The Sterile Insect Technique (SIT) involves releasing a large number of sterile male insects into a specific area, where they mate with wild females, leading to sterility and a reduction in the population. The effectiveness of this method is linked to the ratio of sterile males to fertile males. By releasing sterile males, there is often a significant surplus of sterile over wild males, creating a substantial gender imbalance (more males than females). It's not rare for SIT initiatives to report high male-to-female ratios, with some cases observing ratios as high as 100:1 during efforts to control tsetse fly populations.

In the realm of sexually reproducing insects, female insects typically only need to mate once or a few times throughout their lifetime. Consequently, females tend to be less receptive to frequent mating, often rejecting multiple mating attempts, whereas males are more inclined to pursue multiple mating opportunities. This difference in mating behavior means that males frequently disturb females to secure mating chances. For Aedes female mosquitoes, most only require one effective mating, as the sperm stored in their spermathecae suffices for all the eggs they will lay in their lifetime. Usually, the male-to-female mosquito ratio is nearly 1:1, but in regions where sterile males have been released, a high male-to-female ratio prevails. The effects of an abundance of male mosquitoes on female mosquito behavior and population suppression are relatively unexplored.

A study titled "Mating harassment may boost the effectiveness of the sterile insect technique for Aedes mosquitoes" was published in Nature Communications by Sun Yat-sen University (SYSU), the International Atomic Energy Agency (IAEA), Jinan University, the National Environment Agency (NEA) of Singapore, as well as other institutions, on March 4, 2024. This groundbreaking research revealed that Aedes mosquitoes, primary vectors of dengue, exhibit significant male mating harassment towards females at elevated male/female ratios. This harassment notably reduces both the longevity and blood-feeding of female mosquitoes under laboratory, semi-field, and open-field conditions. For instance, in Aedes albopictus, the mortality rate by day 8 reached 40% at a male/female ratio of 100:1, in stark contrast to just 3.8% in the control group with a 1:3 male/female ratio. Continuous male harassment was documented through video recordings at a 99:1 ratio, where females were prevented from feeding on sugar solution and resorted to hiding at the bottom of the cage to evade male advances. Any attempt to fly attracted more males, trying to secure mating opportunities.

Since the inhibition of feeding under high male/female ratios significantly contributes to the increased mortality rate among females, the study further explored the impact on blood-feeding. Through three distinct blood-feeding methods in semi-field conditions (artificial blood meal, volunteers, live mice), it was observed that female blood-feeding significantly declined only when the male/female ratio exceeded 30:1. Similar findings were noted in an open-field trial with continuous release of sterile Ae. albopictus males over a 1.17-hectare area for more than 14 weeks, resulting in a 40% reduction in mosquito population as per BG-trap captures and an 80% decrease in female biting rates according to human landing assays (HLA). The male/female ratio from HLA was approximately 100:1, significantly higher than the 12.5:1 ratio from BG-traps, suggesting that male clustering around humans, in search of mates, leads to the interception of females by males, thus preventing them from blood-feeding.

In summary, the researchers emphasize that besides inducing sterility in females to decrease mosquito populations, the introduction of large numbers of sterile males can further benefit by reducing female longevity and blood-feeding through mating harassment. These factors, namely female lifespan and human contact, are vital in the spread of mosquito-borne diseases, suggesting that male mating harassment might lower the risk of disease transmission. Therefore, the study proposes a dual-mechanism approach by the mosquito SIT, not only reducing populations but also lessening blood-feeding by influencing female behaviors, significantly enhancing the technique's effectiveness.

Link to the paper:: https://www.nature.com/articles/s41467-024-46268-x