Boxwood and Box Tree Moth Research
An update by Chris Poole
In previous Topiarius, the science article has tried to raise awareness of issues around Buxus plants and how best to deal with them. This time I thought it would be useful to give a brief update on the research that is going on around the world into box blight and the box tree moth (BTM Cydalima perspectalis). There are two main research projects underway that I’m going to focus on, one funded by Euphresco, a trans-national organisation that looks to support collaborative research into plant health and give guidance to evidence based national and international polices. The other, the Boxwood Blight Insight Group (BBIG), is funded by the United States Department of Agriculture which is doing lots of research into the effects of blight and has expanded to cover the box tree moth given its potential impact on the country’s top selling evergreen shrub – Buxus. You will also be pleased to hear that EBTS is involved in both of these research groups.
United States
The first piece of research is looking at the phenology or life cycle of BTM in Western New York state. It is spilt into three parts, firstly the details of its life cycle, secondly how many growing degree days are needed for each stage of its development.
Knowing more about the life cycle timings, the group is going to look at how the information can be used to better control the tactics deployed using the more accurate timings to guide when to take action. For example, imagine an App that would use growing degree days to tell you each time BTM caterpillars were going to be active so that you didn’t have to look through your bushes to find the pesky caterpillars.
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Growing degrees are calculated each day by taking the maximum temperature plus the minimum temperature, dividing the result by two giving you the mean temperature from which you subtract a base temperature. By adding the each of these daily values together you can see how many growing degree days are required for each stage of the development of BTM.
The base temperature is set based on the point at which development starts, which can be approximated to 10c as egg, larvae and pupa development occurs at slightly different temperatures. Degree day calculators/Apps can be found on the web such as the one provided by Syngenta (search ‘Syngenta degree days’)
The third part of this research area is monitoring the presence of natural parasitoids found in Western New York state. During November 2022 the group collected weekly data on the number of moths caught in both sticky paper and funnel pheromone traps. For New York State they found that moths peaked in mid-July and again in early September. With most damage to plants occurring in late July and early August. They noticed that damage was worst on the cultivar ‘Winter Gem’, whilst ‘Green Mountain’ suffered less.
This is interesting because Green Mountain is a cross between Buxus sempervirens and the Korean Buxus microphylla – this might help explain why Europe has been so badly hit by BTM as much of our box is Buxus sempervirens i.e., it doesn’t have the Asian influence. This also ties in with the new blight tolerant hybrids from Herplants in Belgium which were hybridised from Asian varieties and are consequently less appetising to the hungry BTM caterpillar.
58 recipes
To carry out research into BTM you need to have moths and caterpillars available to work with and this gets quite expensive when you are feeding thousands of caterpillars with box plants. So, the US Department of Agriculture team in the Forest Pest Management Laboratory in Buzzard Bay Massachusetts are looking to create an artificial diet that can be used to rear BTM. There are a couple of published diets one from Japan and the other from Spain, both require some box material to enable a complete life cycle. The team have developed their own basic diet, but it only supports growth to near and fully grown caterpillar which in technical terms are 6th and 7th instars. However, they do not then go on to pupate. If you move them from the artificial diet to ‘Winter Gem’ boxwood plants when they reach their 5th instar, they do develop and form pupa which then become moths. This helps a little but most of the consumption of box leaves happens in the late-stage instars, so it doesn’t reduce the cost much. They have tried adding freeze dried leaves, to the meal plan but that doesn’t seem to work either, so they tried to seek out the missing ingredients that were needed to create the fully artificial diet to sustain full life cycles. On their 58th recipe for one of the mixtures, they were successful, rearing caterpillars through to maturity, so they now have a formula that works which should help in maintaining colonies of BTM for future studies.
Croatia
The next bit of research took place in Croatia during 2021 and 2022 and they were looking at BTM mating habits. They found the moths generally mated in the hour before dawn and they remained in copula, joined for between 70 and 90 minutes for the first generation of moths, but subsequent generations spent a shorter time in copula reducing to just under 70 minutes for the third generation.
It was found that females call for males by elongating their abdomens in a straight line, rather than up or down, they also flapped their wings. This behaviour only happened during dusk, not overnight or at dawn and it lasted for around 30-35 minutes.
Mating Disruption
Over the last two years there has been more research going on in the mating disruptor field, led by a US team with field testing in Split, in Croatia. The product that has been developed is called TRECE and it uses the rubber bung lures you might have used in the past. The advantage of this potential product is that it lasts a full season, whereas the already available Box T Press Pro from M2i has to be applied twice in a season. The disruptor was tested over a 70-ha area and showed a 99% trap shut down i.e. male moths couldn’t find the traps because the female moth pheromone was everywhere – they also saw a 75% reduction in caterpillars. This is a very similar result to that achieved with the M2i product Box T Pro Press which is available in Europe.
The team also tested 4 female BTM lures, of which two formulations also attracted males even in the Mating Disruptor treated areas. This is useful as it will help monitor moth levels in these areas. It also has potential to be used to catch females to take them out of a garden environment, however the team are going to look at the range over which it would attract the females.
They have also look at the minimum area that needs to be covered for mating disruption to be effective. Would it work in an urban garden environment, or does it only work for larger areas? The answer is that the larger the area used the better the results. This isn’t a total surprise as in a small set up moths from outside the disruption zone can easily fly in and have a big impact. So in this case bigger is most definitely better!
Zagreb
Next, we move to the scientists at the University of Zagreb who were looking at flight propensity or how far the moths can fly. In their experiments the maximum distance flown by a virgin female BTM was 15.6km and it spent 241 minutes in flight with an average speed of 1.1metres per second. This information will update insights into the migratory activities of BTM.
Just in case you wondered how they measured how far the moth can fly; they use something called a flight mill. Basically, a moth is attached with a dab of glue to a piece of wire balanced on a pin, when the moth flaps its wings, it causes the wire to rotate in a circle which allows the distance travelled to be measured.
Croatia & the United States
We’ve all been used to using the standard green funnel traps with pheromone lures in them, but are they the best way to trap BTM?
A team based in Split, in Croatia and the US Department of Agriculture, California Station ran tests on various trap and lure combinations to see which came out best. The good news for those of us who have invested in the green funnel traps is that these have proved to be the most effective and that the pheromone being used is also the right one. The team also tested four lures designed to trap females and these showed promising results.
The team also ran tests to see how effectively lures would attract moths to traps. The results were interesting showing they can entice male moths in from up to 60-70m or about 10 times the distance indicated on most traps. Not surprisingly they noted wind direction had a significant effect on the number of moths finding the traps.
United Kingdom
EBTS UK has contributed information to the research programme by providing examples of real-life experiences of dealing with BTM. Also, passing on its knowledge gained in using potential products like the frass and thyme oil moth deterrents trialled at Ham House in Richmond and new products such as the mating disruptor from M2i.
We are also contributing to another Euphresco project by funding the purchase of equipment that will be used by the RHS at Wisley. This team will be looking for markers that can be identified in the volatiles given off by blight infected box and BTM infested plants. The idea behind this research is that in the longer term a sniffer device could be created to check for the specific volatiles in plant material coming into the country in shipping containers or lorries to see if it has box blight or BTM. It could also be used in nurseries that are growing box or possibly even by domestic users if the technology was cheap enough to produce.
Canada
The University of Toronto is looking for natural enemies of BTM found in Toronto City Park. These include fungal and bacterial pathogens found in BTM larvae. The hope is to find something natural that can be used against the caterpillars similar to Bacillus thuringiensis that is used in products like Dipel and XenTari. They are also looking at four Trichogramma species (micro wasps) that parasitise BTM eggs to see if they can be used in the fight to control the spread.
Also in Canada, teams in Guelph & Toronto who wanted to find out if Buxus was the only plant that BTM could live on. Using a choice or no-choice feeding experiment between Buxus and Euonymus, it was found that BTM were 15 times more likely to choose the box rather than the Euonymus. Feeding attempts were made on the Euonymus leaves but after 40 days BTM caterpillars only survived and progressed to pupation when feeding on Buxus.
In addition to these feeding experiments they also tested the effectiveness of 3 insecticides and 2 biopesticides to determine their contact and oral toxicity. The most effective insecticide was one based on chlorantraniliprole (a wide spectrum insecticide) which is used in Acelepryn, which unfortunately isn’t currently available for use domestically or professionally on box. But is a product that has been talked about several times for controlling BTM and Syngenta are looking into it’s potential use in this area.
Spain
The Institute for Multidisciplinary Applied Biological Research in Navarra, Northern Spain collected 2132 BTM caterpillars from 43 natural boxwood forests and reared them through to pupation or death. Their research was slightly macabre as they looked at dead larvae and pupae to see what killed them. For example, they look for any parasitoids or bacterial killers. They found one parasitoid that accounted for 1% of mortalities and 106 new strains of Bacillus thuringiensis, which are now being evaluated for possible insecticidal use.
Switzerland & South Korea
A team from the Centre for Agriculture and Bioscience International (CABI) in Switzerland, carried out a literature review looking at BTM on its home turf in China and Japan to see what local parasitoids were present. Unfortunately, due to Covid and ambiguities in sharing material from China, the field trip couldn’t take place. However, BTM is also in South Korea, so the scientists, in conjunction with the School of Applied Life Sciences in Daegu, carried out surveys between May and June in 2022 in both urban and natural stands of box finding 228 different parasitoids in the collected caterpillars. When they looked at the BTM populations for the second and third generations they found a decline in numbers, particularly in the natural box stands rather than urban areas. This is significant because the natural box stands were where the highest levels of parasitoids were found. They were to some extent controlling the population, whereas the urban areas had less biodiversity so had fewer parasitoids.
And finally…
The final piece of research is looking at Sterile Insect Technique. This is where a dose of ionizing radiation is used to sterilise the female moths and reduce the fertility of the male moths. The idea isn’t that we should all have boxes in our gardens that has a radioactive isotope in it designed to irradiate the moths that fly into it. Rather that many moths would be raised and irradiated, these would then be released into the wild. The effect of which should be population collapse.
The researchers have worked out the levels of radiation needed to sterilise the moths, now they need to work out how many moths would be required in an area to cause population collapse.
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Sterile Insect Technique (SIT) has been used effectively a number of times in Africa on insects like tsetse flies and mosquitoes.
