Insects affecting cereals, maize, rice and other dry plant products can be controlled with ozone, writes Janne Hansen of AARHUS University.
To say that the harvest is in the house is not the same as saying that it is safe. An army of small, hungry bugs are ready to make a meal of the food source. Ozone (O3) in low concentrations can potentially replace insecticides to control insects in stored grain. Scientists from Aarhus University together with the company Crop-Protector KS have investigated the effectiveness of ozone in different doses on a range of insects and under different conditions. One of the research objectives of the project was also to develop a machine that uses ozone as a pesticide.
Some insects can do a lot of damage even in dry stored products. Scientists from Aarhus University studied the effectiveness of ozone against insects in their various stages. Photo: Lars Damberg
Grain weevils, lesser grain borers, flour beetles, , several species of moth and others of the same bent can do great damage to stored barley, wheat, rice, maize and other dried plant products if the insects are not controlled. The grain is often stored for many months and in that time the insects can in all secrecy multiply into a very large number.
In the industrialized countries up to 9 percent of the stored grain may be lost. In developing countries, the loss may be 20 percent, and in some instances it may all be lost. The problem is discovered perhaps only in connection with a transaction when the grain has to be transported elsewhere. Pests are a major factor in pricing the grain. Besides the economic losses that result from an infestation, there may also be health problems when the grain is used for food.
One way of controlling such infestations in grain is to spray the grain with insecticide, but there is generally a desire to cut back on this approach. Scientists from Aarhus University have been testing ozone to see if it can be used as an effective controlling agent against pests in grain.
One of the problems with conventional insecticides is that they initially only act on the insects that live freely among the grain. In some species (grain weevils, for example) eggs, larvae and pupae develop concealed within inside the grain kernels, where they are well protected from any chemical sprays, explains associate professor Lise Stengård Hansen from Aarhus University.
It is only when the insects have developed into adults and eaten their way out of the seed that they are exposed to and affected by the insecticide. Therefore, the insecticide needs to be effective for many weeks so that it is still active when they appear. There are not many effective agents available for this purpose, partly because the insects have become resistant to the active ingredients and partly because some agents are no longer allowed.
Ozone, instead of poison
Another option is to use ozone. Ozone is a gas which is highly oxidizing and is used for disinfection in other situations. Microorganisms are, for example, very sensitive to low concentrations of ozone. The advantage is that ozone is an unstable gas and quickly converts to the oxygen molecule (O2), which is harmless. The ozone is produced in situ by an electric high-voltage process and can be used in, for example, a grain silo.
One of the aims of the research was to examine how effective ozone is at different development stages of the insects and find the best combination of dose and duration of treatment. The scientists looked at the effect of ozone on 11 different species of insects that cause damage in dry stored products. Past studies have only looked at the effect on insects that were freely exposed to ozone, but this project included the insect stages developing inside the grains.
They looked at the effect of different temperatures and the physiological effect of ozone by examining the level of oxidative stress and the transcription of the genes involved in the insects’ natural defences against oxidative stress.
The scientists found that in order to kill all the bugs that live freely among the kernels, they had to use 35 ppm ozone for six days. If the stages of the insects living inside the kernels had to be eradicated too, they had to use 135 ppm ozone for eight days.
There are variations in the amount of ozone needed to control the distinct stages and types of pests: adult insects are generally more sensitive to ozone and often die after a treatment with 25 ppm for five days. There is also a variation in the sensitivity for the internal stages, but to eradicate all stages, the high dose of 135 ppm for eight days is used.
The project is supported by The Danish AgriFish Agency under the Ministry of Food, Agriculture and Fisheries and by Crop-Protector KS and Aarhus University.