Bats in buildings

Finding bats in your home can be stressful, but bats are not trying to damage your house or move into your living space. In South Dakota, bats found in buildings are usually using small existing gaps as seasonal roosting spaces. They do not chew holes, enlarge openings, build nests, or chew wiring. However, a large roost can still create problems over time. Accumulated droppings and urine may cause odor, staining, and damage to building materials, especially in warm weather.

The best long-term solution is humane exclusion, which allows bats to leave the structure but prevents them from returning. Exclusion must be done carefully and at the right time of year. Done incorrectly, it can trap bats inside walls or attics, separate flightless pups from adult females, or cause bats to enter the living space while trying to escape.
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Bats often return to the same roost year after year. If bats are using your home this year, they are likely to return in future years unless the entry points are properly sealed. In South Dakota, groups of bats in buildings are often maternity or nursery colonies, where adult females gather in spring and summer to give birth and raise pups.
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Typical roost size in South Dakota may be around 20–30 bats, although colonies can range from fewer than 10 bats to more than 1,000. Because bats usually have only one pup per year, maternity colonies are important to local bat populations and should not be destroyed or sealed in.

Bats are usually seasonal visitors

Many bats using buildings in South Dakota are seasonal visitors. They arrive in spring and leave in late summer or fall to migrate or move to winter hibernation sites. Buildings usually do not provide the stable winter conditions most bats need. The ideal time to bat-proof a building is either:

• After bats have left in the fall, or
• Early in spring before they return

In general, bat exclusions in South Dakota should not be conducted between mid-May and September 1. During this period, young bats may be present and unable to fly. Sealing adults out during this time can trap pups inside and may cause adult bats or juveniles to enter the living space while searching for a way out.

Important exception: Big brown bats (Eptesicus fuscus) may sometimes overwinter in buildings in South Dakota. Buildings with brick, block, stone, chimneys, or foundations may provide more stable temperatures that allow bats to remain through winter.
If you suspect bats may be overwintering in the structure, use a more conservative approach. Sealing access points while bats are still inside can trap them and may cause them to move through walls, ceilings, or other openings into occupied areas of the home.

For more information on bat behaviors, bats in buildings, bat proofing, and timelines to follow visit PennState Extension's booklet: "A Homeowner's Guide to Northeastern Bats and Bat Problems"

How to exclude bats from a building

Large roosts, complicated buildings, or situations where bats may be overwintering should be handled by an experienced bat exclusion professional.

Step 1: Find where bats are entering

Look for signs such as:

• Bat droppings below an opening
• Dark staining around gaps or cracks
• Bats emerging at dusk

If you are unsure where bats are entering, watch the outside of the building at dusk on a warm, calm evening. Cool temperatures, rain, or wind may delay or reduce bat activity, so choose a good weather night if possible.
Common entry points include:

• Gaps around chimneys
• Loose flashing
• Openings along fascia boards or soffits
• Unscreend or damaged attic vents
• Gaps under siding or trim
• Small cracks where building materials meet

Bats can fit through very small spaces. As a general rule, seal openings larger than about one-half inch, except for the active exit point where the one-way device will be installed.

Step 2: Seal other openings first

Before installing a one-way exit, inspect the entire structure during daylight. Use binoculars or a ladder if needed, especially for multi-story buildings. If possible, check the attic with lights off during the day and look for daylight coming through gaps.
Seal all potential openings except the bats’ active entry point. This step is essential. If other gaps remain open, bats may simply move to another part of the structure.
Smaller cracks can often be sealed with quality caulk. Larger gaps may require more durable materials, flashing, hardware cloth, or other permanent repairs appropriate to the structure. Because bats do not chew or enlarge holes, well-sealed repairs are usually effective when all openings are addressed.

Step 3: Install a one-way exit

Once all other openings are sealed, install a temporary one-way exit over the active bat entrance. A one-way device allows bats to leave at night but prevents them from getting back inside.
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One-way devices may include bat cones, tubes, netting, or chute-style exits. The device must fully cover the opening and be sealed tightly around the edges so bats can only leave through the exit end.

Leave the one-way exit in place for 5–7 nights of good weather so all bats have a chance to leave. After that, remove the device and immediately seal the final opening.

Do not use poisons.

Do not poison bats. Poisoning bats is illegal, irresponsible, and ineffective. It rarely kills all bats, does not solve the access problem, and may cause sick or dying bats to scatter into areas where people, children, or pets may encounter them. Because bats are mammals, chemicals harmful to bats may also be harmful to people and pets.
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The safe and effective solution is exclusion: let bats leave, keep them from returning, and seal the structure properly.
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