Talk by Richard Vickers on controlling insect pests without using insecticide
by admin | October 23, 2020 6:47 am
I want to talk to you tonight about a way of controlling insect pests without using insecticide. The technique is called ‘mating disruption’. I’m also going to talk to you about sex because that is what we are manipulating with the technique. It makes use of pheromones, which are defined as chemicals emitted by an individual to send messages to others of the same species. The work is made up from two Greek words – Phero meaning “to carry” and Hormone meaning “to excite or stimulate”. There are a number of different kinds of pheromones but tonight I’ll be talking about sex pheromones, which are a bit like perfumes and are used for attracting mates. Each species has its own unique pheromone and, in the case of moths, are almost always produced by the female.
I work exclusively with moths, although many other insect orders also have pheromones. In moths pheromone is produced and released from a gland that sits at the tip of the abdomen of the female. When the warehouse moth (Ephestia cautella) releases pheromone, or ‘calls’, she curls her abdomen up so that the gland sits above the wing tips. Some species curl the abdomen down and other species don’t do anything obvious at all. The pheromone is carried off downwind and is detected by the male, who then flies upwind towards her. Pheromones are pretty powerful things – males only needs a few molecules to excite them, so a little bit goes a long way.
How can we use sex pheromones? One application is as baits in traps, where a small quantity, similar to the amount that a single female would carry, is evaporated from a substrate such as a piece of rubber tubing. In other words the bait is a synthetic version of the female. Males pick up the scent and come to the trap thinking that they’ll find a female. But all they do is come to a sticky end! Pheromone traps can be used for monitoring and surveillance. For example the Australian Quarantine and Inspection Service (AQIS) uses them around ports to detect the Asian gypsy moth, which is harmful species that can come in by accident. The traps can be used by growers to determine what pest populations are doing. If they monitor their traps on a regular basis they can see whether or not the population is increasing and so decide when to put on control measures or they could use them to indicate whether they have the pest at all.
Mass trapping is a technique that was once thought to have potential as a means of pest control. It was assumed that by trapping and removing enough males, populations would decline and control would follow. However it very rarely works for moths because most males can mate more than once. Even if you trap 80 or 90% of available males, and that is almost impossible to achieve, those that are left behind can still mate with enough females to cause significant damage.
‘Attract and kill’ is a bit like mass trapping except that you use a pheromone in conjunction with an insecticide. The two are put out together as a paste so that when the male is attracted by the pheromone he gets a fatal dose of insecticide. This method is relatively new and is still being evaluated.
Pathogen distribution via moths attracted by pheromone to inoculation chambers is another relatively new technique that has some potential for insect pest control. After the male enters the chamber he gets infected with a pathogen, such as a fungus, which he then takes back out and spreads to others within the population. The pathogen is fatal and causes the insects to die within a few days of becoming infected.
I’ve already mentioned mating disruption briefly and now I’d like to tell you about how we developed the technique for control of a clearwing borer moth in persimmons. It involves releasing large quantities of synthetic pheromone in the orchard. This confuses the males, who simply don’t know which way to go to find the females. If he can’t find a female no mating takes place and we get control.
George Is there only the one signal they use to find a female?
Richard In most cases with moths it is. Some insects use sound and/or sight too: butterflies for example use sight as well as pheromones. But for the moth species we work with it’s almost exclusively odour, at least until the males get very close to the female, when visual cues are sometimes used.
To be honest, we’re not 100% sure how mating disruption technique works – there are a number of possibilities but a likely scenario is that releasing all that pheromone creates a lot of false trails that the males follow without finding a female. It is also possible that with so much synthetic pheromone about the male’s sensory systems become overloaded and are no longer able to detect the small quantities of pheromone released by the female. It’s a bit like us when we come across a bad smell. If we are exposed to it for long enough, our sensory system becomes adapted and we get to the stage where we no longer notice it.
The insect that is creating a real problem in the Persimmon industry is a clearwing moth. Until a year or so ago it was known as Carmenta chrysophanes but then the taxonomists got hold of and changed its name to Ichneumonoptera chrysophanes. It’s much easier to say Carmenta, but tonight I’ll just call it a clear wing moth! Unlike most moths, this one responds to its pheromone during the day, which is a bit of a luxury for us because we like to see how our moths are behaving and with most species that means going out with night vision goggles or infrared light just to see what’s going on.
George Is it a native?
Richard Yes but there are related species in many other parts of the world.
George I don’t think I’ve ever seen one.
Richard No, but you might have seen the damage that they do. In terms of their appearance there’s quite a bit of difference between the male and female, but for most of the moths I work with there isn’t any obvious difference between male and female.
Sheryl What size are they?
Richard About 1cm long. These moths don’t touch the fruit – they ringbark. They’re called borers because the larvae get into the timber itself and cause damage around new shoots as well as old ones. I’ve seen whole limbs that have dropped off trees and in one case in Redland Bay an entire tree had fallen over. I’ve been told by a couple of growers that this insect is limiting expansion of the industry because they haven’t got an effective control measure, other than netting the orchard. The industry had heard of our success with some other fruit tree pests and approached us to see if there was anything we could do for them. There are no chemicals registered for use with this pest and as you can imagine, once the eggs have been laid and the larvae burrow into the tree, they’re almost out of reach of insecticides. New damage is often seen at the base of young shoots (Fig. 1), where small piles of fresh frass are left as larvae burrow in and feed on the layers beneath the bark. Older, larger wounds (Fig. 2), sometimes at the juncture of branches with the main trunk, may also be evident.
Fig. 1. New damage at base of shoot Fig 2. Old damage
The moth has a number of alternative hosts, including several Eucalyptus, Wisteria and Ficus species as well as Alphitonia excelsa. It has also been recovered from galls on Exocarpos cupressiformis. Its distribution is mostly confined to the east coast of Australia, although one specimen has been recorded from the ACT. When this project started I was approached by some growers in South Australia who also have a borer problem. They have provided some of the funding for this project, but unfortunately it appears that the pest they have down there is not the same one that we have in NSW and Qld. I’ve not been able to get specimens of adults from SA so I still don’t know just what is causing their problem.
Conventionally pheromones are identified by extracting the contents of the pheromone gland and feeding them through a gas chromatograph (GC). Then the components are tested in field trials to confirm that they are attractive to the insect. Now as you’ll probably be aware, the persimmon industry doesn’t have a lot of money for research and so when they came to us to see if we could help we indicated that because of the limited budget we would take a few shortcuts in trying to identify the pheromone. Then we would see how we could use it for control purposes. Our initial research was done down at Redland Bay and subsequent trials were done on Ben Jeffers’ place up near Nambour.
Fortunately the pheromone for many of this insect’s close relatives had already been identified, which made our task much easier. It turned out that, almost without exception, their pheromones were confined to 4 or 5 different types of chemicals, so we knew right from the start we had a good chance of making an identification simply by putting out combinations of these components and seeing what we could catch. With each successive trapping trial we refined our blends until we had something that we were confident was an effective attractant. We tried a number of alcohols and acetates on their own and in combination and eventually found that a 90:10 ratio of the acetate to the alcohol caught more moths than any other combination. Then our chemist offered to run a sample of pheromone extracted from the gland through the GC at no charge, just to confirm that the compounds and their ratios that I’d identified in my trapping trials were similar to those in the gland. Sure enough, I had the right components and they were in the right ratio.
The next stage was to see whether we could control the insects using the pheromone as a mating disruptant. This is a typical pheromone dispenser. It’s a polyethylene tube with a wire on one side to tie around the tree and the pheromone column on the other side. This particular pheromone doesn’t have any smell that my nose can detect, although some do. One dispenser contains pheromone equivalent to about 100,000 females! We recommend putting out a 1000 per hectare, so you’re talking about the equivalent of 100,000,000 females. Now perhaps you can see why the male could get a little confused!
Sheryl How much are they?
Richard It depends on the chemicals that go into them. This is now a commercial product and I think they are selling for around $300-$400 per hectare. Bio Control is the agent for the dispensers in Australia.
We set up the trial at Nambour in an orchard with two blocks about 50 m apart. We put dispensers in one (the ‘treated’ block) but not the other (the ‘untreated’ block) and in both we put in pheromone traps. If mating disruption is going to work then males should not be able to find pheromone traps where dispensers are present. And we assume that if they can’t find a pheromone trap then they’re not going to find a female either, although there are some exceptions. What we hoped to see in the treated block was very few or preferably no catches in the traps but plenty in the untreated block. Likewise in terms of damage, we hoped to see much less damage in the treated block than in the untreated block. The trial ran for 9 months – the adults are around for a long period in this part of the world – and we caught large numbers of moths where there were no dispensers but absolutely nothing in the traps where there were.
Dispensers were put out in September and as some damage was already evident when we started the trials, some mated females may still have been present, meaning that further damage could arise even once the dispensers had been installed. What we hoped to do was limit damage to well below that sustained in the untreated block. We conducted two damage surveys – one in December and the final one in the following June. Seventy six trees were inspected in both blocks in the first survey, of which 2.7% were damaged in the treated compared with almost 19% in the untreated block. Damage levels in the June survey were quite high in the treated block – 20.5% , but much higher – 57%, in the untreated.
A feature of this sort of control programme is that it can take a couple of years to get damage down to an acceptable level. We and the growers were quite happy with the result, providing that in subsequent years damage could be reduced even further. We’re now into the second year and have 19 growers using the technique. Time will tell whether we can push damage levels down any further, but the results are quite encouraging. One adjustment we will recommend is that instead of having only one set of dispensers go out into the orchard in the hope that it will last the entire season, we will recommend that a second set go in around New Year.
Data we collected when we first began these trials indicated that after nine months pheromone release rates had dropped to less than 4 mg/hectate/hour. When you think that for at least one species we worked with we had to keep the release rate above 6 mgs per hectare/hour for mating disruption to be effective, it may be prudent to add a second set of dispensers and keep the release rate a little higher.
There’s a closely related pest that gets into currants in Tasmania and New Zealand and we have the pheromone for that. There’s another one that’s a problem in peaches in America. Again it’s the tree that suffers – not the fruit.
Mating disruption is not a system you can use in your backyard – it’s only suitable for reasonably sized orchards.
George I’ve tried wicks but they’re only suitable for fruit fly.
Richard The wicks contain interesting compounds. They’re called psuedo or para-pheromones because they are not actually what the males produce themselves, although the true pheromones may be derived from them after they have been ingested. Fruitfly para-pheromones are very powerful attractants.
Jim Were you using the same materials in your attraction technique in the SA examples or were you trying something different there.
Richard The same compounds – we know it’s a different insect but I’ve not been able to get the adults I need for identification purposes.
Sheryl So what can we do for the backyard – netting? Can you put a piece of wire in and dig them out?
Richard Netting has been tried by some growers and apparently does work. However digging larvae out isn’t likely to be effective. They get pretty well into the tree and you would probably do more damage trying to dig them out. In any case I don’t think you could kill enough for it to be effective. You’re not going to get them all.
George When you dig around, how many do you see?
Richard We tend to see only singles or 2 or 3 so it’s likely that the female lays only a few eggs in any one spot. Little is known of the insect’s life cycle, but the relatively few generations a season (possibly only two) may reflect the food that the larvae are getting. You can imagine that the nutrient value of bark and wood is not as great as fruit might be, so it takes longer for the larvae to mature. The climate, at least in Queensland, means that it is warm enough to enable development to continue for much of the year, which may be why we have such an extended period during which adults are about.
Richard The larvae spend their entire life cycle in the tree. They pupate in the tree as well so there is no immature stage of the cycle that is not on the tree.
Sheryl What’s your next project?
Richard Diamondback Moth on cabbages. I mentioned at the start of the talk that pheromone could be used to bring males in to an inoculation chamber, where they become infected with a fungus that they take out and spread amongst their own population. We ran some trials about 18 months ago to test the concept and they worked quite well. Now we want to scale up. I’ve spent most of today writing a grant application for some funds to finance the project.
George These techniques aren’t going to work on the Fruit Piercing Moths
Richard No. However I believe Harry Faye at DPI Mareeba is about to patent an attractant for the Fruit Piercing Moth. It’s not a pheromone – it’s a food attractant. With this moth the difficulty is that they fly in to the orchard in response to the smell of the fruit, feed on it and then leave. In other words they are not resident within the orchard for very long, which doesn’t give you much time to do anything about them. I’m generally a bit doubtful about the likelihood of food attractants succeeding, simply because there’s so much competition in the form of natural food. For this reason artificial baits have got to be much more attractive to the insect than its natural food.
Sheryl – I remember Peter Young saying that they really like Carambolas so if you put out a bait with these perhaps you can distract them. Is there any other research happening with fruit?
Richard Geoff Waite DPI at Nambour has been working on a pheromone for Fruit Spotting Bug. When the project started six years ago the claim was made that there were six components to the pheromone, five of which had already been identified, but as far as I’m aware there is still no effective pheromone for this pest.
Source URL: https://stfc.org.au/articles/talk-by-richard-vickers-on-controlling-insect-pests-without-using-insecticide/