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By Leonardo “Leo” Salgado and Brian Nault, Department of Entomology, Cornell AgriTech, Cornell University; and Rob Wilson, Division of Agriculture and Natural Resources, Intermountain Research and Extension Center, University of California
Onion maggot and seedcorn maggot are major pests of onion and, if not controlled, can reduce plant stand up to 75 percent (Fig. 1). Since chlorpyrifos was banned a few years ago, insecticide seed treatments have become relied upon as the sole option for protecting onion seedlings from maggots. Seed treatments serve as a preventative approach for maggot control because once onions are infested, there is no way to rescue them (Fig. 2). Some insecticide seed treatments may be more effective against one maggot species than the other. Therefore, choosing effective insecticide seed treatments for one or both maggot pests is critical for protecting the onion crop.
This article provides an update on current and future options for managing maggots with insecticide seed treatments. Additionally, we comment on our experience evaluating one or more insecticide active ingredients in the seed treatment package. The focus is on maggot management in New York’s muck cropping systems where onion maggot abundance is higher than seedcorn maggot. However, we also discuss how these combinations have performed in a northern California onion production system where seedcorn maggot is dominant.
Lumiverd
Lumiverd is a formulation of the active ingredient spinosad, which has been highly effective in controlling onion maggot and seedcorn maggot. In the past, this active ingredient was included in Syngenta’s FarMore FI500 package as Regard SC. In late fall 2022, Corteva (spinosad’s manufacturer) registered Lumiverd on onion. Currently, Lumiverd is the only spinosad seed treatment available for onions.
For over a decade, spinosad has been one of the most effective insecticides for controlling both maggot species. However, in recent years in central New York, we have noticed a decline in spinosad’s performance in our on-farm insecticide seed treatment trials. On one farm, we attribute the reduced performance to insecticide resistance.
Trigard
Trigard is a formulation of the active ingredient cyromazine, which has been available commercially for several decades. Unlike Lumiverd, Trigard is only effective against onion maggot. Since Syngenta no longer has Regard SC, the “new” FarMore FI500 package has replaced Regard with Trigard.
In our New York field trials, Trigard often performs very well by itself, likely because onion maggot is typically the dominant pest. In contrast, Trigard has performed poorly in regions in the western U.S. where seedcorn maggot is most common. This year in our New York field trials, Trigard did not perform as well as in the past. We suspect that the reduced performance was caused by an early outbreak of seedcorn maggot, which was particularly troublesome for many crops in New York this spring. As expected in trials in California this year, Trigard alone performed very poorly.
Cruiser
Cruiser is a neonicotinoid insecticide that contains thiamethoxam. Cruiser is included in the FarMore FI500 package and has been commonly used on onion for well over a decade. Despite being highly effective for protecting beans, peas, corn and cucurbit crops from seedcorn maggot, Cruiser is not very effective against either maggot species in onion.
Sepresto
Sepresto is another neonicotinoid seed treatment, and it contains a 3:1 blend of clothianidin and imidacloprid. Sepresto initially was only available for use on Nunhems’ onion varieties, but it is now available for use on all onion varieties.
Sepresto has not been used very often as a seed treatment option for onion growers in New York. This is likely because FarMore FI500 has been very effective and results from our past studies in New York have not shown Sepresto to be very effective. However, for the second year in a row, our research results in New York have identified Sepresto as the best-performing insecticide seed treatment for maggot control. In previous research trials in New York between 2008 and 2017, Sepresto averaged only 58 percent control of maggots (range in control was 30–91 percent). In California, Sepresto has performed well and is commonly used.
Plinazolin Technology
Plinazolin technology contains the active ingredient isocycloseram, which belongs to a new class of chemistry called isoxazolines. Plinazolin technology is not commercially available as a seed treatment, but Syngenta is hopeful that it will become registered as a seed treatment on onion for maggot control in a few years. Based on our experience with Plinazolin technology over the past couple of years, it has performed better in California than in New York.
Combinations of Insecticides
In recent years in our New York field trials, we have observed that two of the most commonly used insecticide seed treatments (e.g., Lumiverd and Trigard) have not provided an acceptable level of maggot control (Fig. 3). Therefore, we have begun examining combinations of insecticides for maggot control in multiple locations in New York. These studies also were replicated in California, Oregon, Washington and Ontario, Canada. Only results from New York and California were available at the time this article was written.
On one farm in Oswego, New York, this year, Cruiser and Sepresto added to Lumiverd significantly improved maggot control compared with Lumiverd alone (Fig. 3). Results were identical when Cruiser and Sepresto were added to Trigard, which significantly improved maggot control compared with Trigard alone (Fig. 3). However, in all cases, Sepresto alone performed just as well as any of the combinations (Fig. 3). On the other Oswego farm, where maggot pressure was much lower, there was a significant improvement in control when adding Sepresto to Lumiverd, but only a numerical improvement when adding Cruiser to Lumiverd. There was only a small numerical improvement in control when either Cruiser or Sepresto was added to Trigard compared with Trigard alone (Fig. 4).
In Tulelake, California, the best treatment for controlling seedcorn maggot was a combination of Lumiverd + Sepresto (Fig. 5). While this combination did not provide statistically better control than Lumiverd alone, the combination did provide significantly better control than Sepresto alone (Fig. 5).
Guidelines for the 2025 Season
Insecticide seed treatment options for onions are limited and may differ depending on the prevalence of onion maggot and seedcorn maggot in a particular region. Here are some suggestions to consider for the 2025 season. Because Plinazolin technology will not be registered in time for the 2025 season, it is not discussed.
Seedcorn maggot: In regions where seedcorn maggot is the dominant pest, Lumiverd alone and Sepresto alone have performed much better than either Trigard alone or Cruiser alone. Lumiverd and Sepresto appear to be good choices for seedcorn maggot management.
Onion maggot and seedcorn maggot: In most locations in the eastern U.S., onion maggot is dominant, but seedcorn maggot is also present; therefore, treatments must consider both pests. In fields where onion maggot pressure is predicted to be low to moderate, Sepresto appears to be a good option. In fields where onion maggot pressure is expected to be high, Sepresto alone is still a possibility. Additionally, Lumiverd and Trigard are options, but they will need a neonicotinoid partner, either Cruiser or Sepresto. Future studies should examine the performance of Lumiverd + Trigard as a possible option for controlling both pests.
Insecticide Resistance Management
Onion maggot is notorious for developing resistance to insecticides, while resistance development has been less common for seedcorn maggot. Rotating insecticides that belong to different classes has been proposed to slow down the development of resistance in many insect species. Lumiverd, Trigard, Sepresto and Cruiser belong to IRAC Group classes 5, 17, 4A and 4A, respectively. Therefore, to prolong the effectiveness of these products and reduce the probability of resistance development, rotate between active ingredients each year.
For example, where seedcorn maggot is the dominant pest, Lumiverd and Sepresto could be alternated between years. In regions where onion maggot is the primary pest and seedcorn maggot is also present, there does not appear to be a perfect scenario that will enable all classes to be rotated effectively. Using the best performing combinations of insecticides, Lumiverd + Sepresto could be rotated with Trigard + Cruiser or vice versa. Unfortunately, neonicotinoids would be used annually.
Future Plans
Recently, the governor of New York approved new legislation called the Birds and Bees Protection Act. This law will prohibit use of neonicotinoid seed treatments on corn, soybean and wheat beginning in 2029. Many states are watching New York’s implementation of this law to possibly follow suit. While this regulation does not include onion, New York’s vegetable industry is concerned that future regulations may include vegetable crops.
Additionally, large retail grocery stores influenced by customers’ desire to eliminate pesticides that are harmful to bees have adopted pollinator protection policies that require produce they sell to be neonicotinoid-free. Future research will need to emphasize identifying options for maggot control that do not include neonicotinoids.
