Ayman M. Mostafa
Alfalfa weevil (AW), Hypera postica, is the major yield reducing insect pest attacking alfalfa in Arizona and western U. S. during the winter and spring season. The damage caused by this insect coincide with the prime cuts of hay that considered the main feed for dairy cows in Arizona. Because of this economic consequence, growers and agricultural professionals were asking for accurate economic threshold to inform their decision for control measures against this serious pest.
The University of Arizona 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