Aquatic pests in aerial / dry broadcast
Bloodworms
Rice fields may be colonised by blood worms in 2 days, so any delay in treatment can put the crop at risk particularly if it takes several days to flood the field as may be the case when adopting the dry broadcast sowing method.
The advantage of pre germinated seed: Once the secondary root system is developed the plant will be generally unaffected by bloodworms. Therefore pre-germinated seed is vulnerable for a shorter duration. In severe cases, the bloodworms will eat out the seeds before they fully germinate.
Bloodworms can also have a secondary affect by burrowing in the top layers of sediment, which destabilisers the roots of plants that have attached to the soil. Not only will these plants be blown around in the wind, water turbidity will increase. This lowers light penetration and water temperatures resulting in slower plant growth.
Identifying bloodworms
Reports are often received of bloodworm insecticide spray failures but, on closer inspection, these are often confused with aquatic earthworms.
Bloodworms have a distinct head capsule and rarely exceed 25mm (2.5cm) in length. They are a bright red colour that makes them easily recognisable in samples of soil from a rice bay. The aquatic worms are much longer (up to 75mm or 7.5cm in length) and lack the distinct head capsule.
For further information on Bloodworms see page 30-31 of the 2020/21 NSW DPI Rice Crop Protection Guide here
Aquatic snails
The snails feed on the roots of the rice seedlings causing increased seed mortality and impact on yield potential. The effect of root pruning by the snails also delays tillering and, in severe cases, post PI leaning of plants that leads to lodging.
Approximately 50% of aquatic snails entering dormancy at the end of one rice crop will survive and emerge from the soil in the following rice crop.
Snails have evolved a dormancy mechanism to survive dry periods. This is why rice on rice crops are more exposed to snail infestations. Dormant snails become active within a few hours of re-ponding and, as there are limited food sources available at this time, the germinating seedlings are very vulnerable. Infestations of snails and/or snail eggs can also occur from irrigation channels and water used from recycle systems. Snail eggs typically hatch in 7-10 days.
Control should take place when there are more than 5 large or 10 small aquatic snails/m2. Copper sulphate is still the only registered product for aquatic snail control in rice and should be applied at 6-12 kg/ha depending on brand, water depth and organic matter. As copper sulphate doesn’t kill the eggs, a second treatment may be required 10 to 14 days after the eggs hatch.
For further information refer to page 34-35 of the 2020/21 NSW DPI Rice Crop protection guide here
Increasing the sowing rates of seed doesn’t negate the need for control as pests are seldom distributed uniformly across the bays. Snails are more prevalent in the low side of the bays and on the borrow pit edges where there is deeper water and dormancy has occurred.
