Evaluation of Structural Fumigation Efficacy for Management of the Red Flour Beetle
Fumigation of food processing and storage structures with methyl bromide for the control of stored-product insect pests has been a commonly used tool for many years. However, methyl bromide has been identified as an ozone depleting substance, and most of its applications are being phased out worldwide under an international treaty called the Montreal Protocol on Substances that Deplete the Ozone Layer. This process has triggered a need to evaluate the impact of methyl bromide fumigations on pest populations as a baseline, and then to assess the impact of other treatments to determine if they are viable alternatives. The red flour beetle (Tribolium castaneum) is a major pest of wheat and rice mills, and its management has relied heavily on fumigation with methyl bromide. However, there was little information on pest population dynamics and impact of fumigations within food processing facilities. Recent research has focused on the evaluation of fumigation efficacy in commercial mills – of both methyl bromide and an alternative fumigant sulfuryl fluoride – and how the need to fumigate could be reduced through other integrated pest management strategies. Red flour beetle was monitored using pheromone and food-baited traps placed both inside and outside the mills over multiple years at multiple mill locations to obtain the data needed for this type of analysis.

Although superficially wheat and rice mills have much in common and share the same primary pest, the population dynamics of red flour beetle were very different between the two mill types. Analysis of seasonal patterns in pest activity in wheat mills indicated that populations persisted and continued to increase all year round, although the rate of increase was less in the winter. This was because high enough temperatures were maintained inside all year round to support pest development. Immigration of beetles into the mill from outside sources also appeared to be relatively low in wheat mills. Analysis of this monitoring data also identified threshold values below which beetle captures in traps did not change significantly between monitoring periods, suggesting that this might be a useful management target to reduce risk. In rice mills, beetle captures inside were more seasonal, with greatly reduced activity in the winter months. Captures of beetles outside was greater and captures inside and outside the mill were positively correlated. Traps placed in areas where rough rice is stored prior to milling often indicated high levels of red flour beetle activity, and this is a potential source of immigrants into the mill. Unlike in the wheat mill data, inside temperatures in the rice mills followed outside temperatures all year round and thus were not favorable for red flour beetle development or movement during much of the winter. These differences in population dynamics and spatial distribution inside and outside the mill could impact fumigation effectiveness.

The immediate impact of a fumigation was assessed by comparing the number of beetles captured in the monitoring period before treatment with those captured in the period immediately after treatment. This change will reflect the decrease due to treatment mortality, but also immigration of new beetles following treatment. In the wheat mill data, the mean number of beetles per trap decreased by 85% following fumigation with methyl bromide, with most fumigation events providing a high level of pest reduction. Beetle captures immediately after fumigation were positively related with those in the period immediately prior to fumigation, which indicates that the higher pest levels are allowed to build prior to treatment the greater the numbers present after treatment will be, and this can lead to more rapid rebound post-treatment. The season that the fumigation was performed did not impact immediate reduction in captures. This suggests that immigration was not a significant contributor to pest activity immediately after treatment. In rice mills using sulfuryl fluoride as an alternative fumigant to methyl bromide, fumigations led to an average 66% reduction in captures of red flour beetle adults, and the percent reduction was highly variable. Unlike in wheat mills, reduction in captures after fumigation was related to outside temperature conditions, which could be due to the tighter association between inside and outside temperatures and/or the influence on beetle movement into the mill. Thus, this difference in immediate reduction in captures between the fumigants is probably due less to differences in fumigant type and more to differences in pest populations between the two mill types.

Rebound in red flour beetle captures after fumigation was also evaluated, and in wheat mills it was found that time of year fumigation was performed and integrated pest management practices performed within the mill after fumigation both significantly impacted rebound time. However, in rice mills beetle captures tended to increase and decrease independently of fumigations, which made rebound patterns difficult to evaluate. Unlike in wheat mills, rebound rate in rice mills was not associated with beetle capture levels prior to fumigation, which suggests that immigration was a significant contributor to post-treatment levels. Overall, captures of beetles in traps rebounded more slowly in rice mills than in wheat mills. Potential differences in rebound due to differences in egg mortality between the two fumigants could not be assessed because of these strong seasonal patterns in immigration and development in rice mills.

These results suggest a fundamental difference in red flour beetle population dynamics and structure between wheat and rice mills, which in turn impacts fumigation effectiveness. They also suggest how treatment efficacy can be evaluated and how monitoring programs might be used to indicate the need to fumigate. Management of rebound rate and keeping pest captures below threshold levels may provide a way to reduce the frequency of fumigations or eliminate fumigations completely. As methyl bromide becomes less available and as fumigation costs increase, use of monitoring information to guide integrated pest management programs is likely to become of increasing importance.

Red Flour Beetle Interactions with Pheromone Traps
Red flour beetle, Tribolium castaneum, is a major pest of flour mills and other food processing facilities. Pheromone traps that capture walking insects are widely used for monitoring this pest inside food processing facilities to aid in making pest management decisions. These traps are typically baited with aggregation pheromone and food-based attractants, but anecdotal reports suggest that response of red flour beetle to these attractants is not very strong. A current research focus has been on evaluating beetle response to these traps with the goals of improving the attractiveness of traps, selection of locations to place traps, and interpretation of monitoring programs.

Although commercial pheromone lures have long been used for monitoring of populations of flour beetles, no one has ever determined either the site of production in the beetle’s body or the natural blend of components (stereoisomers) in the attractant actually produced by the insect, and there is evidence that the commercial blend is not optimized for maximum attractive potency. We showed that the natural pheromone is produced in the abdominal epidermis (outer layer of the “skin”), and is composed of a 4:4:1:1 ratio of four forms of the pheromone (4,8-dimethyldecanal). We also demonstrated that this natural blend is significantly more attractive than the commercial pheromone blend, which is a 1:1:0:0 ratio of the four forms. Using the natural ratio could lead to significant improvements in flour beetle detection in mills and warehouses. Research is now focused on further characterizing the response of beetles to this natural blend.

How well pheromone and food attractants used in traps work under real world conditions has not been previously evaluated. Using a new experimental protocol that simulates how beetles interact with traps in commercial food facilities, we demonstrated that the beetle’s response was strongest to traps baited with pheromone and food attractant, or with pheromone alone, when air flowed from the trap toward the beetle. The beetle’s positive response to traps baited with pheromone and food attractant extended out to a distance of 90 cm (35 inches), the maximum distance tested. However, under still air conditions, beetles did not respond to any of the tested attractants. Within food processing facilities, traps can be placed in sheltered locations with limited air movement or in more exposed areas with high amounts of airflow, with the result that traps may vary in their attractiveness to red flour beetles. Understanding how beetle behavior is influenced by factors such as attractant type and airflow can help guide the selection and placement of traps within a food facility. Research is now focused on evaluation of additional factors that could impact beetle response to traps.

Variation among environmental and physical conditions among locations where traps are placed inside food processing facilities can potentially influence both the distribution of stored-product pests and the effectiveness of traps at capturing them. Data from a long-term red flour beetle monitoring program was used to evaluate spatial variation in captures among trap locations and to determine if differences in environmental and physical conditions at trap locations might be influencing the patterns of trap captures. Evaluating long-term patterns in red flour beetle captures revealed that while over short periods of time beetles were more likely to be found in certain areas of the mill, over longer periods of time areas of greater insect activity moved around the mill resulting in long-term averages that were more uniform among trap locations. Although the characteristics of individual trap locations were found to be highly variable, only warmer temperatures, higher flour dust accumulation, and proximity of milling equipment were associated with traps with high levels of beetle capture. Results indicate that while the environment appeared to have some influence over pattern in beetle captures it was limited, probably because broader patterns of change in distribution within the mill over time, perhaps related to season or increase in total abundance, were more important. Research is now focused on evaluating some of these factors under laboratory conditions and further evaluation of these factors in other food facilities. (Collaborators: Michael Aikins, Kazuaki Akasaka, Dick Beeman, Yujie Lu, Kenji Mori, Yoonseong Park, Tom Phillips, Altair Semeao, Phil Sloderbeck, Shigeyuki Tamogami, and Jeff Whitworth )