The Influence of Plant Architecture on Biological Control

Just open a seed catalogue, or take a walk through a greenhouse or woody plant nursery and you will find a wide range of cultivated varieties of almost any species of plant you could want to grow. You can select plants for specific maturity dates, fruit size, flower and leaf color, flavor, scent, plant growth habit, and insect and disease resistance. The fact that many of these characteristics may alter the effectiveness of pest natural enemies can greatly complicate augmentative biological control. In this article we will focus on how the gross structural complexity of a plant variety, or its architecture, can affect natural enemies.

Natural enemies usually have to walk or fly short distances on a plant to find a pest, whether they are following the scent associated with its prey or are searching randomly. Differences in the way a plant is put together are bound to affect the ease with which a natural enemy can get around on a plant. To get an idea of the possible variation in plant architecture, just consider the many different ways that leaves and branches can be arranged on a tree or shrub. Leaves can be arranged opposite each other, alternate, or spiraling along a stem. They can be attached close to one another, or far apart. Branches can be arranged horizontally, vertically, or at right angles to one another. If you also consider that leaves vary widely in size and shape you can begin to see how much the searching environment of a natural enemy can vary on a plant.

Two components of plant architecture can readily be determined by the biological control practitioner: plant size and structural complexity. The size of a plant directly affects the total area that must be searched for pests. In the absence of specialized adaptations that help them find a pest, natural enemies tend to be less efficient on large plants because they have a greater amount of surface area to cover than those on smaller plants.

Structural complexity refers to how the surface area on a plant is put together. A plant with few small leaves is less structurally complex than a plant with many small leaves. For example, Trichogramma nubilale can more easily find and parasitize European corn borer eggs on artificial leaves with a simple shape than on those whose area is distributed among smaller leaflets. This suggests that biological control of pests on plants with more structurally complex leaves may require greater numbers or more frequent releases of natural enemies.

We have found plant structural complexity to be a handy mechanism to explain observed rates of parasitism in studies of two ornamental pests. Aspidiotophagus citrina is a wasp that parasitizes euonymus scale. Lower rates of parasitism by this wasp were observed on dwarf varieties of Euonymus japonica with small, closely packed leaves than on large-leafed varieties. Likewise, another wasp, Leptomastix dactylopii, parasitized fewer citrus mealybugs on variegated Coleus than on green varieties. The variegated varieties tend to have smaller leaves that are more densely packed on a plant stem. Rates of parasitism of citrus mealybug on all varies of Coleus were better explained by numbers of leaves on a plant than by the amount of leaf area to be searched.

Recent work with silverleaf whitefly on poinsettia and the parasitoid wasp Eretmocerus californicus provides an example of how increases in plant size and structural complexity associated with plant growth can limit the effectiveness of a parasitoid. At first, weekly releases of 3 female wasps per plant stem gave adequate control of whiteflies on poinsettia plants raised for cuttings. Toward the end of the cycle, however, whitefly numbers increased because the plants were now larger with more leaves to search.

Structural complexity can also affect the searching ability of predators who look for prey on plants by walking along stems, leaf edges, or midribs. Lady beetles and minute pirate bugs are among the groups of insects with this kind of searching behavior. For example, lady beetles reduced aphid numbers on leafless varieties of peas more quickly because the search area was less complex.

Greater structural complexity or more complex leaf structures is not the only factor affecting predtor searching ability, particularly for plants with a slippery wax coating, like mustard greens, kale, broccoli and other cruciferous vegetables. Lady beetles spend more time slipping and falling from plants with large waxy leaves than those with smaller leaves, or with ruffled edges, or hairy leaf surfaces. As a result, predation of aphids on large, slippery-leafed plants is lower.

Can assessing plant architecture be part of developing a biological control program? Yes, but like most other factors affecting natural enemy effectiveness there are no hard and fast rules. To start with, you can use plant architecture to help you choose a natural enemy release rate. Most natural enemy suppliers provide ranges of release rates based on square footage, numbers of plants present, or the perceived pest threat. When the architecture of the crop is simple, and factors like waxy leaves are not involved, you may want to consider using a release rate at the low end of the recommended range. In contrast, when the architecture is more complex, you might start out at the high end. As with other pest management decisions, be sure to monitor the pest density and natural enemies on a regular basis to evaluate the effectiveness of your control tactic.

- Cliff Sadof, Ray Cloyd, and Jinsong Yang, Purdue University

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