NARO confirms pest control efficacy through line selection − Avoiding reliance on chemical pesticides only

A research group led by Principal Scientist Tomokazu Seko of the Division of Crop Pest Control Research at the National Agriculture and Food Research Organization (NARO) announced on January 17 that they have succeeded in selecting and breeding a line of Orius strigicollis (a species of stink bugs used as a biocontrol agent of eggplant and other important vegetable pests, such as thrips) that has a tenacious nature. Additionally, it was announced that this new insect line is more effective than ordinary lines in controlling pests. The stink bug lines were selected for their characteristic of searching longer for food, with their walking activity used as the behavioral indicator. This behavior was demonstrated by comparing the selected line with wild-type lines cultivated at other institutions. The researchers plan to continue improving the functionality of this biological pesticide and have it registered by 2030. The results are expected to lead to the realization of environmentally conscious agriculture that does not rely solely on chemical pesticides. The study results were published in the international academic publication Journal of Pest Science.

On the one hand, if the biocontrol agents are released after the pest population has increased much, they will not be able to keep up with the rate of pest increase. On the other hand, if the pest population is low and there are not enough individuals to feed on, these biocontrol agents will starve to death or escape. For this reason, despite the fact that more than 20 biocontrol agent pesticides are commercially available, their use is limited to a few species.

In this study, the research group focused on the phenomenon of switching food-searching behavior, which has been observed in many insects. The researchers speculated that this phenomenon might be related to failure of establishment. Normally, pests are scattered throughout a patch. To go to these feeding grounds, predators usually perform a nonlinear and cautious "intensive type" search, changing directions slowly and repeatedly in situations where they often find food. However, in situations where they do not find food, they switch to a "wide-area type" of search behavior with fast linear movements to move to another feeding ground.

The research group investigated the possibility that the biocontrol agents would be more likely to become established if they were modified to have a tenacious nature; that is, they would not immediately leave their feeding grounds but would instead search persistently in a "focused" manner.

In predators, individual genetic differences in the time it takes to switch to search behavior have been observed. In this study, O. strigicollis lines with a tenacious nature were selected and bred for use as a natural pesticide against thrips, a difficult-to-control pest. Individuals that have a tenacious nature tend to walk less in a fixed amount of time because they are engaged in "intensive" searching for longer durations.

Therefore, a sensor device for measuring the walking activity of Drosophila individuals in a container was used in this study to record the one-hour walking activity of each of 60 male and 60 female stink bugs. Subsequently, 30% of the individuals with low walking activity were selected, this population was crossbred, and the selection repeated for more than 50 generations. As a result, the amount of walking activity decreased with each generation, with small fluctuations. The breeding of lines with low walking activity (selected lines) was successfully achieved. The stink bugs of the selected lines exhibited approximately three times less walking activity than those of the unselected (wild-type) line.

One adult stink bug (from either the selected or wild-type lines) and one adult thrip were placed in a glass tube that was set on an A4 sheet of paper. The time taken for the stink bug to reach the edge of the paper after feeding on the thrip (giving-up time) was measured using video analysis software. As a result, the giving-up time of the bugs in the selected lines was approximately three times longer than that of the wild-type bugs. The average giving-up time was 158 seconds for the selected lines and 52 seconds for the wild-type lines. Differences in search behavior were observed between the selected and wild-type lines, with the bugs in the selected lines walking in a more nonlinear fashion. After approximately 45 generations, the walking activity of the bugs in the selected lines was three times lower. This selection process took approximately three years.

Beginning in 2018, demonstration trials were conducted in eggplant plots (42 eggplant plants) in a greenhouse (1 are). The bugs from the selected lines stayed on the eggplants longer than the wild-type bugs did and suppressed the increase of thrips. The study showed that the modification of tenacious behavior was effective in improving the establishment of the biocontrol agent.

A test was conducted by releasing two thrips on each eggplant plant in a plot and 42 stink bugs (from either the selected or wild-type lines) on each of two plants near the center. After three days, the numbers of both insects per plant and on six leaves were counted, and the lines were compared. The test was run three times, all with similar results.

Seko said, "We are currently working on the Moonshot Project (ambitious research and development projects sponsored by the Cabinet Office) to clarify the genes related to the establishment and to use them as markers for selection. We believe these efforts will improve the establishment of other predators, including Orius strigicollis. By improving other functions such as eating more pests and being more active at low temperatures, we can create predators that can be used in many purposes, including protection of crops and growing environments where it has been difficult to use predators in the past."

Journal Information
Publication: Journal of Pest Science
Title: Extension of patch residence time of a biocontrol agent by selective breeding contributes to its early establishment and suppression of a pest population
DOI: 10.1007/s10340-023-01696-4