High Maggot Activity in Onions and Other Transplants

Reports from southeastern Pennsylvania and other areas of the state over the past week indicate losses of onion transplants, and in some cases other vegetable transplants (tomato, marigolds, brassicas and others), due to infestations of onion and seedcorn maggots. Samples submitted to the Penn State Insect Identification Lab have been confirmed as seedcorn maggot. Damage has been most severe in onion and other allium transplants, although seedcorn maggot can attack many large-seeded crops.

Based on current reports, this may be one of the most severe maggot years in recent memory, although the reasons for high populations this season remain unclear. Once maggots have damaged transplants, there are no effective rescue treatments. Protection must be applied preventatively to new or replacement transplants.

Using treated seeds (Cruiser 5FS, Gaucho 600, Poncho 600, or Triguard depending on crop) is the most effective preventative measure against root feeding maggots. Depending on your crop, at plant treatments include Malathion, Brigade, or Mustang Maxx. See labels for specific crops and rates. Additionally, there is a FIFRA 2(ee) label for use of Verimark as a transplant tray drench at 13.5 fl oz (398.25 ml) per acre to protect plants prior to field setting. To prepare the correct rate, first convert the labeled rate to a per-plant rate by dividing by your planting density. For example, at 184,464 plants per acre, this equals 0.00216 ml per plant. Next, scale this up to the number of plants in your tray. A 288-cell tray with two plants per cell (576 plants) requires 1.24 ml (0.042 oz) of Verimark per tray. To apply, lightly moisten the tray first, then mix 1.24 ml Verimark in 1 liter (0.26 gallons) of water and distribute evenly over the tray. Treated transplants must be planted in the field within 72 hours of application. Trials conducted at Cornell AgriTech demonstrate that this treatment will provide between 40 and 80% control of onion transplants against maggot damage.

Finally, floating row covers can be used to exclude adult flies from laying eggs immediately after planting but are less effective if maggots are already in the soil.