Caddisflies are aquatic insects that resemble small, hairy moths in their adult stage. While harmless, they become a significant nuisance when they emerge from water sources in large numbers and swarm around structures. The adults are strongly attracted to light, which draws them away from their freshwater habitats and toward homes and businesses. These swarms often leave behind powdery scales and may cause filtration issues in water systems if larval populations are high.
Identifying the Source of the Infestation
Effective management begins with determining the origin of the caddisfly population. Caddisfly larvae are aquatic, spending most of their lives underwater in lakes, streams, ponds, and even slow-moving drainage areas. Homeowners should inspect nearby bodies of water, including decorative ponds, fountains, or areas with poor drainage and standing water. The larvae construct protective, portable cases made of materials like sand, tiny stones, or plant debris held together by silk, making them difficult to spot.
If case-building larvae are found in the water source, the problem is rooted in the aquatic environment. If no larvae are found, but adult swarms are present, the breeding source is likely a larger, off-property body of water, such as a nearby river or lake. The control strategy must shift depending on whether the primary problem is the population’s origin (larvae) or the visible symptom (adults attracted to structures). Focusing solely on removing the flying adults will not solve a problem originating on the property, where a continuous supply of new adults will emerge from the water.
Eradicating Larvae in Water Sources
Targeting the larval stage in residential water features is the most direct way to reduce the overall population. For small water features like ornamental ponds that do not contain fish, biological control agents can be effective. Bacillus thuringiensis israelensis (Bti) is a naturally occurring bacterium that specifically targets caddisfly and mosquito larvae when ingested, while remaining safe for fish, mammals, and birds. Bti is sold in liquid, granular, or briquette formulations and must be applied directly to the water source where the larvae are actively feeding.
Introducing natural predators is another method for controlling larval populations in ponds and water gardens. Fish species, such as koi, goldfish, and mosquito fish, relish caddisfly larvae and can significantly reduce their numbers. Habitat modification also plays a role by removing excess aquatic vegetation, which the larvae use for anchoring their cases and as a food source. Furthermore, ensuring that water is clean and consistently moving discourages the development of stagnant conditions that favor larval growth.
Physical barriers can be implemented by covering water intakes, drains, and overflow pipes with a fine mesh screening to prevent larvae from entering filtration systems or leaving the water source. Treating large bodies of water, such as lakes or major canals, is not typically a residential endeavor and requires professional intervention. For the homeowner, consistent maintenance and targeted biological controls provide a sustainable approach for managing the larval source.
Managing Adult Swarms Around Structures
Adult caddisflies are strongly phototactic, meaning they are intensely attracted to light. Managing this attraction is the immediate solution to reducing swarms near a home. The most effective step is modifying outdoor lighting by replacing high-UV sources, such as white incandescent or fluorescent bulbs. Switching to yellow or amber LED lights, which emit less attractive wavelengths, can drastically reduce swarming adults. Sodium vapor lamps are also a suitable alternative for commercial exterior fixtures.
Reducing the duration of outdoor lighting is equally impactful; turning off lights when not needed or installing motion-activated sensors can help. If adult swarms have already accumulated, physical removal is the most practical immediate solution. A heavy-duty shop vacuum can be used to quickly collect large groups of resting caddisflies from walls and surfaces.
While chemical treatments are generally ineffective for flying insects, they may be used sparingly as a barrier treatment on surfaces where they heavily congregate. These treatments are typically low-toxicity options applied to the immediate perimeter of the structure, such as near doorways or window frames. Strategic placement of insect sticky traps near entry points can also help capture adults attempting to enter the home. Caddisflies are weak fliers, and placing a high-speed fan near a doorway or patio can create an air current barrier that prevents them from landing in the area.
Long-Term Prevention Strategies
Preventing future infestations relies on consistent structural maintenance and landscape adjustments that make the property less appealing to both the larvae and the emerging adults. Sealing potential entry points is a foundational step, which involves inspecting window screens and door sweeps for damage. Replacing standard window screening with a fine-mesh material will prevent the small adults from slipping through gaps.
Landscape management should focus on eliminating sources of standing water and excessive moisture near the structure’s foundation. Ensuring that gutters are clean and that the property has proper drainage prevents the accidental creation of small, temporary breeding habitats. Trimming back dense vegetation, especially shrubs and trees that provide shade, away from the immediate structure will reduce resting spots for the adult insects during the day.
Routine maintenance of any existing water features, such as fountains or birdbaths, is necessary to prevent larval buildup. This includes regularly cleaning out organic debris like fallen leaves and algae, which serve as food and building material for the larvae. If severe, recurring swarms persist, consultation with a professional pest control service specializing in aquatic insects may be necessary. These professionals can assess the large-scale source, such as a nearby community water body, and advise on more extensive, localized prevention measures.