INSECTICIDES, INSECTICIDE PLACEMENT, AND SUGARBEET PHYTOTOXICITY

Robert L. Stoltz and John J. Gallian

Presented at the University of Idaho Sugarbeet Schools on February 15 and 16, 1990, in Burley and Caldwell.

Field observations the last several years have indicated that some insecticides applied at planting may be phytotoxic to young sugarbeet seedlings, especially when the insecticides were applied using the modified in-furrow method. To confirm these observations, three insecticides, Lorsban, Counter and Temik, were tested for their possible phytotoxicity with three different placements at planting. The trial was conducted at the Kimberly Research and Extension Center because there is no root maggot pressure at that location and any damage to sugarbeet seedlings could be attributed to the insecticides. Planting was on 18 April with a Milton planter using the WS-88 large pellet at a 6 1/2 inch spacing. Plots were one row x 30 ft and furrow irrigated. Lorsban, Temik and Counter were applied at both the label rate (Lorsban, 9 oz/1000 row ft; Counter, 8 oz/1000 row ft; Temik, 9.5 oz/1000 row ft) and 1.5 x the label rate. For each application rate the chemicals were applied in a 2-inch band in front of the disc opener (DO), modified in-furrow at the rear of the disc opener (MIF), and in front of the press wheel with a drag chain to incorporate the chemical (PW). Each treatment and an untreated check were replicated 12 times.

Phytotoxicity and plant stand counts were made on 14 June. Phytotoxicity was recorded as the amount of biomass reduction (delayed or reduced plant growth) in insecticide treated plots compared with the untreated check plots, and plant stand counts were made by counting the number of plants per 10 row ft in each plot.

Average biomass reduction and plant stand for each insecticide over all placements and rates are presented in Table 1. The high biomass reduction for Lorsban and Counter can be modified by altering placement as seen in Tables 3 and 4.

Values for the different placement and rate combinations averaged over all the chemicals are presented in Table 2. It is evident that the MIF placement gave higher phytotoxicity than the DO and PW applications, and that phytotoxicity is aggravated by using 1.5 x the label rate.

Phytotoxicity and plant stand counts for Lorsban, Counter, and Temik individually are presented in Tables 3, 4, and 5, respectively. Temik applications caused minimal phytotoxicity and had no effect on the plant stand. Lorsban and Counter were considerably phytotoxic when placed using the MIF application and when applied at 1.5 x the label rate. Damage and plant stand reduction with these chemicals was substantially reduced when applied at the label rate using the PW application method.

Results of this one-year study indicate that Lorsban and Counter applied to control sugarbeet root maggot should be applied using placements other than modified in-furrow (MIF). The margin of safety for application rate effect on phytotoxicity is small for these two insecticides and great care should be taken to ensure proper calibration. Small variations in application rate, band width, and/or tractor speed can easily result in applying 1.5 x the label rate of insecticide.

*14 June sampling date.