In 2022, a rise in aphid populations was observed in many fields across Quebec. In July, an average of 250 aphids per plant were observed in 15 of the 65 fields studied by the RAP Grandes cultures, Quebec’s plant protection warning network for field crops.
Following these observations, because of the critical plant development stage, the recommendation was made to conduct regular field scouting in several of the province’s regions. In the Monteregie area, natural enemies like ladybugs helped reduce the number of aphids, except in a few fields which had to be treated.
The presence of large numbers of aphids can seriously affect the development of soybean plants. Field scouting is therefore essential and must be carried out more frequently the moment the threshold of 250 aphids per plant is reached.
Close tracking of aphid populations as they evolve will help you assess whether their numbers are rising or decreasing, and whether the natural enemies that are present are getting the job done. It will therefore make it simpler for you to determine if an insecticide has become necessary.
Here’s how to proceed:
To start, every 3 to 7 days, select 20 plants at random, well distributed throughout each field. Count the number of aphids on each plant, making sure to check the upper and lower surfaces of the leaves, stems and pods.
If the aphids are too numerous to count, do a quick estimate by counting the number of aphids on half of a leaf, for instance, and multiply by 2.
To treat or not to treat?
There are several criteria that must be met before applying an insecticide to treat a soybean aphid infestation:
- There must be an average of 250 aphids per plant.
- The population must increase by 35% during at least two consecutive 2-3 day periods.
- The soybean plants must be between growth stages R1 and R5.
- The weather conditions must favor proliferation of the soybean aphid (22° to 26°C).
What you should know before applying an insecticide:
- Some treatments may not be approved by your grain buyer.
- Many soybean aphid products cannot be applied when the plants are in bloom, because of the risk to pollinators.
- An insecticide can also kill off the aphid’s natural enemies, increasing the risk of reinfestation by aphids, as well as other pests.
In short, there are many factors to consider before you intervene! The decision must be made on a case-by-case basis, namely field by field. Your Prograin representative can help you make the right decision.
*This is the second article in a two-part series on the soybean aphid.
We invite you to read the first article, Soybean Aphid Damage, if you haven’t already.