Selected Aquatic Insects Can Reduce Stands in Water-Seeded Rice

Previous research at the Texas AgriLife Research and Extension Center at Beaumont on water-seeded rice revealed the possibility that one or more unknown aquatic invertebrate species have been responsible for dramatically reducing rice stands. Preliminary experiments show that there is zero or small reductions in the rice stand of plots when treated with insecticide applied to rice seed, applied to the soil before flooding, or applied to the water surface before emergence of rice seedlings through the water surface. In contrast, plots that did not receive insecticides, or received insecticides relatively late, exhibited significant stand losses. These results suggest that a biological organism impacted water-seeded rice during the early seedling stages and is responsible for stand reductions. Field and greenhouse experiments were conducted at the Texas AgriLife Research and Extension Center to identify the causes of these rice stand losses. In the field, metal barriers were installed around plots, which were flooded and water-seeded (continuous or pinpoint flood). These barriers were pushed down in the mud and served to isolate plots. Some plots were treated with insecticide at various times during seedling development, while other plots were left untreated. Plots were sampled over time for benthic and aquatic invertebrates, and for stand losses. Dislodged seedlings were also counted and removed from plots at various times after seeding (Fig. 1). Organisms sampled were preserved and identified. Plots were harvested and yields compared. During these experiments, a common aquatic beetle, Tropisternus lateralis, caused dislodging of plants and their floating to the surface of the water. This beetle does not actually feed on rice, but feeds on detritus, which may be associated with rice seedlings. We also discovered that another aquatic beetle, Berosus infuscatus, was abundant and was a possible candidate responsible for the stand losses. Both of these beetles were negatively impacted by insecticide treatments. Results of field experiments showed that dislodging of plants occurred when T. lateralis was present and active during a narrow time period of about 10 to 14 days after rice germination, but before seedling emergence through the water surface. Also, insecticides applied after rice emergence through water had little effect on dislodging. In addition, more dislodging occurred in the continuous irrigation system than in the pinpoint flood irrigation. Most dislodging occurred in untreated plots or in plots treated with insecticide relatively late. In general, stand counts and grain yields were lower in untreated plots, but rice displayed an ability to compensate for