Canned aerosols are particularly ineffective in covering which type of surfaces?

Canned aerosols are particularly ineffective on vertical surfaces due to the nature of how aerosols disperse and settle. When aerosol products are sprayed, the propellant helps to create a fine mist that is intended to coat surfaces. On vertical surfaces, gravity plays a significant role; the spray tends to run or drip down rather than adhere effectively. This limits the overall coverage and effectiveness of the pesticide on these types of surfaces.

In contrast, horizontal surfaces present less of a challenge for aerosol application, allowing the product to settle evenly due to gravity. Irregular and textured surfaces might also pose some difficulties in terms of even coverage, but they can often retain a greater amount of aerosol product because their contours provide more areas for the aerosol to cling to. As a result, vertical surfaces are the least effective for aerosol coverage, making this the correct answer.