Part of our back deck is framed and screened. ants and spiders are coming up between the deck boards, even though we spray. Can I caulk the gaps between the boards or is there something better to keep the bugs out.
Caulking won't stop the ants. I doubt that pesticides can stop them. You may have some success in pushing them away from this area, and if you do, consider this a huge victory.
Consult some pest companies who deal with ants. Most pest control companies want long term accounts, and they will recommend just that. You may have to sign up.
If you want to do it yourself: the EPA approved chemicals you can buy at the stores are a joke for the ants. Pest control companies are allowed to purchase stronger chemicals. Those chemicals may only push the ants "from over here to over there"...
The main problems with ants are: 1. Their ability to adopt to whatever conditions are out there: drought, flood, hot and cold. 2. Their out of control birthrate. 3. Their social structure.
Now you know what you're dealing with...
I have to agree with dj1 that no amount of caulk will keep out ants and probably spiders too. If the ants are in the deck I'm surprised they aren't in the house too.
While a pro exterminator may be able to do it better and quicker, I've had good luck using a combination of sprays and granular products. I think the secret is keeping up on the treatments and keeping an eye out for where they are coming from or nesting.
Spiders don't particularly bother me since they take care of other bugs. If they get bothersome I spray, squash or suck them up in a vac.
I suggest that you remove their food and water source and they will die and the rest will leave.
Since you mention they are coming up from under your desk, that sounds like you have debris under the desk, and possibly one of the down spouts are leaking. The debris is most likely pine wood that was used when the deck was built, or you are storing old wood under the deck. Pine has sap in it, which is sweet. Repair the broken down spout and ensure that all the water is draining far away from the house. Clean everything out from under there, and allow the ground to dry. After the ground is dry, spread some boric acid on the ground.
After that, the ants and spiders will be dead and the others will leave.
" I suggest that you remove their food and water source and they will die and the rest will leave. "
The queen doesn't care if you kill thousands of ants, she lays eggs in the millions.
For the best pesticides you can get at retail, go to a rural farm and garden store that sells to farmers or ranchers- not your typical urban stores. Note that some of what they have is not supposed to be used in or around homes, but at least the stuff works and it actually costs less there too. My outdoor favorite since Dursban was taken off the market is intended to be used on feedlots with a 24-hour wait time before animals are allowed back in the area, but you're on your own if you go here!
The spiders are probably after other insects to eat. Ants will come in for food or shelter and sometimes I think they are just nosey. A Sodium Borate solution will repel the ants or kill any that are not repelled. You can order Timbor or Boracare over the internet. If you live where cotton is grown, you can find a Sodium Borate at you local farmers coop around three to 4 weeks before the cotton harvest begins.
You can make your own out of Boric Acid (roach powder) and laundry Borax (20 Mule Team, etc). Bring a gallon of water to boil. Mix in a pound of roach powder and a pound and a half of 20 Mule Team until it dissolves. Add up to one more gallon of water and use a garden sprayer to apply to the deck boards.
If you have a camping stove and an old steel bucket or large metal container that you will never use for food (again), then you can heat antifreeze instead of water, it increases the effectiveness of the Sodium Borate solution. if the antifreeze is used antifreeze or otherwise diluted with water, then don't add more than another half gallon of water in the end, you can add up to one more gallon of water if the antifreeze is straight antifreeze. Be careful when using antifreeze, especially if you have kids or pets, it is extremely toxic. If pets are allowed on the porch, I'd go with straight water.
Various types of baits around the perimeter of the structure and in whatever other nooks and crannies under the deck you can reach. I like to mix different bait stations and granules. That should be the first step to deal with the current infestation. Then once you see the activity go down a good idea would be preventative sprays either from an exterminator or with diy products a minimum of twice per year. Once as soon as it warms up and then again in the fall when most species of insects are looking for a warm space to hibernate for the winter. I bring in an exterminator to do my perimeter and first floor every March. I then use diy products around the outside of the house every few weeks to a month for the rest of the summer and fall. Since I have fallen into the habit of regular preventative treatments I haven't seen any significant activity of any kind in my home.