Water availability is still uncertain, but the probability of maggots infesting onion fields this year is nearly guaranteed. Over the last three field seasons, we conducted several experiments at IREC examining insecticides applied at planting for protecting onion stands from maggot damage. Our 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 2014 weather data. First generation seedcorn maggot flies are emerging right now, and first generation onion maggot are predicted to emerge May 21st. Seedcorn maggot is widespread and most problematic in fields with a lot decaying organic matter especially following alfalfa. Onion maggot is also widespread and infests many fields each year. Onion maggot overwinter in onion cull material and flies 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.
Looking at 2014 fly emergence predictions, maggot larva will attack onions at sensitive growth stages in fields planted now to mid-May. If onions maggot is expected to be problematic, consider planting in early to mid-May to avoid time flies have to oviposit on onion plants.
