Cryptolestes ferrugineus were reared in laboratory cultures on whole wheat plus wheat germ (19:1, wt:wt) and T. castaneum were reared on wheat flour plus Brewer’s yeast (19:1, wt:wt), at 30 ± 1°C and 70 ± 5% RH. The adults of both species used in the experiments were 2- to 3-wk-old. Sexing of T. castaneum was done by identifying the sex of pupae based on the pupal genital papillae in Tenebrionidae (Halstead 1963) and of C. ferrugineus as adults based on a mandibular projection on males (Rilett 1949). Three replicates each of 100 pupae of T. castaneum and adults of C. ferrugineus, were sexed in the laboratory. The mean sex ratio of 100 individuals was 51:49 for T. castaneum and 35:65 for C. ferrugineus (males to females). These values for sex ratio for the two species are similar to those reported by other researchers (Kawamoto et al. 1989; Rilett 1949; Smith 1966).

Certified hard red spring wheat, cultivar AC Barrie, with 5% dockage (finely ground wheat) by weight, and at two moisture contents of 12% and 15% wet basis (wb) was used as food for both insect species. Glass bottles of 224 mL capacity and 10 cm diam with perforated lids were used to store the grain with insects. Each glass bottle contained 150 g of wheat inclusive of 5% dockage. Adult insects of the two species were inserted singly or together at three densities. Hence a glass bottle with 150 g of wheat contained 37, 75, and 150 adults of each species, designated as low, medium, and high insect densities, respectively. For species reared together, each glass bottle contained 37, 75, and 150 adults of each species, for a total of 74, 150, and 300 insects at low, medium, and high insect densities, res-pectively. Adult counts were taken at 2, 4, 6, 8, and 10 wk. Four replicates were prepared for the three initial insect densities and species combinations (two single species and one in combination), and two moisture contents. Separate sets of bottles were prepared for each time interval. This was done to minimize the error due to repeated handling of the same grain at several time intervals of sampling. In all, 360 bottles were prepared with insects in isolation and combination and 72 bottles were sampled at each of 2, 4, 6, 8, and 10 wk. All bottles were kept in a temperature controlled chamber at 30°C and 70% RH. These standard near-optimal conditions allowed for successful population growth (Sinha and Watters 1985) while preventing fungal contamination (Wicklow 1995).

At the various sampling intervals, the bottles were removed, the grain sieved on a #10 screen (2-mm apertures), and adults carefully removed with an aspiration tube. With the adults removed, the remaining immatures and grain were returned to the bottles and incubated at 30°C and 70% RH for 4 wk to determine the number of adults emerging. No other developmental stage of either species was counted. The number of live adults presented in the data are the adults removed by sieving at the end of each respective sampling interval (2, 4, 6, 8, and 10 wk) plus the number of adults emerging from immatures after 4 wk of incubation.

The singly raised populations of C. ferrugineus and T. castaneum were significantly affected by moisture content (F = 9104.46) and initial insect density (F = 104.66) over the span of the expe-riment. Again, the numbers were lower in 12% MC grain compared to 15% MC grain for both species.

Figures 1 to 4 represent the mean adult population (adult count at each sampling interval plus the adults emerging from immatures after 4 wk of incubation); standard errors (SE) are in parenthesis, for the two species under 12% and 15% MC grain rearing conditions. For C. ferrugineus alone, mean populations at all the three insect densities were significantly higher in 15% than 12% MC grain (Figs. 1 and 2). A similar trend was observed for T. castaneum (Figs. 3 and 4). For single species in 12% MC grain, however, mean populations were higher for T. castaneum than C. ferrugineus over all insect densities (Tables 1 and 2). This observation confirms the slightly higher rate of increase of T. castaneum compared to C. ferrugineus (Sinha and Watters 1985) and reflects a greater ability of T. castaneum to survive over a wider range of relative humidity (White 1995). Tribolium castaneum is one of the most cosmopolitan pests of stored cereals and other foods (Sinha and Watters 1985).

For single species in 15% MC grain, overall mean numbers of C. ferrugineus at low, medium, and high densities were 2.3-, 2.2-, and 1.9-fold higher, respectively, than T. castaneum numbers (Tables 1 and 2). Sinha (1975) observed C. ferrugineus adult populations to be 1.2-fold higher than T. castaneum in stored wheat with a 5% dockage level after 12 wk. The lower population of T. castaneum in 15% MC grain can be attributed to a crowding effect which reduces fecundity (Rich 1956; Sonleitner 1961) and to a high incidence of cannibalism in confined laboratory populations of Tribolium (Park 1934). Under extreme circumstances, eggs, pupae, and callow adults have suffered mortality greater than 90% due to predation by larvae and mature adults of their own species (Rich 1956; Sonleitner 1961).

In 12% MC grain, C. ferrugineus adult numbers were significantly higher when reared in combination with T. castaneum than when reared in isolation, at all three densities (Fig. 1; Tables 1 and 2). However, this trend was reversed in 15% MC grain, where C. ferrugineus populations were significantly higher in isolation than when reared with T. castaneum (Fig. 2; Tables 1 and 2). It is possible that in dry grain, C. ferru- gineus 1^st instar larvae were unable to penetrate the wheat germ because of their small size and the intact kernels. Grain damaged by T. castaneum feeding was easier for C. ferrugineus adults to feed on and for larvae to enter. White (1979), in a preliminary study, also observed that the population of C. ferrugineus was higher than T. castaneum in wheat at the end of 5, 10, and 15 wk when reared together using an initial 25 adults of each species at 14.5% MC.

Suresh et al. (2001) observed that adults of T. castaneum caused more mortality of the immatures of both species than C. ferrugineus adults. They also observed that T. castaneum larvae caused higher mortality to their own species than to C. ferrugineus. Our results are consistent with previous observations, that C. ferrugineus has an inhibitory effect on T. castaneum populations. At the experimental densities tested, and under closed conditions where insects could not migrate from crowding pressure, cannibalism may have had a severe impact on the populations observed. In most Canadian conditions, where migration is possible, the effects may be tempered as insect densities are less, except for active “hot-spots.” The density of Cryptolestes spp. found in wheat sampled from farms in northern US states during the summer and fall of 1980 was 45 insects per kg (Storey et al. 1983). Populations of T. castaneum would be expected to decline slowly because of cannibalism and predation by C. ferru-gineus adults on the eggs and larvae of T. castaneum, while C. ferrugineus immatures were largely protected from predation by developing under the seed coat at the wheat germ (Suresh et al. 2001). In this study, T. castaneum populations remained static, showing little if any increase when combined with C. ferrugineus.