Integrated Pest Management of Sunflower Insects
This is the second in a series of articles addressing Integrated Pest Manage-ment (IPM) of insects in sunflower. In the introductory article (August/September issue of The Sunflower), IPM and other currently used terms for agricultural production systems or approaches were defined and their relevance to sunflower production discussed.
We now move into a description of the different aspects of knowledge that are important to a successful pest management plan for the crop. As noted in the initial article, IPM incorporates all appropriate methods from many scientific disciplines into a systematic approach to optimize pest control. All available tactics are considered. Knowledge about the pest, its numbers and the established economic thresholds are required. These aspects comprise the topic of this month’s article.
Pest Identification. Identifying the pest damaging the crop — or those with potential to have an economic impact — is an important first step. The appropriate pest management strategy cannot be decided upon without knowledge of which insect is present in the field.
Correct identification can be difficult because of possible differences in the life stages of insects. Some species, such as grasshoppers and crickets, do not change much physically as they develop; whereas others, such as moths and beetles, have a very different appearance when they are immature (e.g., caterpillars) as compared to when they are adults (e.g., moths).
The different stages can also produce different degrees of injury to the plant. Some adults, such as moths, feed only on nectar or moisture on the plants. Others, such as grasshoppers, feed on plant tissue as soon as they hatch from the eggs through the adult stage. Certain beetles may be pests at all stages or only as larvae.
Identification of a particular pest may be determined by noting the type of damage to the crop, since insects are often specific to one portion of the plant (leaves, seeds, stem). It is also important to know the adult stage of many of the insect pests because control measures may have to be initiated before many eggs have been laid or the immature stages have begun feeding. This is especially important with the red sunflower weevil and the sunflower stem weevil, as those larvae cannot be controlled after they have entered the plant and become protected within the stem.
Identification of the pest insect allows the producer to check the published information that is available on the biology of the pest, its importance, the type of damage it produces — and the different strategies available to reduce losses caused by its feeding.
Economic Threshold. In addition to knowledge about the identification of the insect, the relationship between pest numbers, amount of damage and the cost of management must be considered. Some of the recognized terms used in this regard are economic injury level, economic threshold and treatment threshold.
The bottom line is this: Will the number of insects in the field result in feeding injury that causes an economic loss to the crop greater than the cost of managing the pest?
It may be that if the pest population present is below a specific level, it would cost more to manage than the dollar return from implementing the tactic. The break-even point is called the “economic injury level.” Numbers of pests above that level are worth managing, because the return will be above the cost of initiating a management tactic.
Because there usually is a time lag between the determination of the density of pests that result in injury and the implementation of management strategies, the “economic threshold” is lower than the economic injury level. The use of an economic threshold discourages the producer from treating when not warranted, yet allows him to treat when it is warranted. This saves money, since control measures — often meaning the use of pesticides — are initiated only when numbers of pests exceed the threshold. It has been estimated that monitoring of pests and the use of economic thresholds can reduce pesticide usage by 30 to 50 percent, thereby increasing the producer’s return.
The establishment of an economic threshold is often a dynamic process. There are, with a number of insects, several factors to consider. For instance, with the sunflower beetle, the threshold varies depending upon the stage of plant growth, i.e., larger plants with more leaf surface area can tolerate greater feeding injury before there is any economic loss to the plant.
This brings up another point: the difference in the location where the pest is feeding. Direct pests — those that feed on the portion of the plant which is utilized (seeds) — often have lower economic thresholds than do indirect pests that attack stems or leaves. Plants often have more leaf or stem tissue than is required for seed development, so they can thus tolerate a reduction in plant material before economic losses in yield occur.
The genetic makeup of plants also varies and can be utilized in breeding for resistance to damage. Thus, plants with resistance or tolerance to damage by a certain insect will have a higher economic threshold than those without this resistance or tolerance.
There also are a number of additional factors which must be considered in determining the economic threshold for the pest. Among them are the crop’s projected market price, cost of the management and the plant population. Another key ingredient in the decision is the standard set by the marketplace for crop damage. That ingredient is clearly evident in the different amounts of infested seed tolerated in oil-type versus confection or nonoil sunflower seed.
Often, economic thresholds are set for the adult stage of the pest — a stage that may not cause the damage. Directing management strategies at the adult has the purpose of reducing successful egg laying. A prime example is the sunflower stem weevil. Once its eggs have been inserted into the plant stem, control is not very effective. Those eggs — and their resulting larvae — are protected within the stem tissue.
The same situation occurs with the red sunflower seed weevil, so its thresholds also are set on the basis of adult numbers. In the case of the banded sunflower moth, however, the economic threshold is established for the adult, but control is directed at the larvae before they move into the seeds to feed.
The sunflower beetle has thresholds for both the adult and larval stages of the insect, since each can cause economic damage if densities are high enough.
Monitoring or Sampling. Good management practices require that the crop be monitored for a variety of reasons. Checking for insects is just one part of this process. Of course, a past history of pest problems would make this practice even more necessary.
The identity of the particular insect will determine when and where sampling should take place in the field. This may mean the crop needs to be surveyed only at a specific time of year or when the plant is at a certain growth stage.
The objective of field surveys is to determine if the numbers present can be tolerated or if they exceed the economic threshold, thereby justifying corrective measures. Because most crops are attacked by a variety of pests, monitoring really should occur at regular intervals during the season.
The area of the plant to check is determined by the pest of concern and guidelines established by research into the insect’s biology. An understanding of the biology and movements of the pest also establishes the locations to be monitored within the field. Because they often migrate from last year’s fields, many sunflower insects tend to congregate initially on field margins; so surveys of only these locations would produce an incomplete picture of the true situation. This could lead to a decision that manage-ment should be initiated when, in fact, if other areas of the field also were checked, the average density might be very low. Thus, field scouting strategies usually emphasize moving well into the field and surveying plants or groups of plants at a number of widely spaced locations.
The pheromone trap is a tool used to monitor for pests in a number of crops. The trap is baited with a material based on the chemical used by the female to attract the male for the purpose of mating. Once research has determined the relationship between (1) trap counts, (2) the projected number of pests that will be in the field and (3) damage to the crop from that density of insects, that information may be used for pest management decisions. Guidelines have been established for using traps for sunflower moth management decisions in the High Plains, and research currently is being conducted to provide this information for the banded sunflower moth.
In summary, then, the foundation of pest management consists of (1) correctly identifying the pest, (2) monitoring the crop for the pest of concern and (3) utilizing established economic thresholds to determine whether any action is needed.
In the January issue of The Sunflower, we will discuss the pest management approach known as “biological control.” This method employs the action of parasites, predators and diseases against insect pests.
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