Bug of the Month: The Often Ignored Blackfly Larvae

Blackfly larvae, often overshadowed by their adult counterparts and ignored by anglers, are discreet yet ecologically significant inhabitants of freshwater ecosystems. Distributed globally, these larvae, members of the Simuliidae family, act as essential bioindicators, providing valuable insights into the health of aquatic environments. If blackfly larvae are present in a river system, it’s a great sign that it Despite their unassuming appearance, blackfly larvae play a pivotal role in nutrient cycling, contribute to water quality maintenance, and are particularly noteworthy for their presence in the diet of trout.

Anatomy and Adaptations
Although at first glance, these bugs look similar to the midges we are familiar with, the adaptability and intricate anatomy of blackfly larvae are noteworthy. Possessing slender bodies with bulbous abdomens and specialized hook-like appendages, these larvae firmly anchor themselves to rocks or vegetation. Blackfly larvae also showcase a silk-producing gland that anchors them even more securely to rocks, allowing them to withstand the challenges of turbulent environments.

These bugs often get missed by anglers, even when seining, as they cling onto rocks and other surfaces much tighter than other insects, so aren’t as prevalent in the seine, despite their large numbers in the river. They can also resemble other insect such as baetis and midge larvae at first glance.

The bulbous or swollen appearance of blackfly larvae is often related to their need for buoyancy and stability in fast-flowing water and could be advantageous for their feeding strategy, as it helps create a stable platform for them to cling to surfaces while efficiently capturing suspended particles from the water column. Additionally, their ability to filter organic particles (what they feed on) from the water contributes significantly to nutrient cycling, affirming their crucial role in maintaining healthy rivers and streams we rely on

Ecological Significance
Beyond their structural adaptations, blackfly larvae serve as pivotal components in aquatic ecosystems. As proficient filter feeders, they actively contribute to the regulation of water quality by removing organic debris and algae. Furthermore, they function as a vital food source, forming an integral part of the food chain for various aquatic organisms, especially trout. Though mayfly nymphs (baetis) and midges might be much more prevalent food sources in a water column, studies have shown that trout feed more heavily on blackfly larvae, despite their lower percentage of presence. This means trout key in on blackfly larvae more than most food sources available to them, so throwing an imitation onto your rig is almost always a good bet.

Vector of Diseases
One not-so-nice feature of these bugs is that, in certain regions, blackflies take on a more ominous role as disease vectors. Some species are known to transmit diseases such as onchocerciasis (river blindness) caused by the parasitic worm, Onchocerca volvulus. The larvae themselves become infected with the larvae of the parasitic worms when they feed on infected humans during their blood-feeding phase. When an infected blackfly bites a human, it deposits the parasite under the skin. The larvae then develop into adult worms over time. Adult worms residing under the skin can produce millions of microfilariae, which are tiny larvae. These microfilariae migrate throughout the body, leading to various symptoms and complications, including skin lesions, itching, and ultimately, blindness in severe cases. Luckily these disease issues are primarily in Africa, Mexico, and some parts of Central and South America.

Life Cycle and Development
Beginning as eggs laid in clusters on submerged surfaces, these larvae undergo a complete metamorphosis, transitioning through various developmental stages before emerging as adults. Each stage of their development is intricately linked to nutrient cycling and energy transfer, emphasizing their role in shaping the ecological landscape of aquatic ecosystems.

Conservation Considerations and Fly Fishing Impact
While blackfly larvae contribute substantially to ecosystem health, human activities, such as pollution and habitat destruction, pose threats to their populations. Conservation efforts are imperative to preserve these essential contributors to aquatic ecosystems. Importantly, for fly fishing enthusiasts, the allure of blackfly larvae lies in their role as a favored food source for fish. Understanding and safeguarding the habitats of blackfly larvae become crucial not only for ecosystem health but also for enhancing the appeal of fly fishing experiences.

How to Fish Blackfly Larvae Patterns
This what we really want to know, right?! Blackfly larvae are often found in fast flowing water year-round, so focus on areas with current, such as riffles, runs, and seams. Look for submerged rocks and vegetation where larvae might be present.

Covey Guide Brian Hilbert says, “I typically use blackfly larvae patterns as an ‘in-between hatches’ fly, meaning before a different insect’s hatch starts or after it ends. Though they’re available in the water column year-round, I focus on mainly winter and spring. Flow bumps typical for those times of year dislodge them from rocks and sticks which make them a perfect opportunistic meal for trout.”

When nymphing, cast your blackfly larvae fly pattern upstream and let it drift naturally downstream (dead drift), just as you would fish midge patterns. Allow the fly to sink to imitate the larvae in their habitat, and use enough split shot, or a weighted lead fly, to get your flies down into the feeding zone (lower part of the water column). Be ready to detect subtle strikes and reap the rewards of keying in on this unique food source!

Check out how to tie this simple and effective blackfly larvae pattern with Covey Guide Brian Hlibert: