From UNH Cooperative Extension, this is Over-Informed on Tree Fruit IPM
Hold on to your hats podcast listeners and get ready for a psychosexual adventure because we’re talking about moth sex. Well, excuse me, moth sex pheromones, or the odors moths use to find that special someone and start a family. For decades entomologists have been identifying and synthesizing moth sex pheromones and these odors are incredibly powerful. They really have to be because insects are so small in this big big world. For example I was part of a project years ago involving a moth pest of winegrapes. Just being in a car with the pheromone lures was enough for this pheromone to embed in my clothing that – even after several washes – made me incredibly attractive to these moths. I often found myself in outdoor social situations with one or more very enthusiastic but very confused male moths hovering around me.
IPM specialists use pheromones to monitor pest populations for IPM decision making, to determine how severe a pest infestation is or for proper timing of chemical applications. Pheromones can also be used as another tool in the IPM toolbox to disrupt behavior and therefore disrupt the success of the pest insect in being such a pest. The best example of a widely adopted behavioral control is mating disruption. It’s exactly what it sounds like and orchardist are currently disrupting the mating habits of several species of insects including today’s pest, peach tree borer.
But I’m getting ahead of myself. First the basics: there is a factsheet on the extension website written by Alan Eaton and George Hamilton titled “Peach Tree Borers in New Hampshire” so check that out but I will summarize. Adult moths lay their eggs at the base of peach trees and larvae bore into the trunk, which girdles the tree. This damage leads to a slow decline over a few years, which can often go unnoticed until its too late and that tree needs to be replaced. Upon closer inspection, you might see pupal skins or “exuvium” left behind by new moths that emerge as adults in subsequent years but you’re also likely to see a gummy exudate coming from these injury sites. This is the tree responding to attack – kind of like how we produce boogers to block up our noses from pathogen attack.
Best management practices target egg laying adults during their peak flight period in July or August. Physical barriers like latex paint can contribute but many commercial growers rely on yearly applications of insecticide to trunks to disrupting egg laying. You can use traps to monitor flights for better timing of controls but the insecticides people rely on for peach tree borer are either very expensive or particularly gnarly chemicals. Adding insult to injury, the industry standard of gnarly chemicals probably won’t be available for much longer. But there is a very good non-toxic alternative! Mating disruption! On which I will now over-inform you…
Peach tree borer is native to North America, first described in 1823 by Thomas Say, an American entomologist. For more information on this American pest, named by an American, its only fitting for me to call on a colleague down in Blessed by God, West Virginia.
Daniel Frank, WVU
Anna: How do you know you have a problem with borers?
Daniel: Yeah – it can be tricky because they’re cryptic. Dogwood borer and peach tree borer are usually on the trunks of the tree. Lesser peach tree borer is more in the scaffold limbs of the tree. But often you don’t know you have a problem unless you’re looking at the trees, looking for larval infestations, looking for pupae cases sticking out of bark crevices where they’ve been feeding. It really does take some monitoring and scouting - sometimes getting on your hands and knees – to look at the base of these trees and see. Otherwise you don’t know you have a problem until you start seeing the decline. The tree’s just not healthy looking, maybe not performing well, and you take a closer look to find that’s the borer injury.
But there’ just so many interesting ways to that you can potentially manage them. There’s the chemical route, the insecticide route. People are looking at nematodes as a potential option for control. What I’ve been interested in is behavioral manipulation. Trying to affect the behavior of an insect to the detriment of that pest in some way.
So basically what you’re doing with mating disruption is you’re misdirecting the males. You’re putting those pheromone dispensers out – they look like twist ties – tie those on the branches at a certain rate per acre. Those pheromone dispensers are releasing an artificial sex pheromone that interferes with males looking for mates. They can’t mate. Females can’t lay eggs. That population goes down.
Anna: I’m going to interrupt Daniel here to go a little more in depth on how mate finding normally goes. The moths pupate, when they first emerge as adults, it will take a little while before females are reproductively mature. When a female moth is ready and really feeling herself, she’ll find a nice spot to perch and emit a pheromone. The wind carries plumes of this pheromone through the habitat and that plume is hopefully detected by a male of her species. At the same time, male peach tree borers are out getting their swerve on, literally, swerving around. When males are searching for that special someone, they fly in what we call a casting flight, they kind of meander back and forth, kind of la-di-da, until their antennae pick up an attractive chemical. Detection of this attractive pheromone stimulates a surge upwind for as long as they detect the pheromone. If they lose the scent, they’ll go back to the casting flight until they pick it up again. What a pain, right? But you gotta do it if you’re little and you don’t have great eyes. There are tons of insect species where the male has these ginormous, plumose antennae that have evolved to have lots of surface area and lots of sensory organs to pick up even the smallest amounts of these odors. So you can imagine what happens when you fill the orchard with a whole bunch of this stuff!
Mating disruption has become the industry standard for several pest complexes in western orchards but it’s not widely adopted in eastern orchards. Back to Daniel about why this is so for peach tree borer.
Daniel: When looking at mating disruption for peach tree borer, all the studies were conducted in larger acreages. Generally recommended for more than 10 acres - not less than 5. Of course most of the peach acreage we have in West Virginia is less than 5 acres. I think our average is somewhere around 2 ½ acres.
So I was interested in whether or not mating disruption would be affective in these small block acreages. I started talking with growers to see if anyone was interested in trying this out – I would pay for the mating disruption and they could have a conventional treatment where they could apply chlorpyrifos and manage them the way they normally would. I did find a few takers. One grower set aside 3 blocks for mating disruption and 3 blocks for conventional management, which is chlorpyrifos after harvest.
What we found was that the mating disruption worked. We had trap shut down, the monitoring traps we had in the orchard weren’t catching moths, telling you that mating disruption is working – if the males can’t find traps in the orchard, they can’t find females too. When we did the damage evaluation, we found no significant difference in infestation levels in orchards that were getting the mating disruption versus the orchards getting treated with chlorpyrifos. In some instances, numerically, the infestation was a little lower in mating disruption blocks. So it was quite effective.
Anna: So this is great news! Even though conventional wisdom has told us mating disruption is only for big acreages, we have strong evidence that mating disruption is just as good as the industry standard. Everyone should run out and drop the couple a hundred dollars an acre it costs to get these pheromone dispensers and get disrupting! Well not so fast. There are some caveats. Back to my discussion with Daniel.
Daniel: Some things you have to consider, which you probably wouldn’t in a larger orchard, and that’s immigration of mated females into the orchard. If your orchard is kind of in the middle of nowhere and you don’t have many peaches around, you might not have to worry about that. If you have a lot of peach orchards nearby, you would have to worry about females coming in from those orchards.
Anna: [poorly worded question about wild alternate hosts]
Daniel: Well, peaches are stone fruits, Prunus, and that’s the wild host. Do you have a lot of wild cherry? I’ve actually seen them in a forest. I’ve seen a female fly by and say “that was a peach tree borer! Where’s that come from?” and you look around seeing the wild cherries around and its going after them too.
Some other ways that mating disruption may not be a good fit for a particular orchard. If your orchard is on a strong slope, if your orchard is on the hillside, you’re not going to get even distribution of that pheromone. If you have a lot of gaps, especially in between rows, again you’re not going to get even distribution of that pheromone. That’s really what you need to get good mating disruption to occur. If pheromone is distributed in clumps around the orchard, you’re going to get break down.
Infestation level. If you have really high infestation levels you may need to supplement with chemical controls, at least for the first year. We’ve seen that with other insects. We have mating disruption for codling moth but, if you have really high pressure, you have to supplement mating disruption with insecticides to bring that population down.
So there’s different mechanisms of how mating disruption works and one of those is competitive attraction. Basically following these pheromone sources instead of the female, males are wasting their time. But if you have a lot of insects in the orchard - a lot of females – the chances of them finding a female go up. When you have them in closer proximity to each other, they start using other cues. Rather than olfaction, they might use visual cues, things like that.
Anna: So some caveats but some caveats that we can live with? Daniel mentioned that there’s a range of labeled rates for dispensers that can be adjusted based on pest pressure too. Is mating disruption right for you? Do you still have questions? Tweet at me, I’m @Wallingbug, or shoot me an email, Anna.Wallingford@unh.edu, especially if you’re in New Hampshire, but even if you’re growing tree fruit in the east and have some concerns about trunk borers in general when chlorpyrifos is no longer available. Let me know. It’s something we’re all paying attention to.
That’s it for this week. Thanks to Daniel Frank of West Virginia University and special thanks to Brentwood’s favorite son, Jason Lightbown, who wrote and performed our theme music.