by James A. Bethke
In the last article (you can read it here), I mentioned that true bugs have piercing sucking mouth parts. I thought it would be a good idea to describe what I was talking about, and how it can affect the plants on which they feed. I am going to be somewhat technical below, but I think you will appreciate knowing why there is specific types of damage in your garden or landscape.
Quite simply, a piercing-sucking insect is one that feeds on plants by piercing cells or vascular tissues (xylem and phloem) with their specialized mouthparts and sucking the contents. Due to the direct damage caused by feeding and indirect damage by acting as a vector for many different plant diseases, piercing-sucking insects are some of the most damaging pests in the garden and landscape.
While there is a distinctive mouthpart design for each type of insect, there is a general design for piercing-sucking insects (Fig. 1 above), 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.
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.
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. 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 insecticides are translocated through the xylem, and therefore, a toxic dose is more easily attained by xylem feeders.
Okay, I know some of you will use this article to ease insomnia, but I hope you can see the benefit of knowing what type of insect you are trying to control in your landscape or garden. Further, if you are experiencing plant disease symptoms, it is likely one of these piercing sucking insects are the cause.
James A. Bethke is the Floriculture and Nursery Farm Advisor Emeritus for San Diego and Riverside Counties.
The Floriculture and Nursery Farm Advisor Emeritus conducts applied research, education and outreach programs to improve production and viability of the floriculture and nursery industries in San Diego and Riverside Counties.
Jim's program emphasizes the integrated pest management of major pests of floriculture and nursery production. He collaborates with regulators, growers, and other scientists on advisory committees that set policy based on science.