Over the past week, Sebe Brown, Louisiana State University AgCenter Extension antomologist, has received a number of phone calls about the economic thresholds for threecornered alfalfa hoppers (TCAH) in soybean.
He addressed those questions in this LSU AgCenter column.
Based on previous work conducted by Sparks and Newsom (1984), Sparks and Boethel (1987), and subsequent experiments conducted by the AgCenter in the last five years, the LSU AgCenter recommended threshold is “starting at pod set, 3 nymphs per row foot or one adult per sweep.”
This recommendation is based on published data and experiments conducted in Louisiana at LSU AgCenter Research Stations and Louisiana field locations. Independent of yield loss, TCAH can also cause an increase in the “green bean/stem syndrome” that often is present in soybean fields.
Green bean syndrome’s exact cause is not known, but we do know that stress plays a large part in the malady. Environmental stress compounded by insect- or disease-related stress is often the culprit.
The picture here is from a TCAH experiment conducted at the Macon Ridge Research Station to investigate the effects of TCAH on dryland soybeans. The experiment was terminated at the onset of stink bugs, and all non-target insects were controlled for the duration of the experiment.
The green bean syndrome pictured is directly related to TCAH feeding. The beans on the left were not treated throughout the experiment, the beans on the right were keep free of hoppers.
We also observed significant increases in green bean syndrome in both irrigated and dryland soybeans and observed significant differences in yield in dryland soybeans but not irrigated.
The take-home message is TCAH are economically important insects that have a documented, data-backed economic threshold of 1 adult per sweep at pod set.
If you have any questions or concerns feel free to contact your county agent or me for more information.