There's an old joke that you can tell how long missionaries have been on active service by the way they react to finding ants in the sugar bowl. In the first year, they will throw out the sugar. In the second year, they will pick out all the ants. In the third year, they will pick out most of the ants. In the fourth year, they'll say "Hey, look - extra protein!"
Ants are busy little social creatures that are proverbial for diligence and hard work. They also make picnics a nuisance as they scurry in to carry off any dropped bounty from your food. Ants are all very well outside - unless they try farming aphids on your roses and other plants - as they do a good job of aerating the soil with their nests and clearing up refuse as part of nature's great recycling system. In the house, they are another thing altogether as they get into food. They can sneak in through tiny gaps and can go nearly everywhere.
If you discover ants in your sugar or other dry goods, track them back to their nest. If they have been travelling from your store to theirs for quite a while, this won't be hard, as a regular ant highway will trail back and forth. Once you find the nest, you have two options: deterring the ants or killing the ants.
Killing the ants can be done with a proprietary ant poison, but this is not recommended if you have small children or animals, especially if the entrance to the ants' nest is somewhere that they can reach (which it probably is). Pouring boiling water down the entrance to the nest is an organic option, but you can never be sure that you have killed them all. Alternatively, try a pyrethrum spray, or else a home-made spray of garlic and pepper infusion, although this writer cannot vouch for this.
Ants can be deterred by certain strong-smelling herbs, especially pennyroyal, tansy or southernwood. Stick a spring of this herb, dried or fresh along the trail into your home, preferably at the place they're getting into your house. Don't put these herbs in your pantry, as their strong smell can contaminate your food.
Pennyroyal is also well known for deterring fleas, so much so that the scientific name for this variety of mint translates as "mint that drives out fleas". The medieval method of deterring fleas was to strew this herb on the floor among the rushes. You can't do this with modern carpets, but you can tuck a sprig into linen and towels if you suspect fleas are getting in there. Put sprigs of it into pets' bedding, and you can even rub a handful of crushed pennyroyal over the coat of an animal as a flea deterrent. This is a safe and gentle method of dealing with fleas, but cats don't like the smell much (at least, mine don't). An infusion of wormwood is another herbal option.
But the best way to ensure that your home is flea free is to vacuum regularly. Vacuuming kills fleas that have invaded your carpet. Fleas are most abundant at the end of summer and the beginning of autumn.
Weevils can be kept away from dry goods by bay leaves. Keep dried bay leaves in beans or any other grain (or pulse) used for savoury dishes - it won't matter if the flavour of the bay leaves gets into them.
Clothes moths can be kept away not only with traditional camphor mothballs but also with other strong-scented herbs and spices, some of which smell much nicer than camphor and are more pleasant to have on your clothes. Try cedar or lavender essential oils - the latter smells particularly nice on clothing. I have also read that spraying perfume in a wardrobe works to deter moths, but I have never been willing to be so extravagant with my favourite colognes.