Root-zone application of insecticides
By Rizal A Gatica
Just as scientist of the International Rice Research Institute (IRRI) have found that placing fertilizer 10 to 12 centimeters beneath the soil surface works best for rice, so they have discovered that the same practice is just as effective in the application of granular systematic insecticides. Elvis A. Heinrichs, head of IRRI’s entomology department, says placing insecticides in the root zone of rice plants is much more effective than the spray and broadcast (or paddy water) methods used by most farmers in Asia.
Studies by Heinrichs’ group reveal that plant tissues absorb about 10 times more insecticide when the chemical is applied at the root zone than when it is applied on the soil surface or incorporated with top soil. There are other advantages: the chemical is readily available to the roots; it is shielded from sunshine; it does not undergo volatilization; and it is not washed away by paddy water. These conditions cannot be attained using the broadcast and spray methods.
Heinrichs notes that sprayed chemicals are easily washed off plants, especially during rainy weather. To keep insecticides on plants, a farmer has to spray more than once, thus making the spray method very costly. Similarly, the broadcast method does not provide enough protection because insecticides applied in this manner are washed away during heavy rains and floods.
“So we thought of putting the insecticides in the root zone of plants,” says Heinrichs. “Our experiments show that this method best protects the plants from pest attack. Equally important, it is more economical, requiring only one application of one kilo of insecticide per hectare. In contrast, the broadcast method normally requires four to five applications, and the rate is about four kilos of insecticide per hectare.”
How is root zone application done?
Heinrichs says it’s very simple.”Just put about 12 milligrams of insecticide (Heinnrichs’ group used carbofuran in their experiments) in every gelatin capsules. Two to three days after transplanting, insert one capsule 2.5 centimeters deep and approximately 2.5 centimeters from each rice hill, or four capsules in the center of every four hills. In this way, the insecticide will remain intact in the soil even if the farm is flooded.”
Says Gerardo B. Aquino, a senior research assistant: “The one-capsule-per-hill placement insecticide concentration in the plants during the first 20 days after treatment. This high concentration, specifically in the leaf blades, is significant because whorl maggots and leafhoppers start feeding on the leaf blades at this time.”
Applied at the root zone of the rice plant, the insecticide controls almost all the major rice pest, including the virus-transmitting and sap-sucking leafhopper, brown plant hopper, stem borer, and rice gall midge. The last insect is a very serious pest in Thailand, accounting for a yearly 30 percent loss in total rice production. The pest attacks growing shoots, and because it feeds inside the host plant, is poorly controlled by chemical sprays.
However, root-zone application of pesticides cannot control the grain-sucking rice bug because “no chemical gets to the grain.”
Another advantage of root-zone application is it lessens the quantity of insecticide in paddy water. This may enable farmers to raise fish like tilapia in the paddies.
But the use of gelatin capsules is uneconomical and very laborious. That’s why, says Aquino, “we are trying to find an economical and less time-consuming technique. We have tried using cut paper straw, both ends of which are sealed with paraffin when filled with insecticide. This is effective and less costly, but it is also laborious.”
Now, researchers apply the insecticide directly in the root zone with a machine. To do this, Heinrich says, they use the single-row granular band applicator – a simple, portable machine developed by IRRI’s engineering department. The same machine used by IRRI scientists for fertilizer application experiments, the applicator opens furrows, deposits insecticide or fertilizer (or both) in the soil, and then closes the furrows. There should be no standing water in the paddy so that the insecticide won’t float.
“We’re not completely satisfied with the machine’s performance,” says Heinrichs. “It doesn’t deposit the insecticide close enough to the roots. We want the chemical near the roots so it can be absorbed soon after application. We are now trying to solve this problem with our engineering department.”
Once perfected, IRRI will give the design of the machine to manufacturers. The machine should cost no more than 200 pesos. Since it can be used to apply insecticide and fertilizer in one operation, it will make root-zone application of these inputs easier and more profitable.