Are ladybugs really good for your plants?

I would like to start this article by saying this is based on my personal experience here in Japan dealing with ladybugs in and out of greenhouses particularly concerning potatoes and tomatoes.

My ladybug battles start around the middle of June.  We plant our potatoes here in Japan around the middle of May when the snow has melted away and the soil is starting to warm up a little.  By the middle of June the potatoes are knee high and have already flowered attracting all sorts of insects.  Ladybugs are late on the scene but come in swarms.  I presume they come to munch on the aphids and small insects that are on the potatoes but once they have decimated all other insects they chow down on the potato leaves.  I don’t really concern myself with that because once the beginning of July hits it becomes hot and humid and the potatoes are pretty much ready to harvest anyway.  Now here is where the problem begins.

Tomatoes are planted beginning of June and once potatoes are harvested in early July the ladybugs source of food is gone leaving a hungry swarm looking for its next meal.  This year my tomatoes where planted right next to the potatoes but my rotation puts them different places every year with the same result.  The ladybugs just hop over to the blossoming tomatoes and chow down on any other insects in the vicinity.  Good right!  However the sheer numbers of ladybugs far outweighs any other insects and soon their hunger takes them again to the foliage.  These ladybugs take a perfectly healthy plant and can munch half of it down within days!  Holes in the leaves appear then it turns brown and wilts.  Thankfully I put quite a bit of nutrients into the soil and my tomatoes explode once it gets hot, feeding the hungry swarm and still manage to produce normally but I don’t know what’s going on!

It could be the type of ladybug, the wet, humid weather, disease, soil, my inexperience, who knows how many other factors but I’m pretty sure these ladybugs are doing more harm than good to my garden.  I have always thought that the ladybugs presence was good and beneficial to the garden but here and now I am changing my mind.

Any comments or thoughts would be greatly appreciated!