Integrated Pest Management of Sunflower Insects
This is the fifth in a series of articles addressing the Integrated Pest Management (IPM) of insects in sunflower.
As noted in earlier articles, the ideal management strategy utilizes techniques that require low input costs, are cost-effective and avoid negative impacts to the environment. Cultural controls can meet these criteria. The ideas inherent in this method of pest control attempt to make the environment unfavorable to the pest by reducing its growth, reproduction or survival.
Cultural controls often use farming practices already associated with crop production. Thus, they usually require no additional outlay for equipment, lack deleterious side-effects and are generally quite simple, effective and inexpensive to implement.
However, cultural control measures do have to be applied early; they require detailed knowledge of the particular crop and pest’s biology; and control of the pest is not always complete. Also, attention must be given to recognizing the weak links in pests’ life cycles.
These tactics are generally designed to prevent buildup of the pest rather than relieve an already-present insect problem; so timing becomes very important. Since cultural control methods frequently need to be employed long before the pest or its damage becomes apparent, it often can be hard to quantify the technique’s effective-ness. Sometimes, for example, management is directed at a stage of the insect which is not actually causing the damage, rather than at the pest stage of that insect.
Finally, some cultural control practices are more effective if used across a wide geographic area and combined with other IPM tactics.
There are numerous cultural control methods. Among the most common would be: sanitation, altering planting dates, crop rotation, tillage, trap crops, plant spacing or population, and interplanting.
Sanitation refers to the removal of pest breeding or overwintering sites in order to reduce the number of insects and prevent future damage to crops. The total destruction of crop residue is not always necessary or even desirable, since the impact on soil erosion also must be considered.
Altering or adjusting a crop’s planting date can translate into growing the crop during a period when the pest is not present — or at least avoiding the overlap of the vulnerable stage of plant development with the time of year when the insect is most abundant.
Crop rotation helps reduce overall pest densities over time through the planting of crops which are not subject to attack. Thus, there are one or more seasons when vulnerable plants are not available for insect feeding or breeding. This tactic is most effective for insects that have few alternate hosts or those which cannot move over long distances.
Tillage destroys crop residue harboring overwintering pests by (1) burying the insects to prevent their subsequent emergence or (2) bringing them to the surface and reducing their survival rate.
Trap cropping has been used to lure insect pests away from the main crop and into a small area where they are more easily controlled. This prevents the pests from moving into the crop which is being protected.
Crop spacing can impact pest damage by affecting the growth of the plant, the behavior of the pest as it searching for its food or site to lay eggs, or the effectiveness of natural enemies attacking the pest.
The use of strip cropping or inter-planting has, in some crops, been shown to reduce pest densities and create a habitat that builds up populations of beneficial predators and parasites.
A number of sunflower date-of-planting studies have indicated the potential of planting date variation for reducing damage from the crop’s insect pests. For example, later planting dates have reduced sunflower stem weevil populations in plant stalks and thus subsequent lodging. Seed damage due to banded sunflower moth larval feeding also has been lessened when planting was delayed. Delayed planting will usually avoid the first (and major) emergence of the sunflower midge (although if conditions are favorable, later infestations can be severe as well). In the High Plains of Texas, sunflower planted early or late will avoid a bloom period in early July when the highest densities of sunflower moths occur. Finally, seed damage from the red sun-flower seed weevil has been lower when sunflower has been planted early.
The manipulation of planting dates is a technique requiring knowledge of the biology of the pest, since its effectiveness depends upon not having the preferred plant growth stage available when insect pest numbers are highest. Unfortunately, the life cycles of various sunflower insects are different; so a single planting date strategy will not work for all the pests that attack sunflower.
How can tillage impact some sunflower insects? In South Dakota studies, fall or spring plowing was shown to reduce adult emergence of the red sunflower seed weevil. In Texas, tillage increased the mortality of overwintering larvae of the long-horned sunflower stem girdler.
In both the Northern and Southern Great Plains, however, plowing sunflower stalks proved ineffective in reducing survival of the sunflower stem weevil. The stem weevil larvae are protected in the woody portions of the stalk not broken up by the plow. Adult emergence the following spring can be reduced, though, if the stalks are buried beneath the soil surface.
Recent studies in North Dakota and Minnesota have shown that trap cropping has promise in the management of the red sunflower seed weevil. The planting of an early flowering border of sunflower around a larger sunflower field serves as a trap for the adult red seed weevils, which are attracted to the trap rows and concentrate in the flowering heads.
These trap rows are then treated with a chemical which may effectively reduce weevil populations to the point where there’s no need to spray the entire field. The result would be a much lower cost of insecticidal control.
Plant populations will affect both the sunflower stalk diameter and the percentage of plants lodged by the sunflower stem weevil. The larvae of this insect overwinter in the stalks by constructing chambers in the stem. These chambers weaken the plant; and, if weevil numbers are high, the plant can lodge prior to harvest.
Studies have shown that the density of plant stand has no effect on the numbers of weevil larvae within the stalks. However, as plant population is increased, individual stalk diameter is reduced and lodging increases. It’s important to maintain the structural integrity of the sunflower stalk by keeping its diameter large enough to withstand lodging. This can be achieved by reducing the plant stand. The result will be larger-diameter stalks, no change in insect levels within stalks — but decreased losses due to lodging.
The techniques used in cultural control basically consist of modifications in timing or in the manner of performing necessary functions in the production of the crop. Thorough biological research is required for such controls to work correctly. Also needed is an understanding of the basic life cycle of the pest and its interaction with crop growth and the environment.
Sometimes the slight insect population reduction due to cultural practices will delay the rise of the pest to economic levels, thereby reducing or eliminating the need for more-costly management tactics (such as chemical applications). Using cultural controls may not always provide dramatic results — and it may require a combination of additional IPM practices if benefits are to be maximized. Nonetheless, cultural controls can be an important tool for protecting the crop and minimizing damage from insect pests.
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