Piercing-Sucking Insects and Systemic Insecticides
by James A. Bethke
Quite simply, a piercing-sucking insect is one that feeds on plants by piercing cells or vascular tissues with specialized mouthparts and sucking the contents. Piercing-sucking insects are some of the most damaging pests for ornamental plant producers due to the direct damage caused by feeding — which can cause a variety of plant symptoms including plant death — and indirect damage caused by piercing plant protective tissues and vectoring many different plant diseases.
While there is a distinctive mouthpart design for each type of insect, there is a general design for piercing-sucking insects (fig. 1), which consists of a beak or rostrum that contains a pair of very fine stylets in the center. The stylets are so fine that very little pressure is needed to force them into plant tissues or between cells. As the cross section shows in figure 1, when the stylets are pressed together they form two tubes, or canals. One canal is for depositing saliva and the other is the food canal where food is drawn up into the insect by muscles strengthened for sucking. Normally these canals are enclosed and protected by the mandibular stylets and the rostrum. When the insect wants to feed, it pushes the stylets into the plant tissue in search of a specific food source. Depending on the type of insect, this tissue could be cellular tissue or one of the vascular system tubes, the xylem or phloem. Therefore, because the insect’s feeding habits are selective, feeding on specific types of tissues, where the insecticide is deposited is key to insecticide efficacy.
Figure 2 represents a cross section of leaf tissue. Generally, leaves have both an upper and lower epidermal layer that covers the parenchyma. Within the leaf and stem tissues is the vascular system. The xylem brings water and nutrients from the roots of the plants up throughout the rest of the plant, including the trunk (in the case of trees), stems and leaves. The phloem begins in the leaf tissues, and it conducts food generated by photosynthesis (proteins and carbohydrates) to the rest of the plant. Figure 2 also shows where specific insects and mites feed, and the entry routes where the stylets penetrate the tissues.
The damage and symptoms caused by piercing-sucking insects and the types of tissues they select to pierce and feed upon can be used to somewhat easily identify the insect. For example, cellular plant tissue damage is caused by a cellular-feeding insect, honeydew is caused by a phloem-feeding insect, and salty deposits on leaves is caused by a xylem-feeding insect.
When piercing-sucking insects feed on cellular tissues, they will usually defecate in dark spots, which can be quite distinctive. For instance, most thrips and lace bugs feed on epidermal cells, which results in silvering of the leaves, but the silvering is accompanied by dark spots of defecation (fig. 3). This type of damage differentiates thrips feeding from mite feeding because mites do not leave a distinctive fecal spot. Further, insecticides that deposit in epidermal tissues such as abamectin are much more effective against cellular feeders. With respect to the neonicotinoid systemic insecticides, it has been observed that foliar applications are more apt to deposit in epidermal tissues and cause more mortality of leaf-feeding thrips than a drench application, which disperses throughout the plant via the vascular system and must translocate to the epidermis.
Phloem feeders cause indirect plant damage by producing large quantities of sugary, sticky honeydew, which is deposited on the leaf tissues below the feeding pest. As the honeydew ages, it tends to be covered in sooty mold, a black mold that causes aesthetic damage to ornamental plants. Insects that feed in phloem include aphids, mealybugs, whiteflies, soft scales and certain plant bugs. These insects tend to be host specific because the host tissues carry plant-specific chemical compounds that serve as important host selection cues. For example, this is why aphids may prefer the Tuneful cultivar of chrysanthemum over other cultivars. Some phloem feeders, such as whiteflies, can weave their stylets between cells to reach the phloem, whereas others like planthoppers penetrate cells directly on route to the phloem. Those pests that penetrate cells directly may attain a toxic dose of pesticide prior to reaching the phloem.
Xylem feeders, such as true bugs, leafhoppers and sharpshooters, must cope with negative pressure in the xylem and very low concentrations of nutrients.
This is why sharpshooters have strong muscles for sucking and why they extract extremely large quantities of xylem fluid to attain the necessary quantities of nutrients to survive and develop. The excess water and salts are excreted in large quantities and may leave salty residues on leaves below. It is well known that the systemic neonicotinoid insecticides are translocated through the xylem, and therefore, a toxic dose is more easily attained by xylem feeders.
More About Systemic Insecticides
Systemic insecticides are taken up by the roots or leaves and translocated to all parts of the plant. They are active against a broad spectrum of economically important pests including aphids, whiteflies, leafhoppers and sharpshooters, rootworms and wireworms, planthoppers, mealybugs, soft scales, thrips and phytophagous mites. Systemic insecticides provide many advantages to ornamental plant producers, including persistence in plant tissues, residues that are less susceptible to environmental degradation or wash-off, no unsightly residues, reduced effects to natural enemies, greater worker safety, reduced plant pathogen transmission and no spray drift if not applied as a foliar application. Systemic insecticides can be applied in many different ways such as seed dressing, seed pilling, soil treatment, granular application, dipping of seedlings, chemigation, soil injection and drenching, furrow application, trunk injections, bulb dipping and basal bark application. Systemic insecticides are associated with five different chemical classes (organophosphate, neonicotinoids, selective feeding blockers, tetronic acid and botanical), each with a distinct mode of action. The active ingredients in systemic insecticides include acephate, azadirachtin, flonicamid, pymetrozine, spirotetramat and the neonicotinoids, which include imidacloprid, dinotefuran, thiamethoxam, chlothianidin and acetamiprid.
Factors that can influence the activity of systemic insecticides are solubility, absorption and translocation, which in turn are affected by plant species, plant age, plant growth rate, environmental conditions, soil/growing medium and physiological variations of plants. Recent research indicates that different plant parts are associated with high or low levels of systemic insecticide concentrations, thereby causing differential mortality depending on where the pests feed. For instance, the lack of effective control against mealybugs may be associated with their feeding behavior, as they tend to congregate on plant stems, whereas systemic insecticides may be primarily located within the xylem of stems where the main transport within the plant occurs. Alternatively, the leaves may be an effective sink for systemic insecticides, and movement of systemic insecticides from the leaf xylem to the stem phloem — where mealybugs feed (fig. 2) — may be reduced or non-existent. Research is ongoing to try to understand the interaction between systemic insecticide movement in the plant and insect feeding behavior, and how these dynamics affect insecticide efficacy.
The toxicity of systemic insecticides persists for a variable period of time depending on the plant, its growth stage, the amount of pesticide applied and where the pesticide is applied. For example, in our studies we found that following a foliar application of dinotefuran to poinsettias, the concentrations of insecticide on the leaves were sufficiently high to effectively remove adult whiteflies that were present on the plants at the time of the treatments. However, as the plant grew, the level of insecticide within leaf tissues declined, and it was ineffective in controlling the emerging nymphal population, which led to a second generation of whiteflies. Conversely, when dinotefuran was drenched into the media, it effectively eliminated the parental adults and both the second and third generations of adults and nymphs. Similarly, in a field trial conducted in a commercial vineyard, imidacloprid was consistently detected in the xylem for up to three months after drench application at concentrations known to be effective at managing populations of the glassy-winged sharpshooter Homalodisca vitripennis (Germar). Presumably this would protect the plant from direct damage caused by piercing-sucking insects that feed in the xylem and from indirect damage caused by the diseases they vector.
The use of systemic insecticides, which are mobile in the plant vascular tissues, is a viable management option with many advantages. Systemic insecticides are generally most effective against piercing-sucking insects that feed on the vascular tissues of plants, including aphids, whiteflies, leafhoppers and mealybugs. However, some systemic insecticides are active on mites, beetles, leafminers and thrips that feed on leaf cellular tissue. In all cases, efficacy depends on the intrinsic toxicity of the insecticide to the pest, application rates, method of application and pest feeding behavior.
James A. Bethke is Farm Advisor for Nurseries and Floriculture, UC Cooperative Extension, San Diego and Riverside Counties.
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