Wikipedia, the source of all information, has an entry on this butterfly, also known as the small white (Pieris rapae).
In terms of what concerns us, the butterfly lays her eggs on the underside of Brassicaceae (aka mustards, crucifers, cabbages.) This would mean that anyone with wintered over collards or arugula in the garden should be checking for eggs regularly. Also, anyone who has recently planted seedlings of kale, cabbage, broccoli, collards, Brussels sprouts, etc should definitely be on the watch. According to the Wikipedia link given above (quoting an assortment of scientific articles), the butterflies prefer depositing eggs on isolated plants or on perimeter plants in clusters of plants. They apparently are also aware of the voracious appetites of the caterpillars and lay few rather than many eggs (see below) per leaf although I also saw some imagery on the internet with large clusters of eggs.
|Image by Alpsdake, downloaded from Wikipedia|
If you see the eggs/larvae, the first line of defense is squishing. It appears that soapy water with something spicy - garlic or a hot pepper - will also do serious damage to the larval membrane and/or render the leaves unpalatable to the newly hatched caterpillars.
From my admittedly limited reading, it appears that pesticides aren't particularly effective but there are two strategies that can be implemented to protect your tender plant leaves from being identified as potential nurseries.
First. These butterflies are apparently quite territorial so you can either buy or make small white butterfly shapes and hang them in your row. It's best if they are hung so that they will "flutter in the breeze." I saw one youtube video in which they hung bits of tin foil with the same effect - a breeze appears to be important here as well.
Second. A sacrificial plant. Nasturtiums are considered ideal for this, although personally it makes me rather sad as I'm fond of nasturtiums. But you could plant something that looks rather like a stockaded fort with nasturtiums as the stockade. You do still have to watch for the caterpillars as they can decimate a nasturtium for a midmorning snack and be looking around for the next meal. But it does give you a bit of a jump on the caterpillars.
A third option, which we haven't really explored in the garden, is consciously and deliberately to develop a predator population. This would include some of the wasps and assassin bugs. I believe there are probably some birds as well who would be delighted to snack on the butterflies and perhaps the caterpillars.
There is, as usual, a final option, which is not to plant anything they want to eat. This is actually not realistic for several reasons, not the least of which is that we would run out of things to plant! But, more importantly, the butterflies range over kilometers in a day so unless the entire city participated in a Brassicaceae ban....
The aphids I'm seeing at the moment are a blue-grey carpet in the heart of my collards. A determined attack with soapy water will bring them under control in short order (I hope... it did last year and the year before but you never know.) If you haven't done the soapy water thing before, just put a few drops of regular kitchen dish soap (Palmolive seems to be more effective than the 'organic' kinds I buy at the Coop...) with a couple drops of garlic oil (also available at the Coop and probably other places - I think you can also use chili powder or something hot and unpleasant to the skin)... Put the soap and oil in a spray bottle filled with plain water. The whole thing shouldn't be particularly sudsy when you shake it or feel soapy on your hands.
Like the last fairy at Sleeping Beauty's christening gone awry, it's always a relief to know we have lady bugs in the garden. This link (click here) goes to a slightly rambling six minute video showing lady bug eggs that are just beginning to hatch on the side of a pot. While you probably don't need to pay attention for all six minutes. it is a good source for seeing eggs (yellow ones and a whitish-pink set) and for seeing a newly hatched larva. A somewhat longer (30 minutes) and much more professionally put together video - produced by the BBC is, not surprisingly, much more complete. Replete with images of ladybugs mating, laying eggs, growing through their various stages, and eating their prey, the video could easily have an 'explicit content' warning. It does make fascinating viewing though so click here if you're interested in watching.