Beneficial insects can help us in the vegetable garden in a number of ways. Some of them dine on plant pests directly ("predators"), or by laying eggs in their larvae ("parasites"). Other beneficial insects help by pollinating the flowers that turn into our squash, peppers, cucumbers and many other "fruiting" crops. And there are a few wild species of bees that also eat plant pests, not just nectar.
Whether a bug is a "plant pest" or a "beneficial insect" requires a human perspective - a bug either helps or hinders us in our efforts to grow a vegetable garden. But from nature's perspective, the ideal is really a balance which includes both predator and prey insects. It is this balance we are going for in a healthy vegetable garden, not a predominance of one over the other.
The best way to assure a population of beneficial insects is to attract them from the surrounding environment, and if you give them what they need, they WILL come. You may also be able to buy them at garden centers, but in order to get them to stick around and not just fly off, you will need to provide them with:
There are too many beneficial insects for me to provide an exhaustive list, but here is a table showing a few of the most common ones, and what they either eat or parasitize.
|Beneficial Insect||Helps Control|
|Minute pirate bug||a wide variety of small insects, including thrips|
|Big eyed-bug||a variety of small insects|
|Assassin bug||a wide variety of small to medium-sized insects|
|Damsel bug||a variety of small insects|
|Mealybug destroyer||a wide variety of mealybugs and scale insects|
|Soldier beetle||aphids and eggs & larva of other insects|
|Green lacewing||a wide variety of small insects|
|Syrphid fly||aphids and other small, soft-bodied insects|
|Tachinid fly||wide variety of larva (which ones depends on species)|
|Trichogramma wasp||larva of hundreds of species, including butterflies|
|Praying Mantis||a huge variety of insects, including beneficials|
|Predatory Mites||spider mites|
|Whitefly Parasites||whitefly larva|
Some photographs of parasitic and predatory insects are available at the UC Davis website, which also has a free printable color PDF poster.
Most people don't think of paper wasps or yellow jackets when they think about beneficial insects, but they are actually very helpful because they eat a number of damaging insect pests including cabbage loopers and other caterpillars. (Having said that, I was disheartened in 2020 when I watched a now-scarce Monarch butterfly lay eggs on a milkweed in my insectary garden, which later hatched into tiny caterpillars. A few days later they were all gone... eaten I presume. Next year I may bring them in and feed them to keep them safe, until they can make their miraculous flight to Mexico.)
Beneficial insects should be a part of every vegetable garden. While we all like to see those beautiful photographs of perfect beds with perfect vegetables, a really healthy, diverse and balanced vegetable garden should truthfully be a bit messy. Between the compost bins and leaf piles, the mulches and the stuff planted every which way, all mixed up all over the place, it's photogenic in its own, wabi sabi kind of way...
With pots of flowers and herbs here and there. And saucers of water and pebbles. And a brush pile against the fence. And bird baths. And catnip left growing wild because Angel and Nikki like to roll in it. You get the picture. Go wild (at least little)!
But mostly, have fun.