No sex without aphids

By identifying the sex pheromone emitted by the females, Bérénice developped a manipulation strategy for this exotic species. "The entire reproductive cycle of lady beetles is based on the presence of aphids. Since we know that insects often communicate by means of pheromones, we came up with the idea of sampling the odours emitted by lady beetles in the presence, or absence, of aphids to see what the results would be."

To do so, lady beetles were separated according to their gender as soon as they emerged from the nymphal stage. During the first few days, they were fed pollen and sugar lumps. On the sixth day, aphids were provided to them. "During the experiment, we sampled the odours emitted by the lady beetles in glass chambers. In the glass chambers containing females, we noticed the emission of new components three days after introducing the aphids. This emission was found to increase up to the fourteenth day, before reaching a threshold and decrease around the fifteenth day (see figure below). This doesn’t mean that the females didn’t emit anything before the ninth day. In any case, our analyses didn’t allow us to detect and quantify the emitted compounds before the ninth day. As for the males, they didn’t produce any of these molecules."

The odour resulted from the blend of several molecules that had to be differentiated and identified. The operation was divided into two successive steps. First, gas chromatography analyses allowed the researchers to separate the different components and to measure their proportion and quantify them. This step was followed by mass spectrometry, which allowed them to identify the different molecules in the pheromonal blend.

"After these chemical analyses, we had to test the influence of this pheromone on the behaviour of the lady beetles", the researcher continues. "While females didn’t respond to the odour, males were strongly attracted to it. We reached the conclusion that it was a sex pheromone and not simply an aggregation pheromone. Sex pheromones are produced by one gender to attract the opposite gender with the aim of reproducing." Which leads to new questions. The researchers know that aphids are a key stimulus in the sexual maturation of female lady beetles, and that they only lay eggs in their presence. "It’s a form of adaptation", François Verheggen reveals. "Why would the female lose time giving birth to an offspring that has nothing to eat? But we haven’t identified what informs the lady beetle of the sufficient presence of aphids. Is the female capable of assessing the quantity of aphids and adapting the number of laid eggs, etc. ?"

In any case, the research team from Gembloux was the first to reveal the role of a sex pheromone in the reproduction of lady beetles. To date, no-one was aware that they were emitting volatile pheromones, inducing remote attraction of males. "Indeed, it’s quite amazing that we were the first to discover this pheromone", says the entomologist. "Lady beetles are model insects that are widely studied, and are excellent predators, with a high potential in crop protection."