M.O. Way on Texas Rice: Water Weevils – Especially Tough on Organic Rice

    Rice water weevil

    The 2016 RMTC concluded June 3—what a great convention!!! Thanks to Dwight, Veronica, Bianka and Marcela for organizing and hosting the convention. I am definitely looking forward to next year’s meeting somewhere in the Caribbean.

    I gave a talk at this year’s meeting on insect pest management in organic rice. If you want a copy of my powerpoint presentation, contact me.

    Basically, there are no effective organically-approved insecticides for rice water weevil which I think is the number 1 insect pest of organic rice.

    The rice water weevil lays eggs underwater in the stems of rice plants. After about 4-7 days, eggs hatch and young larvae move to the roots where they prune roots with their chewing mouthparts. After feeding for about 3-4 weeks, passing through 4 instars, larvae construct football-shaped mud cocoons and pupate within (pupae are inactive and do not feed or damage rice). Damage can be severe—if plants have compromised root systems, they cannot take up water or nutrients and support can be little or none—for instance, severely damaged plants can be easily uprooted by high winds and by simply walking near them.

    We have shown that an average of 1 larva per core (4 inch diameter by 4 inch deep plug of mud with rice plants and roots) reduces yield 75 lb/acre. This is a linear relationship, so an average of 3 larvae per core reduces yield 225 lb/acre. Clearly, organic rice is more susceptible to ricewater weevil damage than conventional rice because adults lay eggs earlier in the life cycle of the rice plant.

    In organic rice culture, where farmers employ a continuous or pinpoint flood, rice can be attacked when it emerges through water—maybe at the 3-4 leaf stage. In a delayed flood culture practiced by most southern conventional rice farmers, adults do not begin laying eggs until the “permanent” flood is applied—between 3 and 5 weeks after emergence through soil when rice is actively tillering.

    Thus, organic rice is much smaller at the time of weevil attack compared to conventional rice.

    • Fostering a good stand of organic rice can dilute rice water weevil pressure, so increasing seeding rate and planting when weather is warm to encourage rapid growth and a good stand are sound cultural practices to combat rice water weevil.
    • Planting medium grain varieties can also help because, in general, medium grains produce a massive root system which can compensate somewhat for rice water weevil damage.
    • However, do not drain fields to control rice water weevil. True, draining fields can reduce densities of larvae on roots, but soil must dry to cracking before larvae are affected. Afternoon spring/summer thunderstorms, common in the South, prevent soil from drying. Also, drying organic rice fields can lead to more weed growth, promote blast development and cause loss of N.

    I was recently inspecting an organic rice field near Nome, Texas. Close to the field was a fallow field that was in organic rice in 2015. A lot of stubble was heading out and the field was somewhat flooded due to heavy rains. But, lo and behold, 3 whooping cranes were happily foraging in this field! As you know, whooping cranes are an endangered species. A member of the Houston Audubon Society was watching these magnificent birds. He told me the 3 cranes represented almost 1% on the total whooping crane population in the world. Hooray for rice—kind to wildlife and sustainer of mankind!

    This week’s issue of The Rice Advocate

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