It’s no coincidence three of the 10 plagues of Egypt involved insects of one type or another. Ever since mankind learned to grow and harvest grains, people have waged a pitched battle with the insect world for a share of milled grains.
A century ago, hydrogen cyanide was the fumigant of choice when attacking insect infestations in stored grain. Methyl bromide replaced it, but its days are numbered as a legal fumigant, and alternatives such as sulfuryl fluoride face an uncertain future. Indian meal moths, red flour beetles and other pests still will need to be controlled at flour mills, pasta and cereal plants and other food operations where milled grains are stored, of course. Fortunately, scientists at Kansas State University (KSU), Purdue University and USDA’s Agricultural Research Service Center for Grain and Animal Health Research are pursuing various options. Much of the work occurs in Manhattan, KS, home to KSU, in part because it is home to the Hal Ross Flour Mill, a 340,000-cu.-ft. pilot facility that provides a controlled environment where the efficacy of methyl bromide, sulfuryl fluoride, heat treatment and other commercial processes can be evaluated for controlling insects in all life stages.
Leading the effort since 1999 is KSU Professor Bhadriraju (Subi) Subramanyam. After earning an undergraduate degree from India’s Andhra Pradesh Agricultural University in 1981, Dr. Subramanyam received MS and PhD degrees in entomology at the University of Minnesota. At KSU, Dr. Subramanyam has established an internationally renowned stored-product entomology program, where integrated pest management programs for insects associated with stored grain are developed and evaluated.
The whole point is managing pest levels, not trying to get rid of pests entirely. A fundamental change occurred in the 1970s in California, when entomologists said, “Let’s not use the term ‘pest control’ anymore because it suggests elimination, which is impossible to accomplish.” The idea was to set a threshold level where management began. As long as insect populations are below that level, they concluded, humans and insects can peacefully coexist.
IPM has been successfully applied to field crops, where threshold densities that trigger intervention are defined. But when it comes to stored grain, all we have are USDA standards that are too high to make pest management cost-effective.
FE: What does the research say about the efficacy of heat treatment compared to chemical treatments?
We conducted the first side-by-side comparisons of methyl bromide, sulfuryl fluoride and heat treatment for managing eggs, larvae, pupae and adult red flour beetles in the Hal Ross Mill in May and August 2009 and May 2010. Three times more sulfuryl fluoride than methyl bromide was used.
Temperatures during heat treatment were held between 50°-60°C (122°-140°F). All treatments were limited to 24 hours. All three achieved 90 to 100 percent kills of red flour beetles in bioassay boxes. Egg mortality was less than 100 percent with all three, though the differences were statistically insignificant. The only significant difference was in bioassay filled with flour 2cm deep (3/4 inch). Heat treatment mortality was 90 to 96 percent for certain life stages, significantly less than with the chemicals. This underscores the importance of sanitation for enhancing effectiveness: Flour is a poor conductor of heat, and adult insects will tunnel into the flour.
Air movement is important for heat distribution, and large fans were placed on each floor. Six air exchanges occurred each hour.
We have very good data now on all three treatments. One of the things we’ve learned is that sulfuryl fluoride really needs to be combined with heat. If temperatures are below 80°F, you’re not going to kill the eggs. More gas has to be introduced or treatment time extended beyond 24 hours. The preferred option would be heating the facility to 85°F.
FE: How does the cost of heat treatment compare?
One of my students developed a side-by-side comparison of costs for the three treatment alternatives. Heat treatment costs include propane, equipment rental, transportation and per diem for four technicians. Overall, the cost of heat treatment was competitive. The average cost per cubic meter in the mill was $3.14 for heat, compared to $3.77 for sulfuryl fluoride. Methyl bromide at $1.76 was the least costly, but prices are going up as supplies grow shorter, and it is not a long-term option, in any case.
FE: Have you modeled the cost of heat treatment?
We developed a calculator around 2007, which we make available via license to commercial interests. It provides the heat transfer coefficient for an application, based on building specs such as exposed surfaces, materials of construction, heat loss and infiltration, outside temperatures, number and size of windows and doors and a variety of other attributes. Essentially, it lets you create a variety of scenarios and determine how many BTUs would be required for the applications, all while sitting at the computer.
A rule of thumb is that it takes 7-10 BTUs per cubic foot per hour to conduct a heat treatment. But simply managing the program better can result in significant savings. At a cereal plant with 105 rooms, reducing heat treatment time to 24 hours from 34 hours saved $25,000.
FE: Some of your recent work focuses on infrared radiation applied directly to foodstuffs, as part of disinfestation. What results have you achieved?
Flameless catalytic infrared radiation is working great. Gas-fired infrared radiation was evaluated several decades ago, but those emitters had an open flame, which posed an explosion hazard. Flameless technology using propane or natural gas does not suffer from the same drawbacks, and lab research indicates it is a promising tool for disinfestation, without having an adverse effect on grain quality. Infrared also kills pathogens and can be used to dry commodities. It will be cost competitive: We estimate treatment cost at 1¢-3¢ a bushel.
In a feasibility study of wheat disinfestation with flameless IR heating, we achieved a 100 percent kill rate without adversely affecting the chemical and end-use properties of the wheat. Results are available on my website, www.ksre.ksu.edu/grsc_subi. And it’s green technology, with no VOCs and only carbon dioxide and water vapor as byproducts.
FE: If neither methyl bromide nor sulfuryl fluoride ultimately is available, will heat be the only option?
There is no magic bullet for managing insects either in grain storage or processing plants. For processing facilities, a good sanitation program, coupled with good inbound and outbound inspection and occasional used of a fumigant or heat and fogging, can manage pests.
One commercial project is a fermented bacterial insecticide that Dow Agrosciences markets as Spinosad. It is registered in many countries for use on crops, where it breaks down in a week when exposed to sunlight. In grain storage, it protects against infestation for two years.
At 0.1ppm, 100 percent kill is achieved in the lesser grain borer, a problem insect with stored wheat. It is registered as a grain protectant in the US and has Codex approvals. Unfortunately, Japan has not accepted the tolerance, and until they do, commercial release will be delayed.
FE: Have some promising treatments not panned out?
We studied ultrasound extensively beginning in the late 1990s until 2006, but it only worked well on fleas and Indian meal moths. Coevolution occurs with bats and moths. The meal moth, for example, has a tympanic membrane and can perceive a bat’s signals, then make maneuvers to escape the bat. We built a random sound generator because the moths become habituated to a constant sound, and we found the sound stopped the moths from mating. But a piece of paper can stop the sound waves.
FE: What’s the outlook for development of more alternatives for pest management in stored products?
There are only eight or nine people doing this work in the United States, one of the smallest groups of entomologists working in a specific area, and the group is getting smaller each year. We need more students dedicated to post-harvest protection.