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Monitoring Insect Ecology
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MONITORING INSECT ECOLOGY

A. Introduction

1. Why Monitor?

Insect ecology has multiple meanings. However, we will define it for this discussion in the context of a grower wanting to profit from crops. Insect ecology is the ratio of beneficial organisms to possible pests and the relative demographic patterns these groups will experience through time. The insect ecology, or beneficial to pest ratio, can help growers make pest management decisions which will fulfill their objectives (Chapter 1).

Monitoring insect ecology for decision making is a great way to save money and reduce pesticide use. Most pests do not have established economic thresholds for conventional and organic farmers. Therefore, growers must make “gut” level decisions on if the pesticide application indeed saves enough crop to pay for the application plus a little extra. The beneficial to pest ratio can tell a grower if he/she can easily skip a pesticide application without concern or if a pesticide application is ABSOLUTELY necessary. Of course, there are a lot of gray areas in–between.

A decision to use any pesticide is often impossible to pencil out. Growers must ascertain cost-benefit plus externalities such as economic and social costs. The seven testing guideline questions of holistic management are extremely helpful to decide if one should spray (see Ch. 1). The old adage, "if in doubt, spray" is no longer cost effect, socially acceptable, or environmentally safe.

Deke planting habitat plants close–up of habitat plant
Medium–shot of habitat plants long–shot of habitat plants

2. Insect Ecology Concepts and Patterns to Remember

Insect functional groups are the following: key pests, generalist predators, key predators, parasites, key parasites, ants, spiders, hyperparasites, and decomposers. These functional groups increase and decrease depending on food supply, climate, and many other factors. Any functional group may or may not reproduce at a constant, linear rate.

Pest managers often count the numbers of key pest individuals to make a prediction about future infestations. This “counting” method assumes the pest species will continue producing in similar numbers as they had in the recent past. However, many pests can reproduce exponentially if they are in enemy free space. In enemy free space no predators and parasites are present to damper the reproductive success of pests. Managers must observe functional groups in relation to each other to gather insect ecology patterns and to make decisions.

The demographics of the different insect functional groups must be observed and noted as well to make pest management decisions. If we know which life stages exist within the functional groups, we can more easily see patterns which happened in the past and which may happen in the future. For example, noting the presence of only adult lygus and no nymphs along with several key predators such as orius and big-eyed bugs, indicates that the natural enemy complex is controlling the current lygus population.

Pests, even key pests, a small proportion of them, can be an asset to every grower. Pests are food for primary and secondary predators and are an important for the reproduction of many primary parasites. Secondary food sources for beneficials, or insects other than key pests, may be an even greater assets to a grower. Every farm will have different key pests, alternate food sources, and beneficials. Therefore, there are no standard values to determine the actual ideal number of pests that are needed to sustain an effective natural enemy complex on farm. Pests in relation to the other functional groups must be observed through time so that patterns of natural control and reproduction can be determined.

B. Where and What to Monitor

Insect ecology must be observed by sampling and observing the greatest amount of insects at the site. Simply monitoring key pest numbers is insufficient to make decisions. The sample should be taken from the “hot spots” and where insects are in abundance or where pests traditionally first appear. These areas are, ideally, not in the market crop, but in the habitat and/or trap crop areas. The objective when sampling is to observe as many insect functional groups as possible so that a representative ratio can be observed.

C. When to Monitor

Monitor regularly during the growing season and especially at the first sign of a pest outbreak. Some key pests will show up after a certain pattern of warm climate, such as counting degree days and observing codling moth. Other key pests will be most successful during a certain stage of plant growth, such as aphids, mites, lygus and the pepper weevil. Another pattern of some key pests is to show up at traditional, genetic determinate, times such as many of the fruit flies (Tripedids). Microclimates which provide humidity may help determine when some functional groups are present. Ants will create enemy free space by scouting plants for honeydew producers and are a sign to continue monitoring closely.

D. Insect Monitoring Tools

We usually use an insect vacuum to sample insect ecology. We have not found a better way to sample insect ecology over a large area in a short period of time. For example, a sampling of a hot spot area may require vacuuming 50 sucks which takes approximately 10 minutes plus a few minutes observation with the dissecting scope in the lab. We view the insects preserved in 70% ethanol alcohol and then store them for later reference in labeled jars.

Sweep nets work well in cover crops or habitat areas where physical damage to the market crop will not occur. They are great to use as an educational tool in the field. Insects can be viewed by slowly letting them escape from the net. The viewers can see the insects in several life stages and in relation to their prey and other food sources.

We use hands lenses (10X magnification) and head piece magnifying glasses (6X magnification) to view the small insects in the field. However, there is no comparison to viewing the insects preserved in alcohol with a dissecting scope.

Sticky traps are used in perennial hedgerows and in trees and greenhouses. They must be replaced regularly because they get dusty. They catch a lot of flying insects that are attracted to pheromones and colors.

Pit fall traps help managers view the soil surface insect fauna (figure). The ground predatory insects and spiders are particularly important in controlling key pests which have a life stage in the soil.

Large net traps with alcohol bottles called Malaise traps will catch many large flying insects (figure X).

E. Tips for the Recorder

We developed this monitoring log, or data sheet, to help pest managers get a snapshot of insect ecology. Information on insect ecology will aid managers to predict pest pressures. Pest managers must take into consideration all the consequences of their decisions such as cost-benefit, environmental effects, and social effects.

The recorder should not count the species within the functional groups, rather he/she should mark their presence in relation to the other groups and the demography of the functional group. The monitoring is qualitative, not quantitative (see example below).

The recorder should note in the comments what the key pests are for the plants at that particular time. Often the species within each functional groups will change depending on the time of year. For example, western flower thrips may be in the predator functional group if the key pests are mites (thrips are mite predators).

A recorder must have basic entomological identification skills. It may or may not be helpful to know the species; however, the recorder must know what functional group the species fits into. Mouth parts may be used as one clue to help determine which insects are predators or pests.

Reference:

Metcalf, R.L., and Luckmann. Introduction to Insect Pest Management. 1975. John Wiley and Sons. New York.

INSECT ECOLOGY FIELD SAMPLE MONITORING SHEET

EXAMPLE

GROWER NAME: Mr. Hibbits

WHOLE UNDER MANAGEMENT: 100+ acres of walnuts, family farm, processor/handler of walnuts

GROWER GOAL: profit from walnuts

LOCATION OF SAMPLE: down by the river in dead air space on Rancho de la Vina, taken from 10’ up in trees, mainly from the nutlets

DATE: 7–3–98

METHOD OF COLLECTION: D–sVac insect vacuum with 10’ extension, approximately 50 sucks 6–8 trees sampled.

FUNCTIONAL GROUPS

key pests

secondary pests

parasites majority of insects were aphid parasites, some Brochonids (leaf worm parasites), Metioris hyperparasites–not identified

predators (generalists) green lacewing larva, brown lace wing adult, syrphid fly adult

predators (specific) none

spiders (high food–chain generalist) eragony spiders in good numbers

ants (disrupters) none

decomposers unidentified fruit flies

misc. (unknowns) present

COMMENTS ON PAST AND FUTURE ECOLOGY

Walnut aphids were present before, however the parasites have eliminated them. Food for generalist predators seems scarce with the absence of aphids; however, the generalist are present in large numbers. The need for bloom now is essential because there is no honeydew production. Ants are not disrupting natural biological control nor acting as predators. No husk fly or codling moth observed in the samples or in the traps. We should continue to monitor closely for husk fly and codling moth. Would like to see a low density of aphids as food for the generalists.

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