My hives have screen bottom but currently there is a plastic card board like insert under the screen, should that be taken out for better ventilation or left in place.
Now you've opened a whole hornets nest, the screened bottom board vs non screened bottom board debate.
It was hot and humid yesterday, got up in the mid 90s here. On my left hive I noticed many bees wouldn't go in the hive even when it got dark. Is that normal?
It happens, however it's best especially in spring, if you can run the hive so they can all fit inside. Overcrowding or overheating will encourage swarming. Your picture looks like the hive has plenty of bees, me, I would leave that entrance as open as it is so they have plenty of room to come and go, plus plenty of fresh air to prevent overheating and thus bearding.
I also have the top bee feeder with 50/50 sugar water on the hives so there is no air coming in from the top.
Are you saying the hive also has a top entrance? If so you do not want that open when you are feeding sugar syrup, it makes it easy for other bees to come and rob your hive. My own view is not to have top entrances at all, however there are other opinions on that, all with their own reasons.
From what I can tell they seem to be doing pretty good so far, they stay busy all day. Come and go!
There is nothing more pleasing than looking at a nice healthy beehive, with happy bees, working hard. But as we have been talking varroa mites i'll tell you something. In a typical hive, mite numbers build up during the season, but because there are big bee numbers, it camoflages the effects of the mites. Only in fall, when the bees naturally reduce their population in preperation for winter, does the mite / bee ratio get to a point of danger to the hive, as the bees and brood numbers decrease, but mite population keeps growing.
Point is, this is hard to pick from outside the hive, the bees that are healthy are still working hard and things can look deceptively good. But in fall, you have to use a reliable method to get rid of those mites, so the hive can go into winter healthy, and you will still have bees come spring.
Do an effective mite treatment in fall, and you will not be one of those posts that happen on Beesource that time every year that go <where did all my bees go? A few weeks ago the hive was looking really good, but now it is dead>.