Integrated Pest Management of Sunflower Insects
This is the third in a series of articles addressing the Integrated Pest Management (IPM) of insects in sun-flower. The introductory article defined and discussed the relevance of IPM and other current terms in agricultural production systems or approaches. The second article examined the importance of knowing pest identities, populations and established economic thresholds. This month, we move on to some of the specific measures used to manage or hold pests in check. We begin with biological control — the use of predators, parasites and diseases to reduce insect pest densities.
Biological control as a management tool dates back more than 1,000 years to when the Chinese used ants to control insect pests of citrus. In the late 19th century, predatory beetles were brought from Australia to control scale insects which were destroying the citrus industry in California. That program, still in effect, showed that insects could be successfully utilized to manage other insects, and it helped biological control become an important scientific discipline.
Biological control is defined as “the intentional manipulation of populations of beneficial organisms (natural enemies) in order to limit populations of pests.” In addition to insect pests, the approach also can be used to manage weeds and diseases.
Agents of biological control include predators, parasites, nematodes and diseases (pathogens). Predator insects, such as lady beetles, consume more than one individual insect (prey) during their development. In contrast, parasites generally lay their eggs on or in the pest species, and their young kill and consume the host as they grow. Usually, only one host is needed by the developing parasite.
Parasites are often small and thus easily overlooked. Parasites are not harmful to humans and tend to be specific to one type of host. Predators often feed on a number of different species of pests. Insect-parasitic nematodes are small worms that attack and kill insects that live in moist habitats. They do not harm animals or plants, and a few species are currently being sold for insect control.
Diseases — caused by fungi, viruses, bacteria and other micro-organisms — also occur among insects. Disease outbreaks usually do not occur unless insect populations are very high or environmental conditions are especially suitable for the disease organism. These pathogens are important in the suppression of pest species; some viruses and bacteria can be manipulated to control specific pests and are commercially available. They do not harm nontarget organisms such as humans, animals, plants or beneficial insects.
Natural enemies do not destroy or eradicate all insect pests; but they may prevent the pest population from becoming too high. In some cases, where the natural enemy becomes an established part of the environment, they do this year after year.
There are three broad approaches to biological control: (1) importation, (2) augmentation and (3) conservation.
When an alien species is accidently introduced and becomes a pest, natural enemies are searched for in the pest’s native home. When located, these beneficial organisms are “imported” to the area where the pest is a problem.
“Augmentation” is the release of large numbers of natural enemies to temporarily increase their numbers in a given area. The natural enemies then seek out and attack the pest insect.
The third approach — “conservation” — is concerned with protecting the natural enemies that are already present in an area. Usually, this means the use of farming practices which are less disruptive to natural enemy populations. Reduced or carefully timed insecticide treatments can provide a major contribution by minimizing the destruction of beneficial organisms.
sunflower is one of just a few culti-vated crops that are native to North America. So insects associated with sunflower have evolved with the plant for many centuries, and many have moved to the cultivated crop to feed and develop.
Insects on sunflower include a mixture of plant-eating species, pollinators and natural enemies. Although hundreds of insects have been recorded on sunflower, only a small number have achieved “pest status.” Indigenous or native natural enemies have been a significant factor in preventing many plant-eating insects from becoming economic pests of sunflower. However, even the insect species that have become pests are subject to attack by numerous natural enemies, including predators, parasites and diseases.
Given our limited knowledge about the species of predators, parasites and diseases that attack the important insect pests of sunflower, the best biological control approach is to utilize farming practices that conserve and protect these beneficial organisms.
The example reveals how information about the natural enemies can be used in understanding their role in the population dynamics of the sunflower pests:
One important pest of sunflower is the sunflower stem weevil. Larvae of this weevil cause crop damage by feeding, developing and overwintering in the stem. Parasitic wasps attack both the egg and larval stages of the weevil, with the most common parasite of the weevil larva being a wasp called Nealiolus curculionis. It also attacks the stem weevil in wild sunflower, indicating it also moved when its host transferred to cultivated sunflower. The immature parasites pass the winter within overwintering or diapausing weevil larvae in the sunflower stalk.
Overall, parasitization by this wasp has increased from the levels reported in the late 1970s and early 1980s. Field densities of adult weevils have been quite variable since those years, but the consistent rates of parasitism suggest that N. curculionis effectively locates its stem weevil host regardless of pest population densities.
The ability of the female parasitoid to find and attack hosts is of paramount importance to the success of a given parasite population. Delaying planting has been an effective cultural control tactic for reducing larval stem weevil numbers in sunflower stalks and thus decreasing damage. Studies also have revealed that the parasite was active and capable of attacking larvae of the sunflower stem weevil in sunflower plantings from three different planting dates.
The successful combination of biological and cultural controls helps the producer further reduce potential damage from this pest. Since the parasite appears to have a consistent impact on the mortality rate of the stem weevil in cultivated sunflower, it is important to utilize cropping practices that also conserve and protect these natural enemies of the weevil.
The other important pests of sunflower — such as sunflower seed weevils, banded sunflower moth and the sunflower beetle — also have their own complex of predators, parasites and diseases feeding and attacking the various pest life stages. Conservation of these beneficial organism will help prevent the pest insects from reaching densities that cause economic losses for the producer.
Biological control is one of the safest methods of pest management avail-able. Natural enemies are not toxic, pathogenic or injurious to humans or wildlife. They’re usually self-perpetuating and, because of their specificity, do not harm nontarget organisms. Natural enemies are not polluting or as disruptive to the environment as synthetic insecticide, nor do they leave residues on food.
There are many factors (e.g., crop, pest complex, environment) that can influence the success of beneficial organisms in reducing pest densities to manageable levels. Thus, in many instances, the biological control method must be utilized in conjunction with other tactics. This is consistent with the IPM approach to pest control, i.e., ensuring that the least-disruptive practices are used since they fit best into sustainable agriculture.
Next Month: We’ll discuss plant resistance and how that approach fits into Integrated Pest Management.
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