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Presented to the Canadian Pest Management Society Meeting in Truro
The work I'm reporting here was carried out by a student, working under my
supervision (Dr. Saeed Ba-Angood) as part of a wider study on the bionomics of cereal aphids in Quebec. We investigated two possible cultural control approaches, namely, the use of less attractive cultivars and altering the sowing date, and the detailed results of these have already been published elsewhere (Baa-Angood and Stewart 1980a, 1980b).
The work was conducted on the Macdonald College Farm, Ste. Anne de Bellevue in 1978 and 1979. To investigate the effect of cultivar differences, early maturing barley (cv. Loyola), late barley (cv. Laurier), early wheat (cv. Glenlea), late wheat (cv. Concorde), early oats (cv. Yamaska) and late oats (cv. Dorval) were sown as described by Ba-Angood and Stewart (1980a). Throughout the summer the cereal aphid populations were monitored at weekly intervals by cutting tillers from the plants and counting the aphids present on them (Ba-Angood and Stewart op. cit).
We investigated the effect of altering the sowing date by an in depth study on barley only. In both years the barley (cv. Loyola) was sown on three different dates. Each sowing date treatment was split and one subplot was sprayed with pirimicarb after the flowering stage of the barley. This allowed us to assess the impact of spraying for the different sowing date treatments.
Again the aphid populations were monitored, but in addition parasites and predators were counted and barley yield was determined by estimating the mean number of earheads/m of row, the mean number of grains/earhead and the mean weight of 1000 barley grains (Ba-Angood and Stewart 1980b).
The three major species infesting the cereals were the English grain aphid Macrosiphum (Sitobion) avenae, (Fabricus), the bird-cherry oat aphid Rhopalosiphum padi (L.), and the corn leaf aphid R. maidis. We also found the greenbug Schizaphis graminum (Rondani) and the rose aphid Metopolophium dirhodum (Walker) but only in very small numbers.
We could draw no conclusions as to the desirability of growing barley, oats or wheat in preference to the others. In 1978, wheat had the highest total aphid numbers and barley and oats had smaller numbers However, in 1979, barley showed the greatest infestation and oats were intermediate. We suspect these differences were due to differences in the relative abundance of the three aphid species in the two years. M. avenae was the most abundant in 1978 and the least abundant in 1979. R. maidis was the least abundant in 1978 and the most abundant in 1979. As R. maidis showed an apparent preference for barley in both years, and as it was the most abundant aphid species in 1979, this could account for the heaviest infestation being on barley in that year. Although our studies indicated that wheat was least preferred by R. maidis and that the most preferred host was barley, which agrees with Adams and Drew (1964) we did get populations of this species surviving on wheat, while they did not.
In this series of experiments we did have an indication that the early maturing cultivars (cvs. Loyola, Yamaska and Glenlea) were less infested with aphids than the later maturing cultivars, but the numbers were not significantly different (P=0.05). However, the experiments on sowing date of barley. In both years aphids were most numerous in the later sowing dates. We found no significant difference in the numbers of the major predators (coccinelids) and parasites (aphid mummies) between the different sowing dates (unsprayed) although there was an indication that they were a little more abundant in the earliest sowing date treatment.
We were also interested in the effect of the different levels of aphid infestation on crop yield and here again there is conclusive evidence that the earlier sowing dates gave not only lower aphid densities but higher yields of grain. Note that only in the later sowing date treatment is there a significant difference in yield between the sprayed and unsprayed treatments and also that the unsprayed earliest sown treatment gave a higher yield than the two later sown unsprayed treatments. The other measures of yield also showed reductions for the late sowing date compared to the earliest sowing date. The fact that there was no significant difference in yield between the sprayed and unsprayed treatments for the first sowing date would indicate that under these conditions there was no need to spray.
Our 1979 experiment showed a highly significant reduction in aphid populations for all treatments after spraying, and also there was a further reduction in numbers probably due to aphids flying off the crop. We also observed an increase in predators and parasites at that time. Again there was no significant difference in yield between unsprayed and sprayed treatments for the first sowing dates, and the yields for the unsprayed treatments for the first sowing date were higher than for the other two dates. This trend was again supported by the other indices of yield.
We conclude that the early sowing date is a useful cultural control in itself, and there is little advantage from spraying when this was used. However, although we found no evidence of virus disease in our experimental area, if aphids were acting as vectors this would at the very least complicate the situation.
Adams, J.B. and M.E. Drew. 1964. Grain aphids in New Brunswick II. Comparative development in the greenhouse of three aphid species on four kinds of grasses. Can. J. Zool. 42: 741-744.
Ba-Angood, S.A. and R.K. Stewart. 1980a. Occurrence, development and distribution of cereal aphids on early and late cultivars of wheat, barley and oats in southwestern Quebec. Can. Ent. 112(6): 615-620.
Ba-Angood, S.A. and R.K. Stewart. 1980b. Sowing date and cereal aphid infestations and damage to barley in southwestern Québec. J. econ. Entomol. 73(3): 462-464.
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Ecological Agriculture Projects, McGill University (Macdonald
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