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)".

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