Over-informed on IPM - Episode 019: Onion thrips and bacterial rots

Anna discusses the state onion IPM with Dr. Ashley Leach (previously Cornell AgriTech, now Purdue University), and how resistant cultivars, fertility, and chemical controls integrate to protect high quality onion crops.
Onion Thrips  -Whitney Cranshaw, Colorado State University, Bugwood.org

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Show Notes


Relationship Between Nitrogen Fertilizer Rare, Onion Thrips and Yield in New York 

Onion Diseases


From UNH Cooperative Extension, this is Over-Informed on Vegetable IPM

I am going to jump right in to this week’s topic because I had such a great time catching up with this week’s expert

Now Ashley is currently postdoc in Ian Kaplan’s lab at Purdue University, and I will get to her work out there later but this week is all about her PhD project on thrips attacking onions. Of course, I took out all the cursing and ranting but I’m going to share with you a largely un-edited portion of our conversation.you will tell how much both of us enjoy talking about this particular issue, and I’ll leave it to Ashley to explain:

Ashley: Onion thrips are itty bitty insects, in the order Thysanoptera, that feed directly on onion tissue. This feeding leads to leaf die back, which ultimately can lead to plant die back – so the plant is going to end up dying if you have a really heavy thrips infestation. Even so, if you have more feeding damage, you have less photosynthetic area and we’re going to get a smaller bulb as a result. That’s really the primary complain when we’re talking about thrips feeding, in isolation.

The other problem with thrips is that they’re associated with a variety of plant diseases, plant pathogens. They have viral plant pathogens, which they can vector, including tospoviruses. I worked iris yellow spot virus in onion production. We know there is some kind of relationship between thrips and several species of bacteria that can lead to bulb rots in onion production. There are also associated with fungal pathogens. We just finished up a study where we saw an increase in fungal colonization when we saw an increase in thrips feeding, including Alternaria species that cause purple blotch in onions. So this is a really problematic pest from all of these angles, both from direct feeding and also their association with all these plant pathogens.

Anna: It sounds you’re talking about onion thrips (Thrips tabaci) here. What about other species of thrips feedin in onion? Do you make a distinction? Does it matter?

Ashley: It really depends on where you are in the world. I was conducting my research in upstate New York where it’s Thrips tabaci – that species is driving the bus, its causing all the problems. Sometimes I’ll have growers come up to me and say, Ashley, I have thrips ALL over my onions. I’ll look and they’ll be soybean thrips because there was a soybean field nearby and they were a little lost! Not doing any damage. You can go to places in the southeastern United States and they’ll have a different species complex. They’ll often have Franklinella species that are more dominant. Not to say they don’t have Thrips tabaci there, they just have a more complex species composition than we have in the northeastern states and the mid-West.

Anna: Would it be ok for me to say that any advice you have for dealing with thrips on onions – if its onion thrips or a thrips feeding on onion – we’re going to treat them pretty much the same?

Ashley: Yeah, for the most part. The only place we might get a little ticky tacky might be if we start talking about insecticide resistance. If we’re planning an insecticide sequence for managing resistance that might be one thing. But for any cultural control or non-chemical control in general, they should be pretty universally applicable.

Anna: OK, so what’s your recommendation for scouting thrips in a bulbing onion crop?

Ashley: Well, what I like to do – and I really mean that – we like to start scouting when onion thrips are colonizing a field, sometime in early to mid-June. Then we go in weekly and randomly sample a number plants, depending on how many acres we’ll be scouting. I like to do about 30-40 plants per acre. When we see an average of one thrips per leaf, so for example if there are 6 leaves on a plant and there are 6 thrips on a plant, that’s when we’ll initiate a spray program. Typically that spray program will start out with spirotetramat (Movento). Depending on how bad that infestation is, we might later move to a product like abamectin (Agra-Mek). If we’re having a really hard time controlling this population, we might go in with a heavier hitter like a diamide (Exirel) or spinetoram (Radiant).

Anna: Allright, I’m going to jump in with my obligatory disclaimer here. Pesticides must be applied only as directed on the label to be in compliance with the law. Read those labels, guys. Also, Extension does not promote the use of individual commercial products. Ashley is quite good at mentioning the active ingredient and common trade name of the pesticides she’s talking about but, if you have any questions about the active ingredients of these products, especially if you’d like more information on generic products, give your extension specialist a holler. If you’re in New Hampshire, that’s me! Give me a holler! Back to Ashley for a synopsis of her PhD project:

Ashley: I did a bunch of different things for my PhD. I had really great advisors that let me run in any direction I wanted, so I kind of ran in all the directions.

We as entomologists, we tend to build IPM programs that are solving an arthropod problem, and typically its one arthropod we’re talking about. Well when you’re installing an IPM program for just one thing, that might be a recipe for disaster. Or it might be great! And maybe we can harmonize it with existing problems. When I say pest, I mean weeds, diseases, arthropods.

In onion production we have two big problems: bacterial bulb rots caused by a suit of different bacterial pathogens and onion thrips. Some might argue they have bigger problems with smuts or onion maggot, but those are the heavy hitters in New York and the two we decided to focus in on. We focused on three different tactics, one was more of a cultural control. Looking at different levels of nitrogen fertilization because we know that with increasing rates of nitrogen, we’re expecting increasing thrips infestations and increasing rates of bacterial bulb rot. We looked at two different cultivars, one that was considered more thrips resistant and one that was more thrips susceptible. Lastly, we looked at insecticides. We know insecticide is useful. Growers are going to use this as a very effective way to manage onion thrips.

Anna: I am going to jump here, because Ashley went into much greater detail about the mechanisms of these three approaches but I’m going to summarize a few things.

First, she mentioned nitrogen fertilizers. You may have heard that over-fertilization can lead to more serious pest problems, especially when you are talking about excess nitrogen. There’s a few potential mechanisms that might cause this phenomenon, but the thinking here is that excess nitrogen uptake in the plant might lead to increased reproductive success in the insects feeding on those plants. Insects need protein to produce their eggs. Nitrogen makes up amino acids, which make up protein. More nitrogen, more thrips babies. At the same time, the plant is growing faster which leads to changes in natural chemical defenses or the cell structure is not as great at providing physical defenses. This is the concern and why the Nault lab was interested in investigating.

Ashley also mentions thrips resistant cultivars, but I’ll spare you our conversation about this and just mention that thrips resistant cultivars tend to be the darker blue green varieties and that the resistance is likely a matter of the onions producing chemicals in their epicuticular wax that are yucky to thrips. So they’re kind of sweating out natural repellents. Sounds simple enough, but not so fast. Back to Ashely on what they found:

Ashley: I should be careful how I say this…nitrogen did not impact yield. As long as growers were putting down 60 lbs/acre, we did not find statistically different yields. We did not see it impact thrips densities in any meaningful ways. Even though there is a bunch of literature saying this should be an effective cultural control tactic. We found that, when it comes to onion thrips, the thing that was most effective was insecticide. Using an action threshold. We found that using an action threshold was just as effective as using a calendar-based approach. 

Now getting to what I think is the most interesting aspect of this, the thrips susceptible and thrips resistant cultivars. It did exactly what we would expect in terms of thrips resistance, the cultivar with the resistance had lower levels of onion thrips. We saw that over four years of field trials. BUT, it had 2-10 times more bacterial bulb rot than the thrips-susceptible cultivar. When I tell a grower that, they just say, ok, I’ll just spray the one that’s susceptible. Bulb rot is not something they want to mess around with.

Anna: Remind me again, are the thrips carrying the pathogen. Are they causing damage and pooping it out and rubbing it in, or is that bacteria just in the environment?

Ashley: I get so worked up about this because people want to blame thrips for rot and I just don’t think that’s real. There was a study, that showed two Pantoea species that lead to center rots in onions. Those two species are transferred in the frass of onion thrips. They did some nice experiments that found transmission, something like 30-50% of the time, by thrips carrying the Pantoea. So when you’re talking about Pantoea, there’s a really compelling argument that thrips are causing the problem. However there’s usually a really diverse collection of organisms causing these infections. In New York state, Pantoea is maybe 1 out of 100 cases. It’s just not as prevalent.

Anna: So as far as onion diseases go, everyone’s operation will be a little different. Definitely seek out proper identification if you’re having a problem with your onions, but I’m also including links on the Extension website to some factsheets from Christy Hoepting on several of the onion diseases that Ashely mentions. I’m also including a little summary of the most applicable stuff from the nitrogen work.

And what’s our take home here? Don’t over-fertilize! Definitely don’t under-fertilize! Use insecticides! That’s not a particularly satisfying conclusion. I’ll let Ashley conclude here:

Ashley: I am looking at system optimization. I’m not looking to vilify anyone here. It’s about what we can do with less. We were very fortunate to show, you don’t need as much nitrogen. You don’t need it. So maybe experiment next year with putting less on and see what happens. Same with insecticide. You’re ok with less. Use an action threshold and get the same control from a weekly program. That’s how I would prefer we talk about these things. How can we use less for a program that’s just as effective.

Anna: Couldn’t have said it better myself! That’s it for now. Thanks to Dr. Ashely Leach and thanks to Jason Lightbown who wrote and performed our theme music!