To properly diagnose an insect problem on ornamental plants, it is generally best to visually see the organ-ism causing the problem. However, sometimes it is also possible to rely on “insect leftovers” as a means to determine the cause of the problem. Insects can leave subtle reminders or remains, which indicate that they have been there. Most insect leftovers are a direct result of insects’ feeding on plants; however, they may leave evidence from the normal physiological process of molting as a result of getting larger or changing into another life form. Aphids, for example, leave white cast skins as evidence of molting. These cast skins may be mistaken for whiteflies or dead aphids.
Many insects with piercing–sucking mouthparts produce a clear, sticky liquid called honeydew as evidence of their presence. The reason these types of insects produce honeydew (sometimes in large quan-tities) is that they normally require protein (in the form of amino acids) for development. However, to obtain their normal requirement, they must consume large amounts of plant sap (similar to trying to get the wheat from the chaff). Plant sap contains an assortment of other materials in larger quantities than amino acids. The extra (and there is plenty of it) is then excreted in the form of honeydew.
Honeydew is a problem (other than sticking to cars and to the bottom of shoes) for several reasons. First, it can attract stinging insects (for example, wasps, hornets, and yellowjackets), which may lead to an increased incidence of people getting stung (which is different than paying for gasoline). Second, it attracts carpenter ants, which protect piercing–sucking insects such as aphids from natural enemies (for example, parasitoids and predators). Third, it is an excellent growing medium for black sooty mold fungi, which may reduce the plant’s ability to manufacture food. Chewing insects, especially caterpillars, may leave evidence of their presence in the form of fecal material or frass deposits (the technical term is “caterpillar poop”). This is the excess, similar to piercing–sucking insects, that is excreted, as caterpillars tend to consume more than they can handle (elephants also do this, so we should be thankful that elephants are not an ornamental pest because skid loaders would be standard pest-management equipment). For example, gypsy moth can consume 1 square foot of leaf surface, which results in the production of tremendous quantities of frass (as the old saying goes, “you may be up to your ____ in frass”). The presence of frass makes it difficult to enjoy picnics, especially when you need an umbrella, not for the possibility of rain, but to protect yourself from frass droppings.
Insects such as lace bugs and thrips tend to leave black, hardened fecal deposits on the undersides of leaves. This type of insect leftover is characteristic of these insects and helps in identification.
Many wood-boring insects leave very noticeable leftovers when they infest a plant. This occurs when the larvae are tunneling within the plant or when adults use their chewing mouthparts to create emergence openings. An excellent way to determine if wood-boring insects are the problem is to look for the presence of sawdust-like deposits (also known as wood shavings) at the base of plants or below entry sites.
These are just a sample of insect leftovers from some general insect groups. So, although it is best to have the actual insect causing the problem to make an accurate diagnosis, using insect leftovers is another way to possibly identify the problem.