I have only seen 3 Monarchs and 1 Swallowtail in my garden so far this year. I have all the usual plants, and have tried to keep them watered. I do see lots of wasps around - could this be the reason?
Where are my butterflies?
Wasps will prey on the caterpillars - I don't know whether they bother the adults or not. I have fewer butterflies in dry years - even if my yard isn't dry. I have had very few butterflies in my yard this year - it had been dry and windy. But there were a lot in a drainage easement near my house early summer - moister, weedier, and sunny but down out of the wind. Lately there hasn't even been many even there. Monarchs are fairly long-lived, but Swallowtails are relatively short-lived.
To sum up: Dunno. Some years are just better than others.
It could very well be the reason. I try to keep the wasp populations down by capturing them in small clear glass jars. I don't have access to wasp traps where I live so one of these days, I'll learn how to make them. Some wasps like to eat the eggs and small caterpillars and sometimes the big caterpillars and there are parasitic wasps that wait until the caterpillars have matured to lay eggs in them.
I have common rue plants (ruta graveolens) that are frequently visited by giant swallowtails and black swallowtails who apparently prefer the rue over the lime tree I have.
A month or so ago, I happily noticed that the rue had several creamy colored eggs left by a black swallowtail. On the same day, I noticed a queen monarch laying eggs on the asclepias plants. The next day, I could not find one egg on either the rues or asclepiases. I was so disheartened that they could have disappeared so quickly and completely. My past experience has been that at least 2 or 4 eggs will be left to be able to mature into caterpillars, but not this time. The wasps cleaned up. I have identified 3 predatory/parasitic wasps that appear regularly in the garden so i keep an eye out for them and capture when i can. This happened right before the rainy season started here. At this time all the bugs seem to go crazy finding food. Other times of the year the wasps are a problem, but not so much of one. If I am diligent in my wasp capturing, I can achieve good broods of maybe 30 - 80 caterpillars that make it to healthy butterflyhood. When I am frequently called away from the garden for any number of reason, the wasps move in and have themselves a feast.
I understand wasp traps can be easily found in the States and if i were you, I would get some. Good luck! It is not an insurmountable problem.