Transcript of the Episode and Show Notes
Well Mother’s day is this weekend and this episode is all about moms…maggot moms…more specifically onion maggot moms. Like most mothers, onion maggot moms want what is best for their children. My made sure I got a stellar education and taught me to be a good friend and a hard worker. Onion maggot moms leave their children on onions. It doesn’t sound like much, but thousands of years of evolution have taught her that leaving her kids on onions is the best thing she can do for them. More on onion maggot biology later but first, the basics:
Onion maggot flies lay their eggs at the base of onions and related alliums, like leeks, chives, and garlic. Larvae then crawl inside the plant and feed on roots often right at the soil level. Obviously this feeding, and the presence of maggots, makes infested onions unmarketable. This feeding also encourages entry of fungal pathogens which can ruin an onion bulb even if those larvae die, especially for bulbing onions that need a long storage life. Conventional chemical control is challenging because larvae are so well protected.
Best management focuses on protecting onion plants from egg laying during peak flight times for adults. Onion maggot overwinters in the soil so the first generation flight occurs pretty early, usually late May or early June. A good rule of thumb is to look for flowering wild mustard. You can track growing degree days using the NEWA website, NEWA.cornell.edu, select the weather station nearest you and pick growing degree days from the menu, select base temperature of 40F. Peak flight for the first generation should be occurring around 735 GDD.
You can use yellow sticky card traps to detect fly activity right in your onions are but these flies look so much like other flies, so monitoring using yellow sticky cards is a good way to determine when flies in general are active but probably not a good idea to judge pest pressure. Cabbage maggot adults look a lot like regular house flies and they are almost indistinguishable from seedcorn maggot and cabbage maggot. Also keep in mind that, if you don’t have a problem with onion maggot, and you may not have a problem, a lot of this information in this episode holds true for cabbage maggot. These two species are very closely related and cabbage maggot moms do pretty much the same thing except they have co-evolved with brassicas, and they leave their kids on cabbage instead of onions. Everyone has a different parenting style.
Consult the NE vegetable guide for recommendations on how to protect your onions and brassicas from onion maggot and cabbage maggot but, remember, pesticides must be applied only as directed on the label to be in compliance with the law. So read those labels, every time.
Ready to get Over-Informed? Let’s take a step back in history and review the literature:
As you would expect most of the pertinent literature focuses on understanding adult behavior and oviposition behavior in particular
First described by the number one fly enthusiast himself Johan Meigen in Germany in 1826, cabbage maggot has been around for quite some time and is what we call a cosmopolitan species, or one that is distributed through out the world, pretty much anywhere there’s enough water to grow plants.
As early as the 1920s scientists understood this insect’s behavior could be manipulated with volatile chemicals – or odors. By the late 70s we understood that sulfur-bearing chemicals, which give onions their oniony smell, were particularly important for helping these insects in long distance location of onion hosts (100 m). Flies are more likely to fly upwind and perhaps a little faster with less turning when they smell an attractive odor.
Work in the 80s found that you can lead an onion maggot to onion with those sulfur compounds but egg laying is only stimulated plant odors work together with visual stimuli – the shape and color of an onion stem. This may be part of why later attempts to use behavior against onion maggot by diverting them from growing onions to cull piles didn’t see great success. But a shout out to Rich Cowles and his stimulo-deterrent diversion approach – a term I find superior to other behavioral control terms that are more widely adopted like trap crop or push pull… but leave that to another episodes.
Investigations in the 80s also helped use to understand aestivation or summer diapause behavior. Pupae in the soil stop developing unless soil temperatures are below about 70 degrees F. This would explain why we have such a problem during spring but not so much during summer, even though onion maggot has as many as three generations in New England.
Several investigators looked for natural enemies in the 80s: Ground beetles will eat larvae, if they can get to them. So that’s the same challenge as we see with chemical control. Lab studies investigating the potential for using entomopathic fungi like Beauveria and Metarhizium found moderate infection rates, only about 20-50% infection rates and pathogens generally had more of an effect on pupation rates rather than larval mortality. So that was kind of a wash.
Fast forward to the 1990s and you’ll read about efforts where entomologists exposed females to extracts of male paragonial glands. Essentially they were trying to demonstrate that you could negatively affect female reproductive by exposing her to male hormones, kind of like onion maggot birth control. This approach didn’t exactly pan out but the idea didn’t go away. Commercially available insect growth regulators can depress reproduction in this group. The secret is how you get the females to expose themselves to it…
In the 2000s entomologists worked to understand the winter biology of onion maggot including underground behavior. More recent work has brought more attempts at answering old questions, but with new tools. Transcriptome and mitochondrial genome were published in 2014 and 2015.
And remember how the work done in the work done in the 80s investigating Beauveria, the entomopathogenic fungi, found surprisingly low infection rates considering how effective these pathogens can be in other systems. In a paper published this year – looks like onion maggot had its associated microbiota to thank for repressed infections of Beauveria. Axenic flies, those are flies reared without gut bugs, got sicker than flies with their normal compliment of gut bugs…so eat your yogurt ya’ll. Keep your gut bacteria happy and they’ll do the same for you.
For the most up-to-date information on managing onion maggot, I called someone who would know:
Brian Nault, Cornell University
Unfortunately, there’s no germplasm with any resistance to onion maggot. So there’s been no advancement in onion cultivars that are resistant to onion maggot. Secondly, naturally occurring biological control doesn’t seem to have much of an impact on onion maggot populations. At least what we know so far. So those are two huge tactics in IPM that haven’t really provided much help in managing onion maggot.
Crop rotation is one of the most effective ways for managing onion maggots in onion. Onion maggot isn’t a very good flier, so if you can plant onions ½ mile or a mile from where you had onions growing before, you can really decrease the chance of infestation. The problem is, even though it works beautifully without the use of chemicals, it’s not feasible for the majority of the onion growers. So that’s very unfortunate.
Delaying planting will significantly reduce onion maggot as well. In this case, you’re escaping the maggot in time rather than in space, like you do with crop rotation. So the later you plant, the less likely you are to get onion maggot. But what happens after mid- to the last week of may, the later you plant onions you’re going to end up with a smaller bulb at harvest. And growers can’t afford to have smaller bulbs at harvest.
Most onion growers use an insecticide at planting as their main approach for controlling this insect. It’s a prophylactic approach, but not a bad one because the risk of infestation is extremely high and the value of the crop is extremely high. If you don’t use a protectant at planting and you have an infestation of onion maggot that kills the crop, there’s not enough time to replant.
There are situations where the information on onion maggot activity that’s on the NEWA website is useful for growers who are producing onions organically, who don’t have prophylactic insecticide seed treatments options, or chose not to use them. In that case, by using that model predicting onion maggot adult activity, one could use a row cover or some other physical barrier to protect those onions while the females are out laying their eggs. And then remove that row cover after the flight is over. That can be very effective in onion.
We are currently studying how abiotic conditions are affecting onion maggot populations in fields in New York. We have noticed some areas where onion maggot can be extremely high and other areas where its nearly non-existent, and growers in both areas are using the exact same tactics. We’re wondering if things like soil moisture or temperature, that’s ambient temperature or soil temperature, could influence the levels of infestations. Erica Morety in my program is looking into that.
Another thing we’re looking at is trying to understand the mechanisms for how some of these control tactics work. One of the seed treatments we’re looking at is a seed treatment called spinosad. Spinosad is not known to be systemic but seed treatments provide excellent protection against onion maggot. We don’t know how it works. Perhaps it is systemic? We don’t know that it is. Or its not systemic and its creating some kind of deterrence from the plant. We don’t know that mechanisms yet but we’re hoping to identify it so we can better understand how to use that product, or maybe develop new strategies based on repellence.
This year we’re going to begin a number of projects looking into efficacy of entomopathogenic nematodes and fungi for managing this pest. Unless we find one of these biological controls or alternative controls, I don’t see any budging of getting up off this pesticide treadmill.
