It doesn’t take long for insects to multiply and thrive within stored grain. Even with a single pair of insects – a male and female – the population can explode to more than one million after four months under the right conditions. One adult female can lay eggs continuously and those larvae become adults in less than one month at optimum conditions and lay their own eggs. The result is overlapping generations of insects multiplying within a storage bin – feeding on grain, causing spoilage and other serious issues.
While it seems plausible the Canadian Prairie winters would solve the insect problem, the cold is not as effective as one might think. Researchers at the University of Manitoba’s Department of Biosystems Engineering – led by Fuji Jian, associate professor at the university – have been looking into what temperatures are needed to keep insect populations under control, how quickly they reproduce and how these insects move and behave within stored grain.
“For such small creatures, these insects are a lot cleverer than you might think,” says Jian. “Some species are excellent at finding the places within grain with the exact right temperatures and moisture content.”
Meet the beetles
According to Jian, among the 160 or so identified species of storage mites and insects, there are two main pests in Canada – the rusty grain beetle and red flour beetle. For either pest to survive and multiply within a typical grain bin, the temperature must be around 25 to 35 Celsius (C). “In Canada, most of the storage insect issues actually happen after harvest, through the late summer and into November, because that’s when you have the most ideal temperatures for reproduction,” explains Jian.
Is heat the answer to storage insect populations?
Jian and his team have been looking at the impact of temperature on storage insects. They’ve found that when temperatures climb any higher than the ideal 37.5 to 38 C, insects will move around to try to find cooler spots. If temperatures were to hit over 42 C, most insects wouldn’t be able to survive for more than a handful of days. If you crank up the heat to 50 or 60 C, the insects’ survival plummets to minutes or seconds. However, because growers want to avoid excess heating in their grain bins, controlling insects with higher temperatures isn’t really an option.
Another issue is that high insect populations generate their own heat. “When insects multiply within grain storage, they produce heat. And then when they feed on and damage grain, it also produces heat due to respiration.” says Jian. “At the same time, all that respiration and heat raises the grain’s overall moisture level and causes mold spores to multiply. It’s very much a chain reaction.”
This chain reaction can create numerous, large hot spots within stored grain as insects move from one spot to another to escape rising temperatures, leaving plenty of damaged grain in their wake, adds Jian. “What we’re finding is that as the mould from most hot grain multiplies, the hot spots gradually enlarge and that’s really what influences the motion of the insects in storage,” he says. “The idea that they’re just staying in one spot just isn’t the case.”
Exploring lower temperatures as means of control
If high heat isn’t the answer for controlling insects in stored grain, perhaps cold – or extreme cold – could provide a solution. Another branch of Jian’s research has involved looking at the survivability of different insect species under cold temperatures.
The general rule, says Jian, is the insects tend to slow down and stop reproducing once temperatures get below 15 C. However, Jian and his team have found low temperatures can have vastly different impacts on controlling different insect species. For example, temperatures less than 5 C usually prove fatal for red flour beetle, but rusty grain beetle is a different story.
“Rusty grain beetles are challenging because they’re extremely cold hardy,” explains Jian. “They are very well acclimated to colder Canadian temperatures, which makes them the most concerning storage pest for Canadian farmers.” He points out that rusty grain beetles have been found to survive at temperatures as low as -15 C or -20 C.
“The key in dealing with rusty grain beetles is to use aeration to cool down your grain as quickly as possible,” says Jian. “If you’re fast enough, the beetles might only survive for a couple of weeks. But if that cooling is slow and gradual, then you’re giving them time to acclimate and increasing their chance of survival.”
Grain, he adds, has very low thermal conductivity, meaning that it can be extremely slow to both store heat and release heat. In other words, growers shouldn’t wait for natural, external temperatures to work their magic – especially if beetle populations are already on the rise.
“Grain in the winter will rarely get down below freezing – even if outside temperatures are minus 40,” he says. “If you have a bin that’s 10 meters in diameter and 30 feet tall, the grain temperature in the very centre can still be higher than 25 degrees. Even close to the wall of a bigger bin, you won’t see temperatures go low enough to render the beetles inactive.”
While rapidly cooling grain below 15 C won’t entirely eliminate rusty grain beetles that are already in the storage bin, it can help stop their populations from getting out of control and causing serious issues. “The key with aeration and quick cooling is that you don’t let insects multiply,” says Jian. “If you only have a few insects in your bin and they’re unable to produce more generations, then you generally won’t have a problem and be able to keep your grain quality safe.”
