Soybean aphids—don’t panic, there is no alert yet—like cooler weather than what has been pervasive across the Cornbelt.  They do not do well in heat, but when temperatures are more moderate, they can thrive.  Entomologists used to have a two year cycle for them, but that has been thrown out, after they arrived in a year when populations were supposed to be down.  So every year has to be considered as a potential for soybean aphids.  Have you looked for any?

Before you begin scouting for soybean aphids, brush up on what you are looking for, and learn method of counting that will not require you to count 250 of them, which is the threshold for treatment.  The method is presented by Iowa State University entomologist Erin Hodgson, and is an audio/slide presentation on the Plant Management Network.

Aphids are one- sixteenth of an inch long and have a lime-green pear-shaped body with two dark colored tailpipes.  They will be the only aphids on your soybeans, and may be accompanied by lady beetles and lace wings, which are both predators and look for aphids.  If you find a lot of the predators, you have found more aphids than you want to know about.

Hodgson says soybean fields after the blooming stage should be scouted at least once per week.  She says the current treatment threshold requires counting 38 plants per 50 acres scattered throughout the entire field, identifying at least 250 aphids per plant with the population increasing.  That will give 3-7 days to treat the field before it is too late.

Hodgson developed a speed scouting plan for soybean aphids, which will not determine aphid density, but will help make the decision about whether to treat your field.  It is based on a binomial sequential process.  That requires you to determine if the plant is infested or not, and whether the infestation is more or less than 40 aphids.  Hodgson says with this process you can look at a few leaves and determine if the plant is infested. 

The sequential part of the process means there is a need to examine a base number of plants before making a decision.  40 aphids found on one plant does not mean the field should be treated.  However the speed scouting process requires a minimum examination of 11 plants.  If you found some had 40 or more and others did not, then the process continues to a maximum examination of 31 plants before a decision was made on treatment.  Hodgson says if a decision cannot be made after 31 plants are examined, then the field should be re-examined in 3-4 days.  At that time the aphid population should either be increasing or decreasing.

Speed scouting is not a new economic threshold and the 40 per plant should not be confused with the economic threshold of 250 per plant.  The speed scouting method is derived from the economic threshold.  It is a method of saving valuable time, and should take only 10 minutes to make a decision, compared to an hour for the original counting process.  Hodgson says if a decision to treat is made, it should be confirmed in 3-4 days, to determine whether the population is increasing or decreasing.  She says sometimes the decision is made too quickly.

Aphid populations are highly variable and can be regulated by temperature, humidity, and rainfall, along with predators and parasites, as well migration habits that change the population.  Multiple pests on soybeans can also hurt yield.  Aphids can also develop wings and fly away if conditions get too crowded.

If treating for aphids, the most cost effective treatments are during bloom, during seed set, and the use of a foliar insecticide. After the R5.5 development stage, which is the formation of beans in the pod, treatments can be unreliable. 

One complaint is having too much paper to keep track of, sometimes with more than one sheet per field, if repeat checks are made.  A web-based system has been developed at Kansas State, which can be found at Soypod .  Soypod is designed for use on smart phones or data tablets, and fits on all current software technology.  It keeps track of all fields, re-sample data, and maps out treatment plans.

Summary:
If the weather changes to cooler temperatures, aphid scouting in soybeans should be a priority.  While the treatment threshold is 250 per plant, and increasing, you do not have to count the 38 plants per 50 acres that is recommended.  The use of a speed scouting system will aid in decision making, which requires examination of 11 to 31 soybean plants and determine if they are infested with 40 or more aphids.  Be also aware that populations can quickly increase or decrease, because of predators, weather, and disease.

Source: the FarmGate blog