• Sidebar: Which Bugs Are Good For Your Garden?
On a sunny afternoon in the herb garden, you pause and lean closer to smell a flower, check on a new transplant, and pull a weed. Suddenly you notice dozens of aphids working on the new leaves of your oregano, but before you have time to worry, you see that help is on the way. Other bugs are also roaming the plants, apparently feeding on the pests.
Most gardeners realize that many kinds of insects in a garden can be beneficial in managing the damaging kinds that also occur there. Many can recognize the more familiar ones—lady beetles, green lacewings, even some of the parasitic wasps that dine on garden pests. Not often, however, do gardeners give much thought to the needs or cultivation of these naturally occurring biological controls.
Beneficial insects have environmental needs that parallel our own: a good place to raise their young, food to sustain the young and the adults, a bit of shelter, and freedom from harmful gardening practices. Fulfilling these needs for them can improve the efficacy of biological controls.
The immature stages of many kinds of beneficial insects look quite different from the adults and have different food requirements. A few bugs to munch on is their primary need. Some protection from winds, a bit of mulch, or a place to nest are usually all that they need in the way of shelter. Limiting pesticide applications or selecting products such as soaps, Bacillus thuringiensis, and neem, which have little effect on beneficial insects, can help control pests without wiping out the complementary efforts of natural controls.
Although nearly every garden provides food for the young of beneficial insects, not all of them meet the food needs of the adults. Many adult beneficials sustain themselves on nectar for energy, and some need pollen as a source of protein and vitamins. For example, syrphid flies, whose maggotlike young are among the most effective controls of aphids in a garden, must feed on nectar and pollen before they can produce eggs. The adult stages of many lady beetles, parasitic wasps, and green lacewings similarly depend on flowering plants for sustenance, and the absence of these plants can greatly reduce their effectiveness as biological pest controls.
Not all flowering plants produce sufficient nectar or pollen for beneficial insects. Others are the wrong shape: the deep-throated flowers that attract butterflies, bees, and hummingbirds are quite inaccessible to insects with short mouthparts.
During the past three years, I have worked with graduate students, primarily Mohammed Al-Doghairi, to regularly monitor plants in botanical gardens along the Front Range of the Rockies—in Denver and Fort Collins, Colorado, and Cheyenne, Wyoming— for their use by beneficial insects. We monitored all the flowering plants, about 170 species, at least three times throughout each summer. We conducted a visual survey of the flowers, rated them for the intensity of beneficial insect visitation, and noted the insect species that we saw.
Of the flowering plants that we have surveyed, we identified fewer than one in five plants as attracting beneficial insects. With few exceptions, all of these have shallow flowers that can be easily reached by a nectar-hungry lacewing or lady beetle.
A Bug’s Dream Garden
One way of enhancing continuing biological pest control in your herb garden is to make sure that the plants are ones that the adult forms of beneficial insects will visit and use and that some of them will be in bloom throughout the season. The plants listed in our online chart are the ones that we found to attract beneficial insects here in the Rockies; your own observations will give you an idea of plants that are most useful in your area.
Flowering plants in the carrot family (Umbellifereae) and mustard family (Cruciferae) include many species that are particularly attractive to beneficial insects. Yarrows and a few other members of the daisy family (Compositae) are also useful, as are many mints (Lamiaceae). On the other hand, we found little use by beneficials of plants of the pea family (Leguminosae). This was probably due to the structure of the flowers and the amount of nectar produced, but we found that it is unwise to generalize too much when assessing which plants are important for beneficial insects.
For example, although many members of the figwort family (Scrophulariaceae) were present in our study areas, none was visited by beneficials—except Rocky Mountain penstemon (Penstemon strictus) when it was in bloom in late spring. Closely related plants may vary greatly in their suitability to support beneficial insects. Among cultivars of creeping thyme, crimson thyme (Thymus praecox subsp. arcticus ‘Coccineus’) was highly attractive during June and July, but the cultivars ‘Albus’, ‘Drucei’, and ‘Minus’, all in bloom at the same time, showed little or no use throughout the season. Nectar production, flower color, and the volume of flowers may explain the difference, but clearly, the factors governing whether an insect uses a certain kind of plant are complex and require further investigation.
An herb garden design that is intended to optimize the beneficial effects of such flowering plants should also consider timing of flowering. Peak flowering periods vary widely, and few plants, especially among the perennials, produce attractive flowers for more than a month. A planned sequence of flowering, however, can provide a consistent source of nectar and pollen.
Basket-of-gold (Aurinia saxatilis), a perennial member of the mustard family, is one of the first plants to be readily visited and used by adults of beneficial insects that are active early in the season here in the Rocky Mountain states. Sweet alyssum (Lobularia maritima), Rocky Mountain penstemon, and some potentillas can fill the bill during June, while a great many other species attractive to beneficials begin to flower in late June or early July—various stonecrops (Sedum spp.), wood betony (Stachys officinalis), some asters, and the lobelia commonly seen in hanging baskets and window boxes, Lobelia erinus. Yarrows (Achillea spp.), which are among the most consistently attractive plants throughout the season, are particularly visited in late summer, as are dill (Anethum graveolens) and masterwort (Astrantia major).
As you plan your garden, think of beneficial insects. They ask nothing of you, yet they are well worth inviting to your garden. Interplanting plants that attract beneficials among those, such as roses and vegetables, that attract a lot of pests may give you a higher level of natural control throughout the yard and garden. That will save you time, money, and the need to use synthetic sprays that persist in the soil and water. These plants will not only help you attract beneficials, but also keep them around once they’re there. Providing a sip of nectar or a clump of pollen—“lunch breaks”—requires very little effort, and it will encourage these hardworking insects to spend more time in your garden.
Whitney Cranshaw, an entomology professor at Colorado State University, is the author of Pests of the West (1992) and Bagging Big Bugs (1995), both published by Fulcrum Publishing. He spends a lot of time in his garden, primarily because that’s where the bugs hang out.