Pest Management Influences on Natural Enemies
Excerpt from Fruit Crop Ecology and Management, Chapter 2: Managing the Community of Pests and Beneficials by Larry Gut, Annemiek Schilder, Rufus Isaacs and Patricia McManus
Management practices change the dynamics of the community of pests and natural enemies within the crop. The positive effects like reduced pest numbers and increased yields are obvious. Certain management decisions, however, can have unintended impacts on the community.
Impact of cultural practices
Fire blight of apple and pear was once considered a sporadic disease that usually could be managed by combining cultural and chemical methods. Since the early 1990's, however, fire blight has plagued apple growers to a degree previously unknown. Indeed, modern fire blight epidemics have been an economic disaster.
The "new face" of fire blight has resulted from several changes in the apple crop habitat. Genetic, physical, and cultural factors have interacted to create ideal conditions for growth and spread of the fire blight pathogen:
- The number of trees planted per acre has increased dramatically. This means that fire blight can move more easily from tree to tree.
- More acres are being planted to highly susceptible cultivars, including Braeburn, Fuji, Jonathan, and Rome.
- Size-controlling rootstocks, many of which are highly susceptible to fire blight, are used to achieve high-density plantings.
- Trees are being pushed to bear earlier and training systems are being adopted that are very different from the way apple trees grow in nature. These modern orchards have a high density of apple tissues such as flowers and young shoots that are very susceptible to fire blight. Traditional, low-density orchards also have susceptible tissues, but they are interspersed with a lot of older, woody tissues that are much less favorable to the growth of the fire blight pathogen.
- New tools are being developed to manage fire blight on susceptible varieties in high-density plantings. For example, there are a few size-controlling rootstocks that are relatively resistant to fire blight. Certain plant growth regulators reduce vigorous shoot growth and thereby reduce shoot susceptibility to fire blight. Also, larger trees planted at lower densities will not have the production potential of the more modern orchard systems, but they will be more likely to survive fire blight long enough to yield a crop.
Pesticide impacts on the community
Although pesticide sprays are generally targeted against one or a few pest populations, they often influence other pest and non-pest species. Some insecticides are very toxic to predators and parasitoids. Destroying these natural enemies often results in target pest resurgence or secondary pest outbreaks. Some pesticides have a greater impact on the natural enemies than the target pest. Target pest resurgence can result when the unfavorable ratio of pests to natural enemies permits a rapid increase or resurgence of the pest population. For example, biological control of twospotted spider mite by predatory mites is common in many fruit crops. Insecticides that are applied for control of pest mites and insects are often highly toxic to predatory mites. Some pest mites survive the spray, but most predators are killed. The population of twospotted spider mites is able to quickly rebound, reaching economically damaging levels before its natural enemy can re-colonize from unsprayed areas.
Pre-plant fumigation of the soil is often used for control of black root rot in strawberry. However, if black root rot pathogens are re-introduced in fumigated soil with infected planting material, the disease comes back with a vengeance, presumably because competitive organisms in the soil have been eliminated by the fumigation.
A secondary pest outbreak occurs when a pesticide that was applied to control one pest kills the natural enemies that were keeping a second pest population in check. For example, a complex of predators can be helpful in keeping aphid populations from reaching damaging levels. The broad-spectrum insecticides that are used to control key pests are highly toxic to these predators. As a result, applying one of these insecticides often leads to secondary outbreaks of aphid populations. Pesticide applications can also impact beneficial microbes leading to increased plant disease problems.
Secondary pest problems are not always associated with the destruction of natural enemies. Control of codling moth by mating disruption entails releasing enough sex pheromone into orchards to interfere with mate location, reducing reproduction and subsequent larval infestations. However, using this highly specific tactic in place of broad-spectrum insecticides can also have a significant impact on other potential pests. In disrupted orchards, leafrollers that were kept at non-damaging levels by broad-spectrum insecticides are now only suppressed by natural enemies. Unless natural controls provide sufficient suppression, leafroller populations will increase, sometimes reaching damaging levels.
In similar fashion, minor diseases that are not normally a problem can become important when major diseases are controlled. Since pathogens often compete for space and nutrients, removing one pathogen with a fungicide may benefit other pathogens that are not affected by that particular fungicide. An example of this is the increase in Alternaria infections of blueberry fruit in fields that have been treated with a fungicide against anthracnose fruit rot.