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Detecting hidden insects in wheat

November 30, 1999
By Heather Hager


Feb. 23, 2011 – In Canada, it is prohibited to sell grain that contains live insects. However, some grain storage pests develop inside the kernels as larvae, making them difficult to detect.

Now, US researchers have developed an instrument that quickly detects insects inside wheat kernels. Its potential to detect insect species that are more common in the more northern latitudes of North America is unknown, but if it could be adapted for these species, it would greatly reduce the sampling time and effort required to determine if the grain requires treatment to kill the pests.

The device, called the Insectograph, uses a crushing mill and an electrical current to detect the larvae, explains Dr. Tom Pearson, an agricultural engineer with the US Department of Agriculture’s Agricultural Research Service. He developed the device with the help of colleague Dr. Dan Brabec. The mill has two rollers for crushing the grain, with electrical current running through one and a small gap between them. When there’s nothing between the rollers, there’s no electrical conductance, but when wheat kernels are crushed between the rollers, a small amount of electricity is conducted to the second roller. When a wheat kernel that contains a live, juicy larva is crushed, the bug juice is a better conductor for the electricity, giving a spike in conductance. These spikes are monitored and counted by a computer, which can distinguish between wet kernels and insects by the shape of the spike, says Pearson.

The great advantage of the Insectograph is that it can process a one-kilogram sample of wheat, or approximately 30,000 kernels, in about two minutes. “Since you can assume that the insect-infested kernels are at a low incidence, if you’re only crushing 10 or 20 at a time, the probability of two of the infested kernels from a one-kilogram sample being crushed at the same time is very low,” says Pearson. This also means that the device is more sensitive at lower infestation rates because of the lower probability of multiple infested kernels being crushed at once.

In contrast, the Berlese funnel and other methods of detecting insects used by the Canadian Grain Commission take much longer. In the Berlese funnel method, a grain sample is placed in a large funnel, and a heat lamp warms the grain from above. The heat causes most insects to move downward, away from the heat source, and eventually fall out of the funnel bottom into a collection container. It takes about six hours to do the test because the heat has to percolate through the sample slowly enough not to kill the insects, says Blaine Timlick, stored products entomologist with the Canadian Grain Commission, and receiving the final testing report can take up to two days.

Unfortunately for northern wheat producers, the Insectograph was developed and tested using wheat samples infested with two insect pests of stored grain that are more common in the central and southern United States: the rice weevil (Sitophilus oryzae) and the lesser grain borer (Rhyzopertha dominica). The larvae of these insects develop inside the grain kernel. In contrast, according to Dr. Paul Fields, entomologist with Agriculture and Agri-Food Canada in Winnipeg, the two most common, problematic insects in Canadian grain are the rusty grain beetle (Cryptolestes ferrugineus), which can occur inside wheat kernels, and the red flour beetle (Tribolium castaneum), which doesn’t develop inside the kernel. Another consideration is that larvae of the rusty grain beetle are much smaller than those of the rice weevil and lesser grain borer, says Fields, which might affect their detectability.

Fields notes that another insect, the granary weevil (Sitophilus granarius), can be problematic in some areas of Canada and, like the rice weevil and lesser grain borer, its larvae develop inside the grain kernel. Because it cannot be detected using the Berlese funnel method (because the larva cannot exit the kernel until maturity), it might be a good candidate for detection using the Insectograph. Comprehensive testing of the device to detect these other pest species is necessary to determine its efficacy and accuracy.

In fact, Peterson and colleagues plan to study the Insectograph’s utility for detecting the rusty grain beetle and the granary weevil, which are also pests in the United States. “We’ve recently made some improvements to the device that enable better detection of smaller, or less mature, insects inside grain,” says Peterson. He adds that the device is also showing potential for use with other grains in addition to wheat. “It looks like it will work on rice, and we’re exploring other grains too.” Testing is ongoing.

The instrument is being manufactured and sold by National Manufacturing in Lincoln, Nebraska. According to sales agent Dan Moore, it currently retails at about $15,600 US for the device, cart, waste container, software, and laptop.