After reports last week of alfalfa weevil infestations in the southern part of the District, our ‘early’ spring was confirmed by the presence of hatched weevil across the area in scouting conducted this week.
Results:
Scouting was done in 9 fields this past week. In each field, I randomly cut 30-70 stems and tried to extract all the weevil I could out of each one. The infestation levels were as follows:
While we did see some level of mortality from the last cold snap, it was by no means extensive. At most, I found +/- 20% dead larvae among the live ones. In most instances, it was difficult to find even one.
Analysis:
Keep in mind a couple of things:
1. Typical ‘threshold’ is 50% or one larvae found/every two stems.
2. Most of these larvae are pretty small so feeding damage still appears fairly light in most cases
As was the case last year, weevil numbers really ‘exploded’ right out of the gate in the southern part of the District and the differences you see are a good reminder as to why we scout every field individually. Heat units are obviously an issue, but this Kansas Mesonet GDD calculator report would suggest that’s not all that’s at play:
Those GDD’s just aren’t very far apart, suggesting age of stand, adults remaining in the field last summer to lay eggs and GDD accumulation last fall might have parts to play as well. Alfalfa weevils are a complex pest!
What Next?
• Cold Temperatures: while Tuesday/Wednesday might bring some temperatures damaging to larvae, I think it will be slight and mostly confined to the smallest larvae. While we’ll take anything we can get to help us out, I wouldn’t count in significant weevil mortality from this weather event.
• Weevil Development: keep in mind that alfalfa plant growth is typically more closely associated with ground temperature while weevil development is calculated using air temperature. If we’re above 45 degrees F, the weevil are going to do at least some feeding – even as the alfalfa may not be growing as quickly. At this point, it looks (again) like the plants may not accelerate their growth as quickly as the weevil are.
• Maximizing Product Efficacy: if you want to look at the latest KSU Efficacy trials, you can find them here (RH side of the page towards the bottom under links): https://www.meadowlark.k-state.edu/crops-soils/index.html . This field trial was shared at our Alfalfa Insect Management Meeting a couple of weeks ago by Dr. Whitworth and is a great reminder to make sure you use plenty of carrier:
Drop me a line and let me know what you are seeing or if you have any questions. Have a great weekend!
