The Asian ladybird beetle is mobilized in the fight against aphids

In Belgium alone there are several different species of aphids which attack a wide range of plants including rose bushes, ornamental trees, cereal crop fields, orchards and garden produce. The ladybird is one of their main predators. Why not, therefore, use the destructive power of the Asian ladybird beetle (Harmonia axyridis), which has proliferated in Europe, to defeat these tiny pests? In this area, integrated pest management has taken a giant leap forward. Published recently in the scientific literature, the identification of the insect’s sex pheromone by the Functional and Evolutionary Entomology Laboratory of Gembloux Agro-Bio Tech has constituted a major breakthrough in understanding this ladybird species that is so different from others. However, there is a great deal more work involved in getting from the laboratory phase to the field safely and efficiently. This is the reason why, despite the traditional hazards associated with applied research, work has continued apace at the Gembloux Agro-Bio-tech site.

Over the last twenty years or so, the Asian ladybird beetle (Harmonia axyridis) has established itself in the natural and artificial environments of our countries. At first it was only used in greenhouses to control aphids (and was expected to die when the appropriate season arrived), it accomplished its original mission “too well”. Reputed for its voracity (particularly its larvae), the small beetle showed itself to be capable of surviving low temperatures. More importantly, it has become a very serious competitor of indigenous species of ladybirds, particularly those with two spots (Adalia bipunctata) and those with ten spots (Adalia decempunctata), whose eggs and larvae it devours. In vineyards and orchards, Harmonia axyridis has the annoying tendency to head straight for the mature fruit and to cling to it in the form of clusters of several tens or hundreds of individuals. This alters the taste of the final products during harvesting and processing. Even private individuals in their homes soon begin to tire of this “ladybird from hell” that has come all the way from Asia: while its intrusion into people’s houses may be welcomed at first, this soon changes when it reveals all its habits. It can, for example, hibernate in even the smallest cracks in walls and nooks and crannies of chassis and, in some circumstances, to release a messy substance to which some people are allergic (leading to rhinites, skin rashes, etc.)  

At the Functional and Evolutionary Entomology Laboratory of Gembloux Agro-Bio Tech (University of Liege), we followed the progression of the Asian ladybird in Europe and Belgium on a step by step basis: in Flanders first, then in Brussels and all of Wallonia. We tried to understand the mechanisms behind this very rapid expansion. A few years ago, for example, the researchers at Gembloux demonstrated the role of hydrocarbons secreted by these animals to “mark” the meeting places and encourage their fellow-creatures to assemble there. The research, which in its present incarnation is the AgricultureIsLife platform has concentrated on integrated pest control from the outset. The intention always has been and still is to ban the use of synthetic pesticides or reduce their use to a bare minimum. Integrated pest control prioritizes the use of a combination of various natural preventive and curative techniques over the use of pesticides. These techniques are more natural and by reinforcing each other, succeed in limiting insect infestations and the resultant damage to crops. Such techniques could involve using Harmonia axyridis or any other destructive species for that matter...

Simulation in order to attract

One of the most important recent discoveries with regard to the Asian ladybird beetle, was the identification two years ago of the sexual pheromone involved in its reproduction (See The hunt for aphids, a sexual matter). “We must see reason”, explains François Verheggen, head of work at the entomology laboratory: it would be completely delusional to try to eliminate this animal which has established itself so strongly throughout Europe. And who in their right mind would dare to call for the use of synthetic products which are toxic for the environment and the user? These products would also damage other species. We need to adopt a smarter approach, for example, by trying to use the Asian ladybird beetle to our advantage, notably in the fight against aphids. But this can only happen on one important condition! This time we need to fully understand and control its characteristics. We can no longer play the sorcerer’s apprentice as was the case in the garden products sector in the nineties”!

One of the ways of using this insect where aphids are likely to cause crop damage would be to (particularly by means of the so-called “elaborated” sap, the phloem), to attract the ladybird beetles by substances imitating their sexual pheromone. “Tricking” males into believing that females of the same sex are present. To lure them, in fact, in order to encourage them to move to places where they can freely exert their appetite for aphids. And this must be done as early as possible in the season, as soon as the first generation of invading aphids arrive, or even before they arrive. We have known for a long time that aphids have a very short reproduction cycle which, in favorable weather conditions, can be very short: barely one week. A way must be found to allow the ladybird beetles to attack the aphids before they even have wings and thus prevent them from invading crops and spreading the viruses. 

The sex pheromone and its “double” 

This was the context in which the determination of the exact composition of the sex pheromone of the Asian ladybird beetle made headlines at the end of 2014 in Gembloux and ever since. “The sex pheromone is composed of five terpenes, that is to say, molecules with 15 carbon atoms from the fusion of three isoprene-type molecules. This fundamental discovery, made by Bérénice Fassotte during her doctorate (1), is not the only one that has delighted us”, says an enthusiastic François Verheggen. “A doctor of bio-engineering today, he also discovered that one of the terpenes, beta-caryophyllene, was clearly the most plentiful part of the pheromone of Harmonia axyridis, accounting for 85 % in terms of relative weight. By using olfactometers in particular (NB: devices which serve to test the reactivity of insects to certain odors), he noticed that a mixture limited to four of these terpenes (available on the market at low prices) and excluding the fifth (which is very expensive), was perfectly effective in terms of attracting ladybird beetles”. 

Apart from the creation of this product for manipulating the ladybirds, it was also necessary to apply this product in the correct doses and with optimal continuity, over a period of several days with the added risk of being considered of little interest to farmers. “The identification of the sexual pheromone had already been a considerable challenge”, says François Verheggen. “It should be noted that the ladybird beetles do not release their pheromones in just any circumstances. They need to have the “guarantee” – at this point the underlying mechanism is unknown – that their offspring will have what they need to survive, that is to say, that there is a sufficient number of aphids in their environment. We therefore had to carry out very delicate manipulations. However, once this stage of the identification was completed with success a second more difficult challenge needed to be overcome! This involved designing a sufficiently efficient odor distributor, knowing that the molecules used for attraction are often unstable in the air, in humid conditions and in light, and lose some of their efficiency”? 

Team work

It was here that the Functional and Evolutionary Entomology Laboratory was able to count on the expertise of the Laboratory of Analytical Chemistry of Professor Georges Lognay, also located in Gembloux. With Stéphanie Heuskin as her First Assistant, Bérénice Fassotte designed matrices made up of alginate beads. Sodium alginate is a polymer formed of a chain of carbohydrates fatty acids to which calcium is added. A well-known food additive, (it is sold commercially as E 401), alginate has the particularity of easily contributing to the manufacture of a gelatinous matrix in which various molecules can be encapsulated without releasing parasitic odors.  The two researchers therefore designed two microbeads of alginate which were only one or two millimeters in diameter, containing not only beta-caryophyllene but also another terpene: trans-beta-farnesene. What was the reason for this addition? “In my own thesis, some years previously. I had shown that this molecule, which is very similar to beta-caryophyllene was present in the aphid’s pheromone: it attracts the ladybirds. Stéphanie Heuskin, had used it in her own work of designing alginate beads. By adding terpene to our matrix, we potentially killed two birds with one stone: we had on the one hand a semiochemical (NB: odor) attracting strictly the male ladybird beetles (beta-caryophyllene) and, on the other hand we had a semiochemical mimicking the odor of the aphids and therefore likely to attract both the male and female ladybird beetles (farnesene)".

This “double blow”, in the laboratory, at least, proved effective. “Coupled with gas chromatography (NB: enabling the quantification and identification of the odorizing molecules), a system of odor sampling enabled us to establish that the alginate beads, following a quite brief initial peak, released the two semiochemicals in a very constant manner for seven consecutive days which was conclusive. In other words, our beads released doses very close to what a typical population of ladybird beetles would release in nature in the form of pheromones. We could certainly have increased the released dose a little to increase the attraction but this would be to run the risk of unnecessarily disturbing the male ladybird beetles and induce an undesired behavior. During these manipulations, we also tested the attractiveness of our encapsulated alginate beads on hoverflies (Episyrphus balteatus), a diptera that travels a lot more by flying than the ladybird beetles. Here too, the experiment proved very conclusive”. At this stage, Bérénice Fassotte had therefore perfectly reached the two objective of her doctorate: identification of the sex pheromone of the Asian ladybird beetle and the design of an efficient odor distributor. It should be added briefly that she also completed an international bibliographic review on the ladybird beetle reproduction which was successfully published in the “bible” of this subject area: The Journal of Pest Sciences (2)… It remained at this point to show that this alginate distributor was efficient in the field and not only in the laboratory.  

"Paintball" to the rescue

This is where things went downhill, mainly due to the weather conditions of the summer in 2015. “Based on our inventory experiment with aphidophagous insects (consumers of aphids) in large cultures, we knew that sooner or later we ran the risk of encountering a season where the aphids would be low in numbers and therefore the ladybird beetles as well. This is exactly what occurred! Installed on several wheat and bean plots of the Gembloux Agro-Bio Tech experiments farm, our distributors were only able to yield sparse results which were statistically insignificant. Then in 2016, due to lack of finance, the in situ experiment was unable to be renewed”. Was this the end of the road? Thankfully it was not. The alginate beads are currently being tested in China by my colleague Frédéric Francis with the help of Belgian and Chinese financial aid. There, in the dryer regions, there is scope for hope. It must be acknowledged that the ladybird beetles do not seem to be adapted to the humidity of our regions: when near oils, the terpenes are not water-soluble”. It would take more than this to calm the enthusiasm of François Verheggen who, as we speak, is working on other forms of matrix likely to make good odor distributors. One of these, based on a polymer encircled with a rubber membrane, well-known in the world of integrated pest control, has the disadvantage of not being biodegradable: a solution to this drawback needs to be found. Another idea, developed in collaboration with a French company, consists in designing bigger beads (two or three centimeters in diameter) in which the molecules would be encapsulated and released to keep walnut grove pests away. By means of propellants inspired by “paintball” games, “snipers” placed on the ground could propel them into the treetops thus ensuring homogenous release throughout the targeted arboriculture. 

Unfounded fears

If the experiment conducted in the fields had been able to be done as planned, it would have been interesting to observe the real behavior of the male beetles. It is not enough merely to be attracted to the aphid-infested areas are. They also need to devour the aphids in great numbers which is the aim of the “game”! Another question remains, linked to the interspecificity of species: is the sex pheromone identified in Harmonia axyridis more or less identical to that which is found in other species of ladybird beetles? This is the case with aphids. “We hope to have the answer to this question within two years thanks to the work of Pauline Legrand, who has just begun a doctoral thesis dealing with these questions. 

The question remains as to whether if by “feeding” the Asian ladybird beetle which is already affecting our ecosystems, we are running a non-negligible risk: Will Harmonia axyridis individuals that have been well fed on aphids not have a better chance of reproducing in great numbers thus increasing their chances of survival during winter? Faced with these questions, François Verheggen refuses to give up. “It must be understood that our entire strategy involves displacing already existing ladybird beetles. And this means moving them from places where they live in perfect conditions already (for example in nettle patches) to environments which are less suited to them such as cereal crops. This “forced” displacement is likely to weaken them and to interfere with them. In addition, it will be necessary to attract them to the wheat (for example) as soon as the very first aphids arrive while they are still only present in small numbers. They are therefore not being offered abundant nourishment. Finally, we can reasonably expect that, by heading for the cereal crops due to the odors released, the Asian ladybird beetles will leave more room for the indigenous ladybird beetles whose larvae, it should be remembered, are a prey for the Asian ladybird”! The researchers at Gembloux are far from finished with Harmonia axyridis and with the other ladybird beetles present in nature...

(1) Sexual attraction in lady beetles: fundamentals and applications, Doctoral thesis of Bérénice Fassotte, Gembloux Agro-Bio Tech, April 2016.

(2) The scent of love: how important are semiochemicals in the sexual behavior of lady beetles?, Journal of Pest Science, jui 2016.