Features
SIT for managing the pepper weevil and other challenging pests

The sterile insect technique (SIT) is an innovative strategy to control insect pests without the use of insecticides that could one day be used for the control of routine and invasive greenhouse pests. It has historically been used to control a diversity of insect species that threaten not only humans and animals, but also crop health. Some examples are mosquitoes that vector malaria or dengue in tropical parts of the world, and tsetse flies that transmit parasites responsible for sleeping sickness in humans and cattle. Across the world, successful management programs for multiple Tephritid fruit fly species have incorporated the SIT, which reduces reliance on insecticides, and recently the sweet potato weevil was eradicated from two islands in Okinawa, Japan using this technique.

In Canada, two current and successful SIT programs have targeted field crop pests including the onion maggot (i.e., La Mouche Rose in Quebec) and the codling moth (i.e., the Okanagan-Kootenay Sterile Insect Release program in B.C.);  and now, new research has explored applying the SIT for a problematic greenhouse pest, the pepper weevil.

What is the sterile insect technique and how does it work?
An SIT program essentially involves the release of large numbers of sterilized insects of the same species as a target pest. Following their release, sterile male insects will mate with wild, fertile females which are then unable to produce offspring. Following multiple rounds of sterile insect releases, the wild pest population will decrease to below an economic threshold, often within just a few generations. For the SIT to work, the target species must reproduce sexually, so species that undergo asexual reproduction (called parthenogenesis) are generally not suitable for this management tool. The SIT is also compatible, and in fact, most effective when used in combination with other management tools, such as biological control and conventional insecticides.

Insects are typically sterilized by irradiation using a radioactive source. It is important to note, though, that the irradiated insects themselves do not become radioactive and will not spread radiation once released. Crucially, the sterilized males must remain healthy and competitive against wild males so that they can seek out females and mate with them successfully. 

Cost-efficient mass-rearing and accurate population monitoring techniques are also important for determining when, where, and how many sterile insects should be released, and how effective they are at reducing the pest population. 

Prospects of the sterile insect technique for greenhouse pest management
While the SIT has traditionally been used for outdoor agriculture over large management areas, applying the SIT in greenhouses has begun to receive attention in recent years. Greenhouse environments could potentially serve to enhance SIT efficacy relative to field releases by reducing emigration and thus maintaining high densities of sterile insects for longer periods of time. In fact, a number of key greenhouse pests are already being studied as targets for the SIT as coordinated by the International Atomic Energy Association (IAEA) and the Food and Agriculture Organization (FAO). These include spotted wing drosophila (Drosophila suzukii), the tomato leafminer moth Phthorimaea absoluta (formerly called Tuta absoluta), as well as armyworms in the Spodoptera and Helicoverpa genera. Earlier SIT research for control of Trialeurodes and Bemisia whiteflies as well as the serpentine and tomato leafminers (Liriomyza trifolii and L. bryoniae, respectively) have also been explored. With each study, we are steps closer to applying the SIT in protecting greenhouse crops.

Developing the sterile insect technique for control of pepper weevil

Tray of pupae.

As a recent Master’s student, Jacob Basso and his research groups at the University of Guelph and Agriculture and Agri-Food Canada in Harrow Ont., have studied sterilizing the pepper weevil by irradiation to develop a pepper weevil SIT system, in collaboration with Bruce Power, Nordion Inc, and the Fruit and Vegetable Growers of Canada (FVGC). To this end, pepper weevils were first exposed to multiple doses of radiation as pupae, and the doses that ensured complete weevil sterility (i.e. a lack of offspring) were selected for further study. In this case, both male and female weevils were 100% sterile when exposed to a dose of 110 Grays of gamma radiation.

Irradiated weevils were next assessed for their lifespan and ability to fly. Sterile males have to live long enough to interact with wild females, and they also have to fly to reach them. In these trials, while sterile male flight and lifespan were negatively impacted by irradiation, they still managed to live long enough to mate, and a good proportion were still able to fly.

These irradiated weevils were also assessed for their sperm production and mating competitiveness relative to normal, unirradiated males. In controlled environment tests, sterile males produced much less sperm and mated at a lower rate compared to unirradiated weevils, suggesting that sterile males may be less competitive than their unirradiated counterparts in the crop. 

Optimizing SIT systems and future directions
Despite the limitations we observed in irradiated male pepper weevils, there are a number of ways such SIT systems can be optimized to improve overall sterile male health and competitiveness.  The most evident way is by releasing of a greater total number of sterile individuals. Another strategy to consider is dose fractionation, whereby irradiated insects receive their total radiation dose as the sum of multiple lower doses.

Alternatively, pepper weevils could be sterilized as fully formed adults instead of pupae.  This is because fully formed adults are overall more resistant to radiation than undeveloped juveniles, resulting in improved sterile insect health. Other environmental factors to consider before and during the irradiation process are to chill and expose insects to a low-oxygen environment to slow their metabolism and protect them from unwanted tissue damage. Finally, since irradiated insects tend to have short lifespans owing to gut tissue damage which leads to their starvation, diets of insects such as the pepper weevil could be supplemented with additional nutrients to improve their survival and mating success with wild insects. 

When a combination of these factors are applied to produce relatively healthy and competitive sterile insects, the SIT has a clear potential to manage challenging greenhouse pests such as the pepper weevil. 

While there is still a need to conduct large-scale SIT trials in greenhouses, the prospect of developing this strategy for the sustainable and effective management of greenhouse pests is both exciting and promising.