Fig. 1: Aphid feeding within the phloem sieve tubes

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Insects and mites that feed on horticultural plants have different feeding behaviors. Most insect pests that attack greenhouse-grown horticultural crops have sucking or piercing-sucking mouthparts, which includes: aphids, leafhoppers, mealybugs, soft scales, and whiteflies. Sucking insects feed on a wide range of horticultural crops, such as annuals, perennials, herbs, and vegetables. The vascular plant tissues include the xylem (water-conducting tissues) and phloem (food-conducting tissues). Sucking insects insert their mouthparts into the vascular tissues of plants, primarily the food-conducting tissues or phloem sieve tubes, and withdraw fluids (Fig. 1). The direct damage caused by sucking insects includes: plant stunting, plant wilting, leaf distortion, and leaf yellowing (starting with the lower leaves) (Fig. 2).

Sucking insects feed within the phloem sieve tubes to obtain amino acids that are the building blocks of proteins, and are required for development and reproduction. However, to obtain sufficient quantities of amino acids, sucking insects must ingest large quantities of plant fluids, which contain an assortment of other materials in larger amounts than amino acids. As a result, sucking insects that feed in the phloem sieve tubes excrete copious amounts of honeydew, which is a clear, sticky liquid that can accumulate on plant leaves and stems (Fig. 3). Honeydew is a prime substrate for the black sooty mold (Fig. 4) that inhibits the plant’s ability to manufacture food via photosynthesis, and ruins the aesthetic quality of plants. Ants are attracted to honeydew, so when ants are observed on plants, this is a clear indication that a sucking insect is present. In addition to the direct feeding damage caused by sucking insects, certain aphids and whiteflies; such as, the green peach aphid (Myzus persicae) and sweetpotato whitefly (Bemisia tabaci) vector plant viruses during feeding. However, the viruses transmitted by aphids and whiteflies are associated more with greenhouse-grown vegetables than floricultural crops.

Sucking insects such as aphids, mealybugs, whiteflies, and soft scales may exhibit a high degree of plant specificity because certain plant-specific chemical compounds serve as important plant selection cues. In addition, the nutritional quality of specific cultivars may be correlated with susceptibility. Scientific studies have demonstrated relationships between the concentration of amino acids in plant tissues and attractiveness of plants to aphids and whiteflies. For example, aphids prefer certain cultivars of chrysanthemum (Dendranthema x grandiflorum) (Fig. 5) and whiteflies have a tendency to attack specific cultivars of poinsettia (Euphorbia pulcherrima) over others (Fig. 6). Sucking insects are common throughout the growing season feeding on a wide-variety of greenhouse-grown horticultural crops. However, it is important to act preventatively in suppressing sucking insect pest populations before outbreaks occur. For example, both aphids and whiteflies have a high reproductive capacity with females producing abundant nymphs or eggs in a short-period of time, with the possibility of multiple generations occurring during a single cropping cycle.

Sucking insects that feed within the vascular tissues are susceptible to applications of systemic insecticides. Systemic insecticides are applied to the leaves, stems, and/or growing medium; however, residual activity is longer when applications are made to the growing medium as either a granule or drench. Systemic insecticides tend to be water soluble, which allows the active ingredient to be taken up by the root system and then translocated throughout the plant in the water-conducting (xylem) or food-conducting (phloem) tissues, or both. Once inside the plant, the active ingredient may move back-and-forth from the water-conducting tissues to the food-conducting tissues and vice versa. During the feeding process, a sucking insect ingests a lethal concentration of the systemic insecticide active ingredient and is subsequently killed. For instance, the piercing-sucking mouthpart (or proboscis) of an aphid is inserted into plant tissue and reaches the conductive cells (sieve tubes) through which water and food are transported. As the aphid withdraws plant fluids, it imbibes the active ingredient of the systemic insecticide.

Sucking insects that feed on the leaf underside, such as aphids and whiteflies are also susceptible to insecticides with translaminar (or local systemic) activity. For example, following an application, these materials penetrate the leaf tissues and form a reservoir of active ingredient within the leaf, which provides residual activity even after spray residues dissipate.

Raymond is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits. rcloyd@ksu.edu or 785-532-4750