Louisiana Rice: High Numbers of Rice Water Weevils

Looks like we may have turned the corner on both the weather and the rice crop this week.

Every time we have a period of heavy rain the next several days following that period the heat and humidity combine making for miserable conditions. After a few days the humidity declines and it is just hot. That also means less cloud cover, which is better for rice. Hope this issue addresses a couple of questions.

In the previous two issues of Field Notes, I have commented on the very high numbers of rice water weevils (RWW) in our verification field in West Carroll Parish. Last year on the same farm in a field not far from this field we encountered a similar problem. In both years the fields were planted to hybrid rice at seeding rates around 20 pounds per acre with seed treated with Cruiser Maxx.

Dr. Mike Stout has shown in some of his studies that there is somewhat less efficacy of Cruiser Maxx and Nip-sit Inside in controlling rice water weevils in hybrid rice because of the method of determining dosage of insecticide.

Both of these insecticides are seed treatments and the rate is based on a certain amount per 100 pounds of seed thus seeding rate determines rate of insecticide per acre.

Last year we applied a pyrethroid insecticide after permanent flood when I felt we could not tolerate any more foliar feeding injury. Core samples pulled later revealed very high numbers of larvae. This year when it appeared to repeat the events of last year we applied a pyrethroid insecticide with the second application of Newpath (June 12) just prior to establishing permanent flood. Two weeks ago I reported we still had very high numbers of adult RWW in the field. On July 2, we pulled core samples and found from 6 to 11 larvae per core. Rice is at green ring.

Considering the stage of growth of the plants and the acceptable root systems, draining is not an option and I would not recommend it anyway. Some samples had very early instars and others were large instars and would soon pupate.

The fact that this has now taken place two years in a row at the same general location indicates there is very heavy RWW pressure in that area and causes some concern about insecticide efficacy under these conditions. It also disappoints me that we have had to use a foliar insecticide when a seed treatment was supposed to eliminate that necessity.

Every year I get questions about dimpled leaves. I once read a good physiological explanation of what causes this and unfortunately cannot locate the article or remember exactly what it said.

My best explanation is that when the leaves were emerging from the collar it was tight and/or the process was delayed causing this constriction to form. I have seen this as long as I have been working with rice and have never seen any relationship between it and yield. It is not a disease and it is not caused by an insect and herbicides to not cause it. It is a physiological phenomenon very common in rice.

Clearly, some environmental conditions are involved and your guess is as good as mine as to the specific conditions necessary for it to appear.

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