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Why and How Insects and Mites Feed on Your Plants and Flowers

April 7, 2005
Insects and mites that feed on plants have different feeding behaviors, which include chewing, piercing-sucking, mining, boring, or galling. The majority of insect and mite pests that attack ornamental plants growing outdoors have piercing-sucking or chewing mouthparts. Insects with piercing-sucking mouthparts include aphids, whiteflies, mealybugs, soft scales, and thrips. These insects insert their mouthparts into the vascular tissues of plants, primarily in the food-conducting tissues (phloem), and withdraw plant fluids. This results in plant wilting, stunting, and leaf distortion. A number of insects, including certain leafhoppers and spittlebugs feed within the water-conducting tissues (xylem). Insects with chewing mouthparts include beetles, caterpillars, grasshoppers, weevils, and earwigs (although not considered insects, snails and slugs have chewing mouthparts). Chewing insects physically remove portions of leaves or flowers directly, or consume entire plant parts.

The reproductive portions of plants are attractive to most plant-feeding insects because these portions are more nutritious due to the relatively high levels of protein. However, these portions may contain high levels of secondary metabolites (defensive compounds), which may influence acceptability. Plant leaves usually provide the greatest biomass for insects and are the best food nutritionally--next to reproductive portions. Nitrogen is the primary plant nutrient needed by insects with piercing-sucking and chewing mouthparts. Nitrogen (in the form of protein and amino acids) is very important for insect growth, development, and reproduction. Nitrogen levels are usually higher in younger tissue than in older leaves, and levels decline as plants mature. Nitrogen level is a factor that can limit insect growth and development. In general, plants tend to lack nitrogen in the form that insects can utilize. The dry weight of most insects is between 8 and 14 percent nitrogen; however, plants overall contain only 2 to 4 percent nitrogen. The phloem contains only 0.5 percent or less, and the xylem contains 0.1% or less nitrogen. Reproductive portions (flowers and seeds) and leaves contain from 1 to 5 percent or more nitrogen. Nitrogen is an important component of protein; however, protein concentrations can vary depending on plant type, age, and nutritional status of the soil. Protein is generally higher in reproductive portions, and leaves and stems.

Plants that are overfertilized, especially with nitrogen-based fertilizers, produce succulent growth, increasing susceptibility to plant-feeding insects and mites. The higher levels of amino acids, which are the primary food source used by insects and mites, can increase their reproductive ability. In addition, plants that receive excessive levels of fertilizer may have thinner leaf cuticles, which are easier for insects and mites to penetrate with their mouthparts. Variegated plants, those with white, yellow, or red coloration along with green portions are typically fed upon more by insects because the variegated areas contain more nutrients and fewer defensive compounds than the green portions. Additionally, variegated portions are softer and easier for insects to penetrate with their piercing-sucking mouthparts or to consume.

Insects with piercing-sucking mouthparts that feed in the phloem may produce large quantities of honeydew, a clear, sticky liquid. Free amino acids, essential in the production of protein, are very important to phloem-feeding insects. These insects require protein (in the form of amino acids) for development and reproduction. To obtain the necessary quantities of amino acids, insects must consume large amounts of plant sap, which contains an assortment of other materials in larger quantities than amino acids. The excess is excreted as honeydew. Also, phloem-feeding insects possess carbohydrases such as amylase and a pectin-hydrolyzing enzyme that break down the middle lamellae of plant cell walls. Insects that feed within the phloem (such as aphids, mealybugs, whiteflies, soft scales, and certain plant bugs) tend to exhibit a high degree of host specificity because certain plant-specific chemical compounds tend to serve as important host selection cues. This is why, for example, that aphids may prefer certain cultivars of chrysanthemum to others.

Insects that feed in the xylem, such as true bugs (Order: Hemiptera), must cope with negative tension and very low concentrations of nutrients in the xylem fluid. As a result, these insects feed faster as the water potential becomes more negative, and they extract extremely large quantities of plant fluids, which is one reason why xylem-feeders tend to be larger than phloem-feeders (I bet you didn’t know that!!).

Insects with piercing-sucking mouthparts that feed within the food-conducting tissues are susceptible to systemic insecticides applied to the leaves, stem, or soil. Systemic insecticides are generally very water soluble, which allows them to be distributed into roots and leaves. Also, plants do not readily metabolize systemic insecticides. In general, the active ingredient is taken up and moved throughout the plant (translocated) in the xylem, the phloem, or both. Additionally, once inside the plant, the active ingredient may move back-and-forth from the water-conducting tissues to the food-conducting tissues or vice versa. As an insect feeds, it takes up a lethal dose of the insecticide and is killed. Systemic insecticides include acephate (Orthene/Precise) and imidacloprid (Merit).

Insects with piercing-sucking mouthparts that feed primarily on the underside of leaves, such as whiteflies, are susceptible to insecticides with translaminar properties or local systemic activity. These materials penetrate leaf tissues and form a reservoir of active ingredient within the leaf. This provides residual activity against plant-feeding insects. Examples of insecticides with translaminar properties include abamectin (Avid), acephate (Orthene), and spinosad (Conserve).

Leafminer larvae feed between the leaf surfaces in the mesophyll layer of cells. Generally, this protects the larvae from applications of contact insecticides; however, products with translaminar properties are effective, as they are capable of entering the leaf.

Spider mites, including twospotted spider mite, Tetranychus urticae, do not feed in the vascular tissues. Twospotted spider mites primarily feed on leaf undersides within plant cells and obtain food by removing chlorophyll (green pigment) with their styletlike mouthparts. Because spider mites don’t feed in the vascular tissues, they are not susceptible to systemic insecticides; however, spider mites are susceptible to insecticides/miticides with translaminar properties, such as abamectin (Avid) and etoxazole (TetraSan).

Chewing insects, in general, are nonselective in their feeding behaviors—they typically ingest macerated whole-leaf tissue. However, some are more selective. Chewing insects are typically not controlled with systemic insecticides. Insecticides with contact and stomach-poison activity are more effective in controlling insects with chewing mouthparts.