Citrus Bud Mites
Citrus bud mites are microscopic insects that feed on both flower and leaf buds. They mostly attack citrus species, with the ever popular eureka lemons and Washington oranges being particularly susceptible but they will attack a few other species as well.
Because they're so tiny, you'll never see the bugs themselves but you'll know if they attack your plants because affected leaves and fruit are deformed right from the get go. Leaves may also turn black and die and fruit may sometimes fall before it matures. Usually, even on susceptible plant species, only a very small number of fruits and/or leaves will be affected and the fruit is still edible. Attacks usually occur in summer.
To prevent pest attack, avoid planting citrus trees too close together - separate them by other trees, bushes or garden beds. Large aromatic bushes such as rosemary and trees with aromatic flowers or foliage are particularly good companions that will mask the smell of the trees so it's harder for the pests to find them.
Treatment is usually unnecessary but if you do need to treat it, white oil, neem oil or any other horticultural oil will be effective.