A swarm of bees in May is worth a load of hay, But a swarm of bees in July is not worth a fly.

A swarm arriving in May is portrayed as valuable because much of the productive season still lies ahead. The load of hay gives that value a concrete measure.

A swarm arriving in July is portrayed as having little value because less time remains for it to become productive. The contrast makes timing, rather than the bees themselves, the deciding factor.

Use the saying to emphasize that an opportunity can be valuable when it arrives early but of little use when it comes too late. The comparison works best when there is a limited season or window in which something can become productive.