What characteristic is essential for an effective Gravid trap?

An effective Gravid trap is specifically designed to attract female mosquitoes that are seeking to lay eggs. This is integral to the trap's function, as female mosquitoes are the ones that require a blood meal to produce eggs and are also attracted to standing water or specific odors that indicate a suitable place for oviposition.

The trap uses a combination of visual cues, scents, and water to create an environment that appeals to gravid females, making it more efficient in monitoring and collecting these specific mosquitoes for population studies or control measures. By understanding the behavior and reproductive habits of these female mosquitoes, the Gravid trap can effectively contribute to mosquito management strategies.

The other characteristics presented, such as capturing larvae or attracting males, do not align with the primary purpose of the Gravid trap, which focuses solely on promoting the capture of egg-laying females. Similarly, while ease of transport can be a practical consideration for using the trap, it is not a defining characteristic that contributes to its effectiveness.