Production of webbing, greater fecundity, faster rate of development, and greater ability to thrive on foliage injured by feeding enabled populations of T. telarius to suppress those of P. ulmi on peach foliage in a greenhouse and insectary. However, these inhibitory factors were non-operative in an orchard environment in southwestern Ontario.The tendency of the two species to populate different levels of a host plant, and of P. ulmi to utilize the upper surfaces of leaves more than T. telarius, each decreased the intensity of competition. The suppression of P. ulmi in an insectary or greenhouse was delayed but not prevented by the above factors.

Can. Ent. 105: 449-458 (1973) The effects of various levels of infestation of the corn leaf aphid, Rhopalosiphum maidis (Fitch), on yield of field corn were examined from 1965 to 1970 inclusive when all plants whose yield might have been influenced by factors other than aphid feeding were excluded from the data. A possible relationship between moisture stress in plants and degree of aphid injury was also investigated.A light infestation (maximum of 400 aphids on the tassel) caused average yield reductions up to 8.3% when plants were under moisture stress, but nil to negligible losses when moisture was adequate. Moderate infestations (many hundreds of aphids on part of the tassel) produced average losses up to 34.8% under drought conditions and up to 11.8% when moisture was abundant. Severe infestations (many hundreds of aphids on most of the tassel) caused average yield reductions ranging from 43.2 to 91.8% when plants were under water stress and up to 58.9% even with abundant moisture. Very severely infested plants (many hundreds on all of the tassel and whorl leaves) were usually barren or had ears with just a few kernels regardless of moisture conditions.Most injury from aphid feeding appeared to occur before and during pollination. No particular combination of temperature and rainfall favored aphid buildup.

There was no evidence that the corn leaf aphid, Rhopalosiphum maidis (Fitch), can overwinter in southwestern Ontario. An alate with a small colony of nymphs was found on 26 April, indicating that migration from the south starts early in the spring. Early migrants colonized winter barley, but not winter oats, rye, or wheat. Colonization on field corn did not occur until over a month after plant emergence. Only small numbers of alatae initiated infestations on corn and the large variation in size of aphid infestations observed at pollination was due to differences in the longevity and fecundity of these few early attackers. The whorl leaves which enclosed the tassel before pollination provided a very favorable environment for rapid development of the aphids. Trapping of alatae in yellow pans of water at the periphery of a corn field in the fall showed that a minimum temperature of 13°C was required for flight. At a constant temperature of 25.5°C and a light:dark photoperiod of 14:10 h, the average prereproductive, reproductive, and postreproductive periods for 29 aphids were 5.9, 15.8, and 9.6 days, respectively. The average number of nymphs produced/female was 68.2.

Two principal pests of apple and peach trees in southwestern Ontario are the European red mite, Panonychus ulmi (Koch), and the two-spotted spider mite, Tetranychus telarius (L.). Usually only P. ulmi is present on the trees in large numbers during the spring and early summer, at which time T. telarius lives predominantly on the ground cover. In August, many T. telarius move from the ground cover to the trees, and large numbers of both species may then be present. In mite control investigations in an apple orchard at the Harrow Research Station, T. telarius moved onto the trees, especially in the central plots, regardless of the acaricides previously applied or the numbers of P. ulmi that were or had been present. This phenomenon posed the question of whether the residual effects of acaricides applied to the trees were mainly responsible for the size of P. ulmi populations in the central plots, or whether competition resulting from the ingress of T. telarius was partially responsible.

In a 3-year study, one annual application of a 10% granular formulation of disulfoton in the soil at a rate of 2 lb actual toxicant per acre controlled M. persicae throughout the entire period that aphids were most prevalent in pepper fields. Some control was observed within a few days of treatment, but effective control did not occur until 2 to 3 weeks after placement of the insecticide. Irrigation increased the initial uptake of disulfoton, but after 2 weeks the degree of control was essentially the same in irrigated and nonirrigated plots.Analyses of fruit samples from treated plots showed that there were no detectable amounts of disulfoton or its oxidation products 42, 50, and 60 days after treatment. These observations suggest that the required interval of 90 days between treatment and harvest could be reduced. Appreciable amounts of the insecticide or its oxidation products were present in the leaves on all sampling dates.