Critically endangered dragonflies, Libellula angelina inhabit Okegaya marsh in Iwata, on the western side of Shizuoka Prefecture in Japan. Their population had decreased sharply since 1999 because of the explosive increase in the population of red swamp crayfish (Procambarus clarkii, hereafter referred to as crayfish). Crayfish not only prey on dragonfly larvae but also cut and graze macrophytes that serve as habitats and refuges larvae. We have hypothesized that the introduction of aquatic plants less susceptible to crayfish disturbance could provide safe larval habitat and thereby support the persistence of L. angelina in environments where crayfish are present. According to the previous research conducted by Taniguchi (2024), Tobita (2024), and Hokiyama (2025), Sagittaria trifolia ‘Caerulea’ might be relatively resistant to crayfish disturbance. Therefore, we investigated the effects of crayfish on dragonfly larvae and tested whether S. trifolia var. less susceptible to crayfish could provide safe habitats for the larvae.

[Experiment 1] We examined species-specific differences in crayfish impacts on dragonfly larvae. Three species (Sympetrum baccha matutinum, Rhyothemis fuliginosa and Crocothemis servilia) ecologically similar to L. angelina were used in 2024 and 2025, with additional species, Orthetrum spp. representing different life cycles and habitat from L. angelina in 2025. Plots either contained S. trifolia var. or were left unvegetated, and crayfish divided into two size groups (4–7 cm and ≥7 cm) were released in the treatment plots. Larvae in control plots survived the longest and successfully emerged, whilst all crayfish treatments resulted in drastically reduced survival duration, often less than one day, irrespective of plant presence or dragonfly species. These findings indicate that S. trifolia var. provides limited refuge for larvae and that crayfish exert strong negative effects across multiple dragonfly taxa.

[Experiment 2] We evaluated larval survival duration under high-density vegetation using S. trifolia and S. trifolia var. Pantala flavescens were released in plots with crayfish divided into two groups the same as Experiment 1. Despite dense planting (25–30 petioles), larvae were preyed within one day in all crayfish treatments, regardless of crayfish size, whilst larvae in control plots survived until the end of the experiment.

Collectively, these results demonstrate that S. trifolia and S. trifolia var. do not independently provide safe habitats for dragonfly larvae in the presence of crayfish. Further research should explore combinations of macrophyte species, including submerged macrophytes, to determine how aquatic plant diversity may enhance larval survival and contribute to the long-term recovery of L. angelina populations.