The Influence of Elevated Temperatures on a Residual Insecticide and Inert Dust to Disinfest Empty Bins Prior to On-farm Grain Storage

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• 2015 annual report

Commodities

• Agronomic: corn, wheat

Practices

• Education and Training: extension, technical assistance
• Pest Management: chemical control, integrated pest management, physical control, prevention
• Sustainable Communities: sustainability measures

It is good practice for farmers to clean their grain bins before storing newly harvested grain. Unsanitary bins can harbor unwanted insects that infest grains, including the red flour beetle (Tribolium castaneum (Herbst)), the saw tooth grain beetle (Oryzaephilus surinamensis (Linnaeus)), and the lesser grain borer (Rhyzopertha dominica (Fabricius)). Due to the fact that it is difficult and labor intensive for farmers to clean under the perforated floor of steel grain bins, where many insects thrive due to the presence of grain dust and broken kernels, it is necessary to employ other methods to control stored-product insects prior to refilling the bin. The use of elevated temperatures has been previously documented as an effective method to kill stored-product insects within 2-4 hours when temperatures of 50-60°C are reached and maintained in empty bins. Previous research has shown that the efficacy of residual insecticides or inert dusts against stored-product pests can be increased after application followed by heat treating the bin with temperatures below 50°C. Laboratory trials will be conducted to determine the efficacy of a low toxicity insecticide (Diacon) and the inert dust, diatomaceous earth (DE), on concrete dishes to simulate the floor of a grain bin against 20 adults of T. castaneum, and then held at 28, 32, 36, 42, 44 and 46°C for 4-24 h. Based on the results, Diacon or DE will be chosen for further validation in field trials using empty 3000 bu bins, with and without heat treatments. Bioassays containing adult insects will be placed inside the bins to determine the efficacy of treatment. The end result of this research will be to provide guidelines to farmers about the optimal time to apply residual insecticides or inert dusts to bins prior to storage based on temperatures inside the bins. In addition, the relationship of Diacon or DE in combination with heat treatments on the mortality of stored-product insects will be determined.

Introduction:

Several species of stored-product insects have been reported from empty bins (Chao, 1954; Wright, 1991; Reed et al., 2003; Arthur et al., 2006; Hagstrum et al., 2008). Removal of grain and grain debris from empty bins prior to storing newly-harvested grain can help in reducing insect numbers in the newly-stored grain (Reed et al., 2003; Arthur et al., 2006). Some stored-product insects are long-lived and removal of residual grain and grain debris, which serves as their food, may not be sufficient to control them. The use of an approved insecticide (Arthur and Subramanyam, 2012) after sanitation of empty bins is shown to provide effective control of insects. Bridgeman (1994) conducted tests with an amorphous silica (Dryacide®, A & R McLaughlin Private Limited, Wembley Downs, Western Australia) applied to storage surfaces at 6-8 g/m² in four 100 m long x 15 m wide x 5 m high rectangular storage structures in Australia. Treatment efficacy was verified by using 30 flour-baited cardboard traps (Wright, 1991) in each storage facility. Trapping three weeks before sanitation and three weeks after sanitation showed no significant difference in the percentage of traps with beetles and psocids.  However, after application of Dryacide®, trapping over the next 11 weeks showed a decrease in the percentage of traps with beetles and psocids from 18 to 3% and from 90 to 40%, respectively.

Clean, empty bins can also be treated with several alternatives to chemical insecticides. Two approved alternatives to chemical insecticides include the use of diatomaceous earth or DE and the use of high temperatures (Subramanyam and Roesli, 2000; Tilley et al., 2007; Subramanyam et al., 2011), or a combination of DE and heat (Dowdy and Fields, 2002). 

There are numerous studies documenting the effectiveness of DE dusts against stored-product insects, mostly on grains (McGaughey, 1972; Korunić et al., 1996; Subramanyam and Roesli, 2000; Kavallieratos et al., 2005; Vardeman et al., 2007; Kavallieratos et al., 2010).    There are limited published studies that examined the efficacy of heat treatment of empty bins against adults of stored-product insects.  Tilley et al. (2007) reported 100% mortality of adults of the red flour beetle, Tribolium castaneum (Herbst), lesser grain borer, Rhyzopertha dominica (F.), and rice weevil, Sitophilus oryzae (L.), by raising temperatures of the bin’s floor to a minimum of 50°C for up to 2-4 h. Moog and Maier (2007) reported 77-91% mortality of adults of the maize weevil, Sitophilus zeamais (Motschulsky), when exposed for 3 h at 55°C in the plenum area of empty bins.  The mortality of T. castaneum adults in the plenum area at this temperature and exposure time was 72-87%.  However, similar exposure in areas 1.83 m above the plenum resulted in 100% mortality of both species. The authors inferred that the lack of uniform distribution of hot air at the plenum may have resulted in less than 100% mortality of beetles.

Previous research has shown that heat treatments in combination with DE increased mortality of stored-product insects. Dowdy (1999) reported mortality of unfed and fed adults of T. castaneum adults exposed to untreated glass Petri dishes and dishes treated with 5 g/m² of four DE dusts at 34 and 50°C and 65% r.h. The DE formulations used were Concern® (Necessary Organics, Inc., New Castle, Virginia, USA), Natural Guard® (VPG Co-op Gardening Group, Inc., Bonham, Texas, USA), Insecto® (Natural Insecto Products, Inc., New Castle, Virginia, USA), and Protect-It® (Headley Technologies, Vancouver, British Columbia, Canada).  Exposure of unfed insects for 15-30 minutes to 34°C alone resulted in 0-1.3 and 42.5-55.0% mortality, when mortality assessments were made 1 d and 7 d after exposure, respectively (Dowdy, 1999). A similar exposure just to 50°C resulted in 1.3-28.8 and 51.3-65.0% when assessments were made 1 and 7 d after exposure, respectively.  Adults that were fed or had access to food showed reduced mortalities that ranged from 0-1.3% and 0-56.3%, irrespective of whether observations were made 1 or 7 d after exposure.  Protect-It® was the most efficacious dust producing 91.3-100% mortality of unfed adults after a 15-30 minute exposure to 34 and 50°C when mortality was assessed 1 d after exposure.   The mortality with the other three DE dusts was greater at 50°C compared to 34°C, and the mortality of adults ranged from 8.9-76.3% based on mortality 1 d after exposure.  However, all dusts produced 97.5-100% mortality at both temperatures when mortality of adults was assessed after 7 d.  Mortality of adults that were unfed never reached 100%, irrespective of the temperature, exposure time, and post-mortality assessment time, except for adults exposed for 30 minutes to Protect-It® at 50°C.  These results suggest that sanitation, in conjunction with heat and DE, is more effective than heat alone or DE plus heat. Additionally, this study also showed delayed mortality effects associated with heat alone and heat plus DE. Fields et al. (1997) reported that in an oat mill, completely mortality of adults of the confused flour beetle, Tribolium confusum Jacquelin du Val, occurred when temperatures reached 47°C after 32-38 h, but in the presence of DE complete mortality of adults occurred when temperatures reached 41ºC. Dowdy and Fields (2002) evaluated heat in combination with application of Protect-It® applied at 0.3 g/m² to second and third floor surfaces of a pilot flour mill subjected to a heat treatment against T. confusum adults.  The benefits of DE were only evident on the south side of the second floor where temperatures did not quickly reach 47°C. At the end of the heat treatment, adults exposed to partially and fully treated DE floor surfaces had 50 and 75% mortality, respectively, compared to 15% mortality of those exposed to heat alone. The combination of heat plus DE can kill insects quicker than heat or DE alone. Ebeling (1994) showed that the time required for 100% mortality of the German cockroach, Blatella germanica (L.) at a temperature of 43.3°C in the presence of a silica aerogel, a synthetic silica, was reduced from 147 to 41 minutes. 

To our knowledge, there are no published studies that investigated the combined efficacy of DE and a range of temperatures on concrete surfaces, such as those found in empty bins. The combination of these treatment methods would involve lower energy inputs to obtain temperatures lethal to insects (Fields et al., 1997). Eliminating stored-product insects in empty bins prior to storage of newly-harvested grain, along with additional integrated pest management methods, such as bin sanitation, can increase the profitability and quality of stored grain in a more sustainable and environmentally friendly manner.  

In the present investigation, laboratory experiments were designed to examine the influence of five temperatures below 50°C, two DE application rates to concrete arenas, and four exposure times on mortality of T. castaneum adults.  Concrete arenas in 9-cm Petri dishes simulated the floor of empty bins.

The research determined the optimal temperature at which to apply DE, a sustainable method, to disinfest empty grain bins prior to storage of newly harvested grain. Farmers should plan to apply DE to the concrete floor of the storage bin when temperatures are greater than or equal to 42ºC inside the bin. Diatomaceous earth can be placed at a concentration of 5 g/m2 and a fan can be used to distribute it over the floor of the bin.