Hemiptera plant. Phloem sap, which has a higher concentration of sugars and nitrogen, is under positive pressure unlike the more dilute xylem sap. Most of the Sternorrhyncha and a number of Auchenorrhynchan groups feed on phloem. Phloem feeding is common in the Fulgoromorpha, most Cicadellidae and in the Heteroptera. The Typhlocybine Cicadellids specialize in feeding on non-vascular mesophyll tissue of leaves, which is more nutritious than the leaf epidermis. Most Heteroptera also feed on mesophyll tissue where they are more likely to encounter defensive secondary plant metabolites which often leads to the evolution of host specificity. Obligate xylem feeding is a
Coccus hesperidum all the nutrients they need, the scale insects ingest large quantities of sap. They then secrete the excess sugary fluid as honeydew. This is attractive to ants which often tend the scale insects, driving away predators. The brown soft scale does not normally kill the host plant, but the loss of sap probably causes it to grow more slowly and crop less heavily. The main disadvantage to the host is the sooty mould which grows on the honeydew. This reduces the area of leaf available for photosynthesis, and spoils the appearance of the plant, its flowers and fruits. Traditionally, brown