Insects can foresee storms and high winds

Animals alter their sexual behavior when there is a drop in atmospheric pressure, a common phenomenon before rain and strong winds, reports a study conducted in Brazil.

By Elton Alisson

Agência FAPESP – In India and Japan, there is a popular saying that “when ants carry eggs, sparrows play in sand, then it will definitely rain.” In Brazil, a proverb states, “When humidity in the air rises, termites and ants leave their nests to mate.”

A study published in the October 2 issue of PLoS One proved that insects foresee changes in weather, as demonstrated by behavior modifications. The study was undertaken by researchers at the Luiz de Queiroz School of Agriculture at Universidade de São Paulo in Piracicaba (ESALQ-USP Piracicaba) in partnership with Universidade Estadual do Centro-Oeste (Unicentro) in Guarapuava and Canada’s University of Western Ontario.

The researchers observed that the beetle Diabrotica speciosa (popularly known in Brazil as the “Little Brazilian” or “Patriot beetle” because it bears the colors of the national flag), the potato aphid (Macrosiphum euphorbiae) and armyworms (Pseudaletia unipuncta) can detect a decrease in atmospheric pressure that is usually a sign of rain. When these insects notice a drop in atmospheric pressure, they modify their sexual behavior, exhibiting a reduced desire to court and mate.

“We found that insects can in fact detect changes in the weather through a drop in atmospheric pressure, and they anticipate and seek shelter to protect themselves against poor conditions, such as storms and high winds,” said José Maurício Simões Bento, professor at the Department of Entomology and Acarology and one of the authors of the study. Bento is also a researcher at the National Institute of Science and Technology on Semiochemicals in Agriculture – a FAPESP-funded INCT – and the National Council of Scientific and Technological Development (CNPq).

“These animals are certainly better prepared to face sudden changes in the weather that will probably occur with greater frequency and intensity in the world over the coming years due to global climate change,” explained Bento.

The researchers selected three species of insects – the Little Brazilian beetle, the potato aphid and the armyworm – that belong to distinct insect families and vary significantly in body mass and morphology.

The Little Brazilian beetle has a robust structure and a hard shell. It is therefore more resistant to severe conditions such as heavy rain and high winds. The potato aphid has a more fragile structure and is therefore less resistant to extreme weather events.

Given the evidence that insects adjust their flight and feeding behaviors according to changes in wind speed, the researcher decided to evaluate the effect of atmospheric conditions specifically on the courting and mating behaviors of these three species when subjected to natural or experimentally manipulated changes in atmospheric pressure.

The experiments under natural conditions (without manipulating the pressure) and controlled conditions in laboratories revealed that females, when they detect an abrupt decrease in atmospheric pressure, for example, reduce or simply stop manifesting production of what are known as “mating calls” during which they release pheromones to attract males.

For their part, the males show a decline in their sexual activity, do not respond to female stimuli and seek shelter for protection from the changes that may occur in the coming hours. When the bad weather ends, the insects resume their courting and mating activities.

“This behavior of a momentary loss of interest in mating hours before a storm represents an adaptive capacity that at the same time reduces the probability of injury and death – since they are small organisms that are very vulnerable to adverse weather conditions such as storms, heavy rain and high winds – and ensures reproduction and perpetuation of the species,” Bento explains.

Experiments

The experiments under natural conditions were conducted at ESALQ and INCT of Semiochemicals in Agriculture, both in Piracicaba. The researchers utilized a y-tube olfactometer, placing the female on one of the two lower extremities, leaving the other as a control (that is, vacant). An air current was then sent through the system, carrying the pheromones to the main extremity, where the male is located.

It is thus possible to determine whether the male is being attracted to the pheromone emitted by the female, depending on the direction he takes when the air current with the pheromone is released, i.e., the extremity where the female is located or the other end (the control).

In the experiments with the little Brazilian beetle, the researchers found that, under stable or growing atmospheric pressure, the insect walked normally toward the tube where the air with the female pheromones was being released. By contrast, under conditions of a drop in pressure, the insect presented less interest in and movement toward the female.

The group also observed that the males showed little effort to mate when kept in direct contact with the females under conditions of lower atmospheric pressure. The behavior, according to Bento, can be explained by the insect’s sensation of risk to its life.

“It is as if, faced with imminent risk, these animals put survival in first place because it guarantees perpetuation of the species and put mating at a lower priority because it is an activity that can be resumed after bad weather passes,” he evaluates.

The researchers also evaluated the number of times that the potato aphid and the armyworm responded to the mating call of their respective females under different atmospheric conditions. The results were similar to those obtained with the Little Brazilian beetle.

The behavior of the insects studied was significantly affected by changes in air pressure, as determined by the researchers using data furnished hourly by the National Meteorology Institute (Inmet).

When the data provided by the institution indicated a decrease or sudden increase in atmospheric pressure in the region of Piracicaba, the researchers began experiments to record changes in calling and copulation behaviors and compared their findings with those obtained under stable pressure conditions.

“In this manner,” Bento explained, “we managed to verify that the sexual behavior of insects varies due to the effect of atmospheric pressure, since all other conditions, like temperature, humidity and light, were controlled in the experiments.”

After observing the changes in the sexual behavior of insects under natural conditions, the ESALQ group formed a partnership with Canadian colleagues in the Biology Department at the University of Western Ontario to conduct new behavioral experiments in the laboratory, specifically a large-scale barometric chamber that allows scientists to control atmospheric pressure in addition to temperature, humidity and light. According to Bento, testing conducted under controlled pressure corroborated the observations made under natural conditions.

Extensive phenomenon

Bento pointed out that the fact that all three species of insects analyzed in the study modified their sexual behavior in response to alterations in air pressure suggests that the phenomenon may extend, in general, to other insect species and that these animals have adapted to deal with severe meteorological conditions.

“There have been other scientific works suggesting that changes in animal behavior due to changes in the weather, but they were conducted with a single species and could not be generalized to other species,” said Bento. “Our study showed that, among insects, this phenomenon seems to extend to other species.”

The group of ESALQ researchers is now investigating the mechanisms used by insects to detect changes in atmospheric pressure and develop adaptive behavior to interrupt mating upon sensing changes in the weather.

The article Weather forecasting by insects: modified sexual behaviour in response to atmospheric pressure changes (doi: 10.1371/journal.pone.0075004), by Bento et al, can be read in PLoS One at www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0075004.