Water availability is still uncertain, but the probability of maggots infesting onion fields this year is nearly guaranteed. Several experiments were conducted at IREC from 2011 to 2013 examining insecticides applied at planting for protecting onion stands from maggot damage. The results show seed treatment with OI500 (spinosad) was the most effective all three years. The next best labeled option was Lorsban applied in furrow at the maximum label rate. In untreated plots, onion stands were reduced at least 50% from maggot feeding compared to those treated with an insecticide.

During the study we trapped maggot flies to determine when adults emerge and deposit eggs in onion fields. The results from 2011, 2012, and 2013 are shown in the figure below. All three years, seedcorn maggot and onion maggot flies were found in the traps. Researchers in New York and Canada have developed degree day models to predict fly emergence based on air temperatures. Using Tulelake weather data, we compared their predicted emergence dates with fly counts from 2011-2013.  Most years, the predicted emergence date came close to matching elevated fly counts. 

Below is a graph showing predicted emergence of seedcorn and onion maggot using 2015 weather data. Winter air temperatures have been extremely warm and peak first generation seedcorn maggot flight occurred last week (17 days earlier than 2014 and more than a month earlier than historic averages). First generation onion maggot flight is predicted on May 9^th (12 days earlier than 2014). Seedcorn maggot is widespread and most problematic in fields with a lot of decaying organic matter, especially following alfalfa. Onion maggot is also widespread and problematic in most onion fields. Onion maggot overwinter in onion cull material and flies are attracted to deposit eggs at the base of onions plants. Onion maggot is most problematic in fields located within 0.75 miles of old onion fields with cull material left in the field.

Since seedcorn maggot larvae have already started laying eggs in fields this year, seedcorn maggot larvae will be feeding all April and early May when onions are sensitive to maggot damage. Onion maggot fly emergence may peak early enough in 2015 that fields planted in early to mid-May will have yet to emerge during onion maggot egg-laying. Unfortunately, the second generation of seedcorn maggot may occur early enough for seedcorn maggot to damage plantings in May.