Ayman M. Mostafa
We received some calls about incidences of high populations of alfalfa weevil during the last couple of weeks. Our field scouting showed that populations of alfalfa weevil varied across the alfalfa production areas in the state.
The UA Cooperative Extension Field Crops IPM Program conducted multi-year research to investigate alfalfa weevil thresholds. Our results indicated that an action threshold of an average of 2-4 large larvae per sweep justifies the application of control measures. This threshold can be dependent on hay price and cost of treatment.
Sweep net samples should be conducted in at least 4 areas in the field (5 sweeps / area). It is important to use the proper monitoring methods for alfalfa weevils to determine their threshold levels. Using the right pattern for sweep net sampling is equally important. After many observations of sweep net sampling techniques in alfalfa, we’ve noticed that different “swing patterns” resulted in different insect pest counts. The economic thresholds we use to monitor and justify treatments for alfalfa insect pests are based on 180° sweeps. Using other sweeping techniques may give different results and may lead to either spraying too late or unnecessary spraying. As stated earlier, this threshold is based on counting large alfalfa weevil larvae.
Remember that alfalfa varieties with rapid spring growth may be more tolerant of weevil damage and early harvesting can mitigate damage. It is important to note that surviving larvae under windrows may reduce subsequent alfalfa growth.
It is also critical to correctly distinguish alfalfa weevil larvae and adults from other insects that could be in the field at this time of year. We developed a short 1-page publication to help with that: http://cals.arizona.edu/crops/cotton/files/EAW_ShortFc.pdf