It’s that time of year again. We have received reports of low numbers of soybean aphids being found in a few fields. Their appearance is on schedule and our crop is right in the middle of its vulnerable growth stages (R1 to R5). Will this year be a problem or will we have smooth sailing? Only well conducted, comprehensive field scouting will tell the tale. Here are a few quick tips:
Remember that every field is a drama unfolding – Soybean aphid scouting isn’t just about hitting a threshold. It’s about monitoring an ongoing battle between the soybean aphid enemy and our allies – the natural predators. The natural predators do so much every year to take care of this problem for us. We only want to go forward with the insecticide option if our allies have lost the battle.
Good Scouting requires multiple visits – If we are going to monitor a battle, we need to sample the field repeatedly over time to see who’s winning. We want to track soybean aphid numbers to see if they are rising, stabilizing or falling and we can only do that by scouting a field at least three times at intervals that vary from a week to 24 hours apart depending on how rapidly the situation is changing.
Remember the action threshold and resist the temptation to modify – The action threshold (AT) or economic threshold is the average number of soybean aphids found per plant. For soybean aphids this threshold is 250 aphids per plant and rising. The actual economic injury level or EIL (point at which yield loss from aphid feeding is equal to the cost of control) is 674 aphids per plant. The AT is a conservative value that gives the grower lead time to get an insecticide application completed. Once the AT is breached the EIL will be reached quickly. These thresholds represent one of the most thoroughly researched and verified thresholds in the agricultural industry. They work and no halving or quartering of the thresholds are required or valid. We’ve done the math on this.
Train you eyes for aphid counting – Counting individual soybean aphids is virtually impossible unless numbers are very low. The best way to deal with counting them is to train your eyes for estimating the numbers seen per leaf. There are a lot of good resources available online to help you become an accurate and rapid estimator of aphid numbers.
Take enough samples – When scouting for soybean aphids, it’s never one and done and you can’t assess the field from the truck or driveway. Multiple samples are needed across at least half of the field. Really, the more samples the better. Some recommend sampling five spots and looking at six plants per spot of a total of 30 plants sampled. That’s a minimum number. If you can convince yourself to sample 10 spots and 10 plants per spot, even better!! Pull plants and inspect them thoroughly including the undersides of leaves and the stems. Remember that small “hotspots” with high aphid counts can be found in fields, particularly close to field edges. A few hotpots does not constitute an infestation. After all sampling is done, the threshold is based on an average of all counts.
Observe the plants and the predators – Whenever entering a field to scout, let your hands move through the plants as you walk. You are looking for the feeling of stickiness which tells us we are dealing with aphids. The more sticky your hands feel, the more motivated you should be to scout. Also, remember to do at least a qualitative inventory of natural predators (lady beetle adults and larvae, green lacewing larvae, hover fly larvae, minute pirate bugs, etc.) every time you sample a field. Get a handle on how the predator numbers are changing relative to soybean aphids numbers. Lady beetle adults and larvae, are an obvious and easy to count indicator species, to start with.
Click Here for More on Soybean Aphid Scouting
Click Here for a Handy Resource to Train your Eyes for Estimating