Sampling and Monitoring

ENTOMOLOGY
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Sampling and Monitoring for Pests and Their Natural Enemies
Various methods have been devised to sample, or estimate the numbers of, arthropods on trees and shrubs. Many of these procedures were first developed for use in agricultural crops, but several have resulted from investigations on landscape ornamental pests. The objectives of sampling or monitoring are to detect the presence or absence of pests; quantify abundance of pests and their natural enemies; and follow the progress of an arthropod population through time by regular, periodic sampling. The goal of monitoring is to reach a decision as to whether, or when, a pest population requires control action. All sampling procedures share certain characteristics:

They use a common sampling unit, such as leaves, terminals, beats, or minutes.

The unit chosen must be consistent with the feeding habits of the pest population under observation. Do not select leaves as the sampling unit for scale insects that occur predominantly on twigs. Similarly, do not count aphids on the younger leaves if they occur mostly on the older leaves.

The number of samples taken must be adequate. What is "adequate" must be determined on a case-by-case basis, and by time and equipment constraints. Pests are seldom distributed uniformly over a tree or shrub. Similarly, every tree in a group of trees of that species will not be infested to the same degree. Generally, the number of samples taken from each plant at each interval is held constant over the entire sampling period, and over the entire group of plant samples.

The sampling procedure must be standardized. It helps if the same person does all the samplings. If two or more persons are involved, they should check one another in a preliminary sampling exercise to determine that their sampling methods and results are the same.

Written records of arthropod counts are kept by date, location, and person sampling, with a brief description of procedures used.

Because some insects and mites are quite small, the person sampling or monitoring pests and their natural enemies should carry and use a 10x hand lens.
Methods of Sampling
Counting Insects on Plant Parts
Each sample is pruned or pinched from the plant and the number of arthropods present is counted immediately without magnification, or under magnification using a hand lens. The number of samples taken from each plant usually ranges from five to 25.
Variations of this method include estimating pest numbers in 10s or 100s, when counts are high, or taking more samples (around 100) and recording only whether pests are present or absent on each unit.
Counting insects on plant parts is effective for sampling aphids, spider mites, and other arthropods that do not readily fly or drop from the plant when the sampling unit is removed. Sometimes only the immature insect stages are counted, if the adults fly readily when the sample is taken.
Time Counts
The person responsible for sampling counts the number of insects seen during a one- or two-minute visual search of the plant. Several such timed searches are made in different parts of the same plant (if large). This procedure is useful for large insects, such as caterpillars, or for egg masses of insects on tree trunks or limbs. The plant is not damaged, and the insects counted are available for re-counting at the next scheduled sampling. Because it is difficult to count insects and keep track of time simultaneously, two persons are required for best results, unless an electronic alarm watch can replace the second person. The time-count is not a useful sampling method if the insect population is high because the insects can't be counted fast enough.
Beating Samples
A sampling tray is held horizontally just beneath plant foliage, and the foliage above is struck sharply a standard number of times (2 to 5) with a short stick or the other hand. Arthropods falling to the tray are immediately counted and then shaken off. This process is repeated several times around the periphery of the plant. An attempt is made to standardize the density of foliage beaten. The tray may be one square foot in surface area, or as small as a five- or six-inch circle (pads of paper or plastic disposable pie plates have often been used).
The trap surface is usually white to contrast with the insects being counted. This procedure has been used to sample such pests a psyllids, certain aphids, plant bugs, and spider mites.
Fecal Pellet Collections
Lepidopterous larvae, like catalpa caterpillars, oakworms, and datanas, produce relatively large, solid, dark, fecal pellets, most of which fall to the ground beneath the plant. Using three to five shallow pans, paper cups, or sticky cards deployed beneath the foliage of infested trees or shrubs, counts of pellets give an estimate of the larval population in the tree or shrub. The size of the individual pellets indicates whether the caterpillars are young or more mature. Collection traps are usually deployed for a 24-hour period each week. Traps are put out at times when no rain or sprinkler irrigation is expected within the 24-hour period.
Some tent-making and leaf-rolling caterpillars tend to deposit their fecal pellets such that few fall to the ground. The fecal trap method is not useful for estimating the number of these pests.
Attractant Traps
Devices containing synthetic or natural attractants and that physically trap the insects attracted to the device are useful for sampling several moth and beetle pests of ornamentals. They will trap only the mobile adult stage and, in the case of most sex pheromone traps, only the male of the species.
Sampling and Monitoring Natural Enemies
When sampling for pests, the person sampling should also look for:

Predators, such as ladybird beetle adults and larvae, syrphid fly larvae, lacewing larvae, and spiders.

Evidence of parasitism, such as aphid "mummies," darkened greenhouse whitefly pupae, and scale insects with exit holes of the parasites.

Signs of insect diseases, such as blackened, dead caterpillars and dead, discolored aphids infected with fungi.

The impact of biological control can be estimated by counting the number of natural enemies per sampling unit, then calculating the ratio of affected pests to healthy ones. If signs of biological control are apparent, delay any insecticide treatment. Sample again in a week for pests and natural enemies. If the natural enemy population is increasing faster then the pest population, consider no insecticide treatment. If natural enemies are still active but the ratio has increased in favor of the pest, and pest number or plant unsightliness are approaching unacceptable levels, then consider the following measures when an insecticide is applied:

If possible, use a material that is selective to preserve natural enemies (e.g., Bacillus thuringiensis for caterpillars), or a material that is minimally disruptive to natural enemies (e.g., insecticidal soap, horticultural oil, or systemic insecticides).

If the use of selective or minimally disruptive materials is not possible, treat only the plants that are in immediate need of an application, leaving the untreated plants to serve as a reservoir for natural enemies.

Various honeydew-producing insects, especially soft scales and aphids, are protected or guarded by ants, whose only interest is the honeydew food source. Ants interfere with parasite and predator activity. Therefore, the presence of ants on plants infested by honeydew-producing insects should suggest control of the ants by insecticidal treatment of the base of the plant only. This is another kind of selective use of an insecticide. Sticky bands may be used instead of insecticide treatment, e.g., for walnut datana and spring cankerworm adults.

References:
Pinkston, K. 1994. Insects and Mites Affecting Ornamentals. Oklahoma Cooperative Extension Service, Oklahoma State University. p. 53-54.

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