Generally, these insects are associated with silt loam soils in a crop rotation where rice follows soybeans. Lespedeza worm larvae, not adults, feed on the rice plant stem between the seed and the soil surface. In most cases, there are stand reductions in the rice as a result of the larvae feeding. A rice stand is established, then the larvae begin feeding and you begin losing rice plants.
When examining a rice seedling that has lespedeza worm damage, it is evident that the larvae eats the soft plant tissue including the vascular tissue which is the tissue that moves water and nutrients throughout the plant. The remaining vascular tissue looks similar to a strand of thread that connects the young plant leaves to the seed. However, last year in some extreme cases the larvae actually ate the emerging rice tissue and there was seedling damage before rice emergence.
Once lespedeza worms are discovered in the field and damage has occurred, it is difficult and expensive to try to fix this insect problem. A common approach is to "drown them out" or pump the field up and hold water on the young rice. Sometimes this may help but it can also hurt the young damaged rice.
Then there is the idea that if some stand loss occurs you can "sweeten the stand" by flying in rice seed in the thin areas. This generally doesn't help because of the age or plant development stage difference in the remaining rice and the flown in seed. Then if you have a thin stand you could add DAP (18-46-0), urea, ammonium sulfate, or all three fertilizers and flush again.
The bottom line: all these potentially corrective measures to fix the lespedeza worm problem can cost up over $40.00 per acre and still not "fix the problem". The $40.00 per acre is bad enough but the "problem field" syndrome that is always in the back of your mind can be a real mental issue to deal with.
The best ways to deal with this pest problem in rice involves Icon as a preventative seed treatment. Icon was first labeled for rice water weevil control as a replacement for granular Furadan. The Icon label has been expanded to include control of lespedeza worm.
The labeled rate range for Icon is 0.025 to 0.050 lbs active ingredient per acre. Generally, where yield loss potential in extreme situations is great from rice water weevils, such as early-planted water seeded rice, the 0.05 lbs ai/A rate is utilized. However, in drill-seeded rice where rice water weevil potential loss is considerably less but you want a preventative treatment for lespedeza worm the 0.025 lbs ai/A rate is adequate.
The next obvious question is what is this 0.025 preventative treatment going to cost me. The answer is $11 to 12 per acre. Remember, up to $40.00 per acre was spent last year in trying to remediate damage from the grape colaspis. The $40.00 per acre spent last year was in many fields spent trying to correct or make the most of the problems. But actually the total cost is considerably more when you include yield loss from the stand losses due to the lespedeza worm damage.
Icon does bring something else to the table besides water weevil and lespedeza worm control. We worked with the product several years before it was labeled and noted visual growth differences in the field with Icon treated seed. It appears to be a plant growth regulator effect when rice is treated with Icon. We compared gibberellic acid treated rice seed (Release, N-Large) to rice seed treated with both Icon and gibberellic acid treated seed and noted the beneficial effects of early plant vigor, color and emergence from the Icon.
Last year's growing season was somewhat unusual because of the cool conditions that slowed the early growth and development of the rice seedling. Generally, when the seedling rice develops to the three-to-four-leaf stage, the damage from grape colaspis is less severe. However, who knows what the growing conditions will be once the rice is planted.
So the bottom line is, as always, the grower makes the call. Do you pay for Icon seed treatment and not worry about grape colaspis or water weevil damage or roll the dice. Those who spent the money last year on flushing, fertilizer, time, aerial application and cost, the answer is a no-brainer – treat and sleep.
Ronnie Helms is a partner in G&H Associates in southeast Arkansas.