Author: Urban/Small Farm IPM Specialist, Department of Extension Plant Sciences, New Mexico State University.
There are many naturally occurring predatory and parasitic insects (and related species) that can help reduce pest populations on farms and in gardens. These biological control agents include various parasitic wasps and flies, minute pirate bugs, hoverflies, green lacewings, ambush bugs, ladybird beetles, crab spiders, and predatory mites1. In order to maximize the impact of these beneficial species, they should be protected from exposure to pesticides by practicing integrated pest management (IPM)2. They can also be encouraged by growing flowering plants that provide nectar and pollen, since such resources can help sustain them when prey is scarce and can increase their lifespan and level of egg production.
Mixtures of so-called “insectary” plants can provide nectar and pollen all season long if properly maintained, but they must be selected with care. Because beneficial insects differ in the size and structure of their mouthparts, not all flowers are equally accessible (or valuable) to all species. The flower mixture should contain a diversity of plant species with different bloom periods and flower sizes, structures, and colors in order to benefit the maximum number of beneficial insects (Figure 1). Several companies offer commercial seed mixes designed for this purpose, but such mixtures do not always perform consistently well in different regions or climates, and preliminary trials with three such mixes gave poor results in New Mexico.
Figure 1. Insectary mixtures in full bloom.
When selecting species to plant, keep in mind that the main aim is to produce an abundance of different blooms for as much of the growing season as possible. Ideally, the species selected should be cheap, readily available, easy to grow, and quick to bloom. Annual species have the advantage over perennials of being quick to establish and produce flowers, and are also easy to rotate in different areas of the farm or garden in successive years. Similarly, while native plants have the benefit of being well adapted to New Mexico conditions, their seed can be hard to find, expensive, and/or difficult to germinate. For these reasons, a mixture of common garden flowers and herbs often works well for enhancing populations of beneficial insects; such plant species are also more likely to be available from organic seed suppliers for certified organic producers. Plantings intended mainly for pollinators (native bees and domesticated honey bees) require a different approach3.
Seed Mixes and Cultural Guidelines
Research conducted at NMSU’s Los Lunas Agricultural Science Center has shown that the following mixture–comprised of several quick-growing garden annuals–can significantly increase populations of several groups of important predatory and parasitic insects, and can therefore be a valuable component of an IPM approach. Seeds of the suggested plants are all readily available, relatively cheap, and easy to grow. In our experiments, this mixture survived well in both sandy and clay soils and provided continuity of bloom from late June to early October with minimal care (i.e., regular watering and weeding during establishment). This mixture can also be used as a starting point to develop more complex blends by adding additional species (with appropriate adjustment of seeding rates). Within reason, the more species the better, since a greater diversity of plants will benefit more species of insects. If additional species are added to the core mixture, however, it is best to select single-flowered varieties rather than the more ornamental “double” ones since the nectar and pollen in the latter are often difficult for insects to access. It is also sensible to avoid species that are in the same botanical family as your crops. Normally, this is rarely a problem, but sweet alyssum, for example, is part of the cabbage family (Brassicaceae) and may attract pests of such crops (e.g., flea beetles, harlequin bugs [Murgantia histrionica], and Bagrada bugs [Bagrada hilaris]); hence, this species should be omitted if brassica crops, such as arugula, mustard greens, kale, Chinese cabbage, etc., are being grown.
Our suggested mixture consists of California bluebell (Phacelia campanularia), buckwheat (Fagopyrum esculentum), dill (Anethum graveolens), plains coreopsis (Coreopsis tinctoria), garden cosmos (Cosmos bipinnatus), and sweet alyssum (Lobularia maritima) (Figures 2 through 7). With the exception of California bluebell, all will flower for most of the season in New Mexico. California bluebell will usually set seed and die by the middle of summer, but is included because it is one of the earliest species to bloom and hence may provide valuable early season forage for beneficial insects. In our trials the following species performed poorly and hence are not recommended, although they may perform better in other areas: California poppy (Eschscholzia californica), cornflower (Centaurea cyanus), coriander (Coriandrum sativum), garland daisy (Chrysanthemum coronarium), nodding catchfly (Silene pendula), tidy tips (Layia platyglossa), and English or pot marigold (Calendula officinalis). We also found that buckwheat, although a very valuable insectary plant, can be highly susceptible to root rot caused by the fungus Rhizoctonia solani (especially in sandy soils); avoid this species if your soil is known to contain this pathogen.
Figure 2. California bluebell (Phacelia campanularia).
Figure 3. Buckwheat (Fagopyrum esculentum).
Figure 4. Dill (Anethum graveolens).
Figure 5. Plains coreopsis (Coreopsis tinctoria).
Figure 6. Garden cosmos (Cosmos bipinnatus).
Figure 7. Sweet alyssum (Lobularia maritima).
The insectary plants should be sown in spring as soon as the danger of frost is past, and if possible in advance of the crop(s) to be protected. The seeds in our suggested mixture vary greatly in size, making it difficult to use a mechanical seeder; for small areas, therefore, the seeds can be mixed with dry peat and broadcast by hand over a prepared seed-bed. The mixture should be lightly incorporated into the soil prior to watering. Since some of the seeds are very small, mixing them with a much larger volume of peat helps ensure a more even distribution across the bed and makes it easy to see which parts have been sown. For larger areas, an alternative approach would be to mechanically sow each species in single rows within a wider bed, although this would require more space and more seed.
Reputable seed companies should be able to provide suggested seed rates for different species. In our plantings, we sowed each species at twice the recommended seed rate (to compensate for difficult spring germination and establishment conditions) divided by the total number of species in the mix. It may be necessary to adjust this basic guideline for individual species in different situations; garden cosmos, for example, can be too vigorous on clay soils, and under these conditions it should be sown at half the normal seed rate to avoid it dominating the mix and shading out the other, lower-growing species.
The following example shows the calculation to determine the weight of seed needed for one species (dill, in this instance) in a mixture with five other species (i.e., as part of a six-species mix). The calculation is for one row 10 inches wide by 50 feet long. Table 1 shows the weight of seed needed for each of the remaining species in the mix to sow this area. Once all the seeds are weighed, they should be combined and thoroughly mixed with 8 to 10 liters of dry peat for broadcasting by hand. Since seed rates are normally expressed in pounds per acre, this has been used as a starting point, but the ultimate weights are also given in grams to facilitate weighing small quantities.
Table 1. Weight of Seed Needed for Each of the Six Species in the Suggested Core Mix for One Row 10 Inches Wide by 50 Feet Long (0.000953 acres).
(lb per acre)
(lb per acre)
seed rate for
(lb per acre)
seed rate for
(ounces per acre)
by row area
(= weight of seed
needed for one
row, in ounces)
for one row
Example Calculation of Weight of Seed Needed
Area: One 10-inch-wide row (0.83 ft) × 50 ft = 41.5 ft2 = 0.000953 acre
Normal seed rate: 8 lb per acre
Twice normal seed rate = (2 × 8) = 16 lb per acre
Twice normal seed rate divided by total number of species in mixture = Fractional seed rate = 16 ÷ 6 = 2.7 lb per acre
Fractional seed rate in ounces = 2.7 × 16 = 43.2 ounces per acre
Fractional seed rate multiplied by row area = 43.2 × 0.000953 acre = 0.041 ounce (1.16 grams). This is the amount of dill seed needed for a single row 10 inches wide by 50 feet long, as part of a six-species mix.
1See Pocket Guide to the Beneficial Insects of New Mexico, available at http://aces.nmsu.edu/ipm/documents/beneficial-insects-booklet-final.pdf.
2See Integrated Pest Management (IPM) for Home Gardeners (NMSU Extension Circular 655), available at http://aces.nmsu.edu/pubs/_circulars/circ655.pdf.
3See USDA-NRCS Plant Materials Technical Note No. 71 (rev.), Revised Pollinator Plant Recommendations for New Mexico, available at http://www.nm.nrcs.usda.gov/technical/tech-notes/pmc/pmc71a-rev.pdf.
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Printed and electronically distributed May 2013, Las Cruces, NM.