Understanding insect and mite pest 'leftovers'

Features - Pest & Disease

Honeydew, frass and cast skins can help growers assess pests causing plant damage.

November 29, 2017

Fig. 1. Honeydew on the top of a leaf Fig. 2. Black sooty mold on upperside of leaf Fig. 3. Ants tending scales. Ants protect scales from natural enemies such as parasitoids and predators. Fig. 4. Small caterpillar frass associated with the diamondback moth larva
Photos: Raymond Cloyd

Insect and mite pests cause direct damage to greenhouse-grown horticultural crops, including vegetables, by feeding on plant parts including leaves, flowers, and fruits. Direct damage symptoms may be used to determine what insect or mite pest is responsible. However, insect and mite pests may leave behind subtle reminders or “leftovers,” which not only provide evidence of presence, but can also be used to help determine what pests are associated with plant damage.

To properly diagnose an insect or mite pest problem on horticultural crops, it is generally best to visually observe the insect or mite pest causing the problem. However, sometimes insect or mite pest “leftovers” may be a way to assess the actual pest causing the problem. Most insect pest “leftovers” are a direct result of feeding on plants such as honeydew or fecal material (also referred to as frass or insect poop); however, some insect and mite pests may leave evidence based on the normal physiological process of molting (cast skins) as a result of increasing in size or transitioning into another life form. These “leftovers” may be found on the top of leaves or on leaf undersides, depending on the insect or mite pest.


Most insects with piercing-sucking mouthparts, such as aphids, whiteflies, and mealybugs, will produce honeydew as evidence of their presence. Honeydew is a clear, sticky liquid commonly found on the top of leaves (Fig. 1). The reason why these insect types produce honeydew — sometimes in very large quantities — is that insects with piercing-sucking mouthparts require protein (in the form of amino acids) for development. However, to obtain the required amount of nutrition, these insects must consume large amounts of plant fluids. Nonetheless, plant fluids contain an assortment of other materials in larger quantities than amino acids, and the extra material (in which there is plenty) is subsequently excreted as honeydew.

Honeydew is a problem for several reasons. First, honeydew serves as a substrate for black sooty mold (Fig. 2), which can coat leaves and consequently reduce a plant’s ability to manufacture food via photosynthesis. Second, honeydew attracts ants that protect insect pests from their natural enemies (parasitoids and predators) as well as tend to and spatially distribute insect pests on plants (Fig. 3).

Fig. 5. Large caterpillar frass affiliated with the tomato hornworm Fig. 6. Caterpillar frass on leaf associated with two caterpillar larval stages Fig. 7. Thrips frass on leaf underside inside sunken tissue Fig. 8. Shore fly frass on leaves
Photos: Raymond Cloyd

Fecal material

Insects with chewing mouthparts such as caterpillars may leave evidence of their presence by means of fecal material, also called frass, which is similar to the excretion (honeydew) of piercing-sucking insects. Caterpillars, in general, tend to consume more plant material than what is actually needed for development; consequently, the excess is excreted out as fecal material. Frass may come in various sizes (small and large). For example, the frass in Fig. 4 is considered “small caterpillar poop” and is associated with the diamondback moth (Plutella xylostella) caterpillar, while the frass shown in Fig. 5 is considered “large caterpillar poop” and is affiliated with the tomato hornworm (Manduca quinquemaculata). Furthermore, the texture of frass can be used to determine the particular life stage (young vs. old larvae) involved. For instance, the frass shown in Fig. 6 indicates two caterpillar larval stages fed on the same plant leaf. The frass on the right that is more fluid-like and resembles split pea soup. It is associated with the young caterpillar. The frass on the left, which is more textured and hardened, is affiliated with the older, more mature caterpillar. In addition to caterpillars, other insects, such as thrips, shore flies, and leafminers, may provide evidence of their presence by means of frass. Thrips create sunken tissues on leaf undersides during feeding, and small darkened fecal material can be observed in these sunken areas (Fig. 7). Shore fly adults leave black fecal deposits on the top of plant leaves (Fig. 8) that can impact the aesthetic quality of plants and their subsequent marketability. Leafminer larvae cause direct plant damage by creating serpentine tunnels in plant leaves, but they also excrete fecal material that may be visible within the tunnels (Fig. 9).

Fig. 9. Leafminer larval frass inside tunnel Fig. 10. Aphid cast skins on top of leaf Fig. 11. White cast skins affiliated with aphids molting on top of leaf Fig. 12. Twospotted spider mite white cast skins on leaf underside Fig. 13. Leafhopper cast skins on leaf underside
Photos: Raymond Cloyd

Cast skins

Aphids will leave behind cast skins on plant foliage as evidence of molting (Fig. 10). These cast skins may be mistaken for whiteflies or dead aphids (Fig. 11). In addition to aphids, plants heavily-infested by the twospotted spider mite (Tetranychus urticae) or leafhoppers may have cast skins present on leaf undersides (Figs. 12 and 13).

In conclusion, this article has described various insect and mite pest “leftovers” including: honeydew, fecal material, and cast skins that may be used to help identify the pest causing plant problems. Although it is best to actually see the insect or mite pest in order to make an accurate diagnosis, using insect or mite pest “leftovers” is another way to possibly determine the culprit responsible for the problem.

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