Evaluation of several common seed treatments for efficacy against seedcorn maggot in sweet corn, 2023

This trial was conducted at the University of Wisconsin’s Arlington Agricultural Research Station (coordinates: 43.314159, −89.335899) on a Plano silt loam soil in 2023. Two separate plantings were established sequentially, intended to evaluate efficacy against the first (overwintering) and second generations of the seedcorn maggot (SCM) life cycle, respectively. Sweet corn (Zea mays) hybrid seed (Seminis SV1339SK) was pretreated and provided ready to plant for this trial. The first planting was established on 4 May, and the second planting on 5 Jun, each consisting of 5 replicates of 7 treatment plots arranged in an RCB design. Each plot measured 2 rows (5 ft) by 30 ft and was planted at a 2 seed/ft rate by a 2-row tractor-mounted Almaco cone seeder. Replicates were separated by 5 ft of bare ground. Both plantings were timed to align with a predicted adult SCM flight: the first-generation adult flight was predicted to peak at 360 °F growing degree days (base 39 °F, upper 86 °F), and the second-generation flight was predicted to peak at 1080 GDD, based on a model published by UW-Madison Extension. The first planting, completed on 4 May, coincided with 377 GDD, and the second planting, on 5 Jun, occurred at 1,155 GDD, so both plantings were timed within a few days of the expected peak of the SCM adult flight and maximum seedling risk due to oviposition.

All seed, including the untreated check treatment, received a fungicide program including 42-S Thiram (5 fl oz/cwt), Apron XL (0.32 fl oz/cwt), Dividend Extreme (5 fl oz/cwt), Maxim 4FS (0.08 fl oz/cwt), and Vitavax 34 (3.6 fl oz/cwt). All treatments excluding the untreated check seeds (Trt 1) also received an insecticide as part of the seed treatment (product and rate are indicated in the results tables).

For the first planting, seedling damage was assessed on 22 May and 30 May (18 and 26 days after planting, respectively) by counting the total number of emerged plants in each row and by digging 10 successive plants from the center of the first row and recording the number of plants with SCM damage or larvae present. Stand counts for the second evaluation were adjusted to account for the 10 plants dug during the first evaluation. Damage ratings are reported here as the percentage of counted seedlings damaged. Emergence and SCM damage were evaluated for the second planting on 21 Jun and 29 Jun (16 and 24 days after planting, respectively) in the same manner as the first planting. Evaluation data were analyzed in R version 4.3 (R Core Team, 2023). Crop emergence ratings are reported as percentage of total seeds planted in each plot that emerged by the evaluation date, and damage ratings are reported as percentage of 10 rated seeds per plot with damage.

Emergence in the first planting was high (82–88% 18 DAP, 83–92% 26 DAP) across all treatments with no statistically significant differences between treatments (Table 1). SCM damage was moderate, with mean damage ratings ranging from 2% (Entrust high rate) to 14% (Poncho), though due to high variation between replicates, no statistically significant differences were present between any treatments. The mean number of larvae found ranged from 0 (Reatis) to 2 (Poncho) per 10 plants, but there was no statistically significant difference between treatments. Compared to the first planting, emergence in the second planting was slightly higher (83–91% 16 DAP, 87–94% 22 DAP) but more consistent across treatments (Table 2). Mean SCM damage was low, ranging from 0% (Cruiser, Lumivia, Entrust low rate) to 6% (Poncho, Reatis) relative to the first planting. The number of larvae found per 10 plants was also very low, usually only 0 or 1 per treatment across all replicates. Low SCM pressure may be explained in part by unusually dry weather leading to unfavorable conditions for the insect.¹

Footnotes