30% or more of plants supporting aphid populations of 250 or more aphids per plant in growth stages R1 (flowering) through R5 (early pod fill). This threshold has been tested in many states to the north of Missouri where soybean aphids often cause very severe problems. It is based on an economic injury level of 1006 aphids which is the actual number of aphids per plant where yield loss from this pest equals the cost of control. The 250 economic threshold number allows producers a few days to apply control measures before actual economic yield losses occur. Damage from soybean aphid is more severe on the early growth stages such as R1 (flowering) as compared to later growth stages through R5 (pod fill). Problems with this pest will decrease significantly as many soybean plats move past the R5 stage of growth.

Although low numbers of aphids initially migrated into the northern half of state about four weeks ago, these populations have expanded in Missouri with the presence of cooler than normal temperatures and the collapse of most beneficial insect populations Cool temperatures occur naturally, but the collapse of the beneficial insect population may be the result of several factors such as the unnecessary application of insecticides during early season when producers are applying herbicides or later in the season when they apply fungicides. The collapse of beneficial insect populations have been observed in many states, but in Missouri this has a dramatic effect as beneficial insects such as insidious flower bugs and many species of lady bird beetles often keep low to moderate populations of pests such as soybean aphid from increasing to pest levels. The soybean aphid problems now being experienced are also the result of late planting of soybean which provide aphids with soybean plants in the early stages of growth much later in the growing season than in more normal years.

Figure 1: Adult soybean aphid (wingless).

Producers are encouraged to scout for the aphids in order to determine pest status in their fields. Soybean aphids are best seen with a ten times or larger magnification lens (Figure 1). They are small teardrop shaped, lime green to yellow aphids which have black eyes, black legs, and two black tipped cornicles (tailpipes). This aphid species is the only aphid commonly found in colonies on soybean plants. They remove plant juices using their piercing-sucking mouthparts and may be found feeding on soybean foliage, stems and pods. If 30% or more of plants support aphid populations of 250 aphids or more in soybean at growth stages R1 to R5, then treatment of this pest is justified. Numerous insecticides are labeled for control of soybean aphids on soybean. Most offer protection for 14-28 days depending on weather conditions. It is important to read the pesticide label and follow all directions, precautions and restrictions listed.