The tobacco hornworm, above, and the tomato hornworm feed on tomato plants. A new edition of “Garden Insects of North America” will help you tell them apart. Both become sphinx or hawk moths. (David Shetlar)

Reference books form a wall around my little desk. Three years may pass before I revisit one on bamboos, for example, but as I tell my colleagues: "When I need it, I really need it."

One book that gets opened a lot more frequently than that is "Garden Insects of North America," published in 2004. If I want to see what sort of creature the tomato hornworm becomes, I turn to Page 147 and find a large and handsome moth, the five-spotted hawk moth.

Insects haven't changed that much since the book was published; dragonflies, we can assume, rode on the backs of dinosaurs. But there are dynamic shifts in how insects affect our gardening, for better or worse, as species once a problem fade from the scene and others arrive, perhaps because of climate change or the unintended effects of global trade.

Not all these aliens are detrimental, by the way. "Garden Insects" author Whitney Cranshaw says the arrival of the European paper wasp has significantly reduced pest populations in northern Colorado, where he lives.

"To me, it's a game changer in this part of the country," he said.

But what has changed too in the past 14 years is our common understanding of insects. Yes, there are still too many people who reduce the insect universe to one of "bugs" that must be annihilated. But gardeners have never been more ecologically minded, and the idea that we must shelter pollinators is now instilled in every grade-schooler, which is all to the good.

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We became aware of widespread and mysterious declines in honeybee populations, particularly with the advent of colony collapse disorder. We learned that overwintering populations of monarch butterflies have dropped. If you were paying attention, you'd know that bumblebees and other native bee species have been declining alarmingly.

A lot of people are worried about the damage of agricultural pesticides, especially neonicotinoids, to non-targeted insects. There are lots of dire news stories generated by scientific studies, but one report from Europe last year was particularly unsettling.

In western Germany, biologists tracking flying insects over 27 years at 63 locations found a decline of about 80 percent in the population by total weight. The causes are open to speculation, with agricultural pesticides high on the list. The results, if extreme, are consistent with other studies of species decline.

If you might think it time for an updated version of "Garden Insects," Cranshaw has obliged with a fully revised second edition put together with David Shetlar, a professor of urban landscape entomology at Ohio State University. The page count has grown from 672 to 704, with more than double the number of color photographs (thanks to the advent of digital imaging).

More important, the book has been reorganized to be more useful to the gardener, amateur and pro. The pest insects are now grouped by the type of damage they cause, rather than scientific alliances, and there is a new chapter on "good bugs" — insects that we rely on for pollination, to prey on "bad bugs" or to aid in turning yard waste into humus.

Underpinning the book is the idea that to know an enemy or friend, you must first be able to identify it. Most of us know that ladybird beetles — ladybugs — devour aphids; perhaps not so many of us realize it's the ugly larvae that do most of the hunting. As a boy, I thought the common hover fly was a clownishly bad mimic of a bee. But Cranshaw and Shetlar tell us that its larvae "are particularly important in controlling aphid infestations early and late in the season, when many other predators are not active."

One of the main aims of the book, Shetlar said, was to get people to understand the life cycles of an insect. He and Cranshaw don't guide the reader on control. There are practical reasons for this — approved pesticides come and go — but there are other grounds as well. Smart gardeners confronting pests ask themselves: Is the infestation bad enough to warrant action? Is it being caused by stresses I'm placing on the plant? Will the insect damage have any long-term effect on the plant? Can I just hose the creatures off?

Releasing predatory insects has become a keystone of pest control in organic agriculture, and commercial insectaries raise "beneficials" by the millions. Surprisingly, given our long-standing devotion to kill-everything chemical pesticides, it was in the United States that this precision biological approach was first employed. The vedalia beetle was brought in to the citrus orchards of California in the 1880s to take care of a serious pest, the cottony cushion scale. "It provided the first clear demonstration, worldwide, of the potential value of biological control," the authors write.

Cranshaw and Shetlar bring us other garden invertebrates, including arachnids, mollusks, crustaceans and earthworms.

Some of these animals can be highly destructive — sawflies on pines, Japanese beetles on roses, gypsy moths on shade trees, for example — and some are a bloody nuisance to us, especially the mosquitoes and ticks.(The book steers clear of these parasites.) But it helps to understand the totality of their presence. These creatures together provide vital services to us and the environment by keeping down pest populations naturally, by aiding in the decomposition of organic matter and by pollinating the plants.

We can help sustain this world by having lots of different pollen- and nectar-bearing plants and by leaving places for beneficial insects to find shelter.

Most of all, we can keep insecticide use to a minimum, know exactly what we are trying to kill and avoid harming any six-legged bystanders.

Shetlar said he is encouraged by a younger generation that is questioning norms (and not just in the garden). "They're asking, 'Do I really need some company to come in and spray some pesticide in my back yard?' " he said.

Fundamentally, I think the idea that insects are either bad or good is a false, binary way of looking at it. Paper wasps are scary creatures and induce fear when they build their nests in the eaves, but they take care of many pests in the garden. The parsley worm will munch away at your parsley, dill and fennel, but then turn into the beautiful black swallowtail.

The hardest thing for us to comprehend is that the rest of the natural world isn't placed there for our benefit. This is a particularly hard notion for the gardener to swallow; a big part of nurturing plants is for our enjoyment. Remember, though, that a snail may think it has as much right to exist as you do, and it has a God-given house to prove it.

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