Toxicity of diatomaceous earth (Protect-ItÔ) to red flour beetles and confused flour beetles (Coleoptera: Tenebrionidae): effects of temperature and relative humidity
Dr. Frank Arthur

Introduction: Diatomaceous earth (DE) is a natural product composed of fossilized diatoms (aquatic phytoplankton) from freshwater or marine sedimentary deposits. They absorb waxy fats and oils (lipids) from the insect cuticle and may also physically cut and damage the cuticle. This inhibits insects ability to retain water, and they die from dessication. Factors such as the target insect species, environmental conditions, and exposure interval can affect the response of insects to DE. The objectives of this test were to determine: 1) effect of temperature and relative humidity on mortality of red flour beetles and confused flour beetles, 2) variation in mortality between the two species, and 3) if mortality would continue to occur after the beetles were removed from the treated environment.

Methods: Studies were conducted with Protect-ItÔ, a DE formulation of marine origin. The label rate for use as a surface treatment is 5 grams/m², or 0.15 oz per yd². Adult red flour beetles (RFB) and adult confused flour beetles (CFB) were exposed without food to the label rate of Protect-It for different time intervals at 72, 81, 90°F, and 40, 57, and 75% RH. Beetles were exposed in 3.5 in diameter Petri dishes lined with filter paper. After exposure the beetles were removed from the treated dish and held in new dishes without food for 1 week. Mortality was determined after exposure and after the 1-week holding period. Results are summarized using selected examples from the different phases of the study.

Results: Red flour beetles exposed at 81°F, 57% RH (Figure 1). Mortality increased as the exposure interval increased. Also, beetles continued to be affected by exposure to DE even after they were removed from the treated environment because the 1-week mortality was usually much greater than initial mortality. Red flour beetle versus confused flour beetle, 81°F, 57% RH, 1-week mortality (Figure 2). Mortality was much lower for confused flour beetles than red flour beetles exposed for 8, 16, and 24 hours, then held for one week. Results were similar at other temperature-RH combinations, indicating that the confused flour beetle was less susceptible than the red flour beetle. Confused flour beetle, 81°F, 40, 57, 75% RH, 1-week mortality (Figure 3). As RH increased, mortality decreased, particularly at the 8, 16, and 24-hour exposure intervals Mortality at 75% RH was usually much lower compared to mortality at 40 and 57% RH. Confused flour beetle 57% RH, 72, 81, 90°F. Mortality increased as temperature increased, and the effect was more pronounced at the lower exposure intervals (Figure 4).

Conclusions: Mortality of both beetle species increased slightly as temperature increased. However, as relative humidity increased, the diatomaceous earth became less effective because longer exposure intervals were required to produce complete mortality. Also, the confused flour beetle was less susceptible to the DE formulation than the red flour beetle. These results show that both the internal environmental conditions within a processing mill or storage facility and the particular target pest species could affect the performance of diatomaceous earths when they are applied as surface treatments.

(Figure 1)	(Figure 2)

(Figure 3)	(Figure 4)