Predatory Aquatic Insects: Threats to Fry in Ponds
Dragonfly nymphs, water tigers and giant water bugs are predators, not parasites. In ponds and fry tanks they can decimate small fish. Here is how to manage them.
Not every creature that harms fish is a parasite or a disease. In ponds, tubs and unfiltered fry tanks, a range of aquatic insects and their larvae are active predators that hunt and eat eggs, fry and small fish. They are most dangerous exactly where small fish are least protected, so understanding them matters for anyone breeding or raising young fish outdoors.
The main culprits
Dragonfly and damselfly nymphs
Dragonfly nymphs are voracious predators, eating most living things smaller than themselves. They hunt with a remarkable weapon: the lower jaw has a huge, extensible labium, armed with hooks and spines, folded under the body at rest and struck out at great speed by hydraulic pressure. Their staple diet is insect larvae, but they also feed on tadpoles and small fish. Since most of a dragonfly's life is spent as an aquatic nymph, they can lurk in a pond for a long time.
Diving beetles (water tigers)
The larvae of predaceous diving beetles are known as water tigers for their voracious appetite. They have sharp mandibles and, immediately on biting, deliver digestive enzymes into the prey to suck out the liquefied remains; the larvae especially take on animals of the same or bigger size, such as fish and tadpoles. The adults are predators too, and because they are able fliers that seek out light reflecting off water, they readily colonize new ponds.
Giant water bugs and other true bugs
Giant water bugs are among the largest aquatic insects. They grab prey with raptorial front legs and inject a powerful proteolytic saliva, and are aggressive predators that stalk, capture and feed on fish, amphibians and aquatic invertebrates. Some can disperse by flying. Backswimmers and water scorpions are smaller relatives that also take fry and tiny fish.
How they get in, and the risk
These insects arrive in two main ways: as eggs or larvae hidden on live plants and in pond water, and as winged adults that fly in and lay eggs in a tempting body of water. The threat is concentrated on eggs, fry and nano fish, which are within the size range these predators can subdue. Adult fish in a filtered tank are rarely at risk.
Prevention and control
- Inspect and quarantine new plants and rinse them before adding, to avoid importing eggs and larvae.
- Cover fry tanks and rearing tubs with fine mesh to stop flying adults laying eggs.
- Net out visible nymphs, beetle larvae and bugs by hand during routine checks.
- Avoid raising valuable fry in open ponds where these insects breed freely.
- Screen pond inflows and be cautious with water transferred from wild sources.