Ok, I'm mildly obsessed these days with aphids. Whether I'm in the garden or out on the balcony, abura mushi (Japanese for aphid) are everywhere. Trying to do battle organically and sensibly is my current challenge, and I thought I'd share some of what I've learned so far about why they find my plants so delectable and what can be done about it.
I'm still researching this, but what I've learned thus far is that they are attracted to the color yellow, which means those giant zucchini blossoms must seem ideal. It also explains why nasturtiums are listed as a trap crop.
They are also encouraged by high amounts of nitrogen in the soil. High levels of nitrogen encourage a fair amount of new growth, which is wear aphids often best like to feed. Despite their tiny size, they need a great deal of the heady brew offered by new growth where the greatest amount of amino acids are present. I worked a fair amount of composted chicken manure and old coffee grounds (both excellent sources of nitrogen) into the beds before planting, which may subsequently be contributing to my aphid problem.
Humid weather (welcome to Tokyo in summer!) is an ideal environment for them, too. Keeping plantings and plants a bit less dense so air can flow and sunshine can penetrate are, as I've mentioned before, pivotal. I'm slowly beginning to learn the art of judicious pruning on tomatoes, eggplants, and now zucchini.
Companion planting is high on the priority list. The practice of mixing herbs and flowers in with vegetables serves to attract beneficials, repel pests, or act as a trap crop. Bringing in beneficials (like a praying mantis or two) to eat aphids and keep their numbers down means more vegetables at harvest time and healthier plants. Companion planting effectively puts up a big welcome sign for pollinators, too, which again means more vegetables at harvest time. Fennel, parsley, cilantro, dill or other herbs that bloom in umbel (umbrella-shaped) blossoms are nearly irrisistable.
Aphids and other pests find mint, peppermint, onions, and garlic repellant. Planting these among vegetables or concocting a spray from them will help ward off garden evildoers. Those that decide to enter the garden anyway will get munched on by the beneficials.
There are some plants aphids adore more than vegetables, and sad as it is to say nasturtiums are one of them. Planting a trap crop around the garden means a lack of those lovely blossoms for summer salads, but it helps ensure the survival of other vegetables. Some determined aphids will, of course, still make it through, but it shouldn't be more than a good dousing with water or a praying mantis or two can't efficiently dispatch.
A Few Good Resources
I wanted to learn a thing or two about aphids, and this article from Vegetable Gardener.com offers good basic information on the bug, what it likes, and how to avoid it as well as get rid of it.
Perhaps offering more information than one could possibly ever want to know about aphids is this article from The Earth Life Web. Great links, references, and explanations will help the aphid obsessed and infested gardener get to know this pest.
The best site I've found yet about organic pest control techniques, this article from OISAT on trap crops offers an incredibly useful table of crops, their companion crop, and the targeted pest. This article from Hobby Farms is a nice companion piece, too.
This fact sheet from Colorado State University's Extension Office offers a handy table for making insecticidal soaps, and Organic Matters Magazine also offers a short but useful article on aphids and remedies such as onion and garlic spray.