4 Managing Soils To Minimize Crop Pests
AGRICULTURAL PRACTICES THAT PROMOTE HEALTHY SOILS are a pillar of ecologically based pest management. Good soil management can improve water storage, drainage, nutrient availability and root development, all of which may, in turn, influence crop-defense mech- anisms and populations of potential beneficials and pests.
In contrast, adverse soil conditions can hinder plants’ abilities to use their natural defenses against insects, diseases, nematodes and weeds. Poor soils can cause plants to emit stress signals to potential attackers, height- ening the risk of insect damage. For more information about improving your soil quality, see Building Soils for Better Crops, 2nd Edition (Resources, p. 104).
Healthy Soils Produce Healthy Crops
A healthy soil produces healthy crops with minimal amounts of external inputs and few to no adverse ecological effects. It contains favorable bio- logical, physical and chemical properties.
A biologically healthy soil harbors a multitude of different organisms — microorganisms such as bacteria, fungi, amoebae and paramecia, as well as larger organisms like nematodes, springtails, insect larvae, ants, earthworms and ground beetles. Most are helpful to plants, enhancing the availability of nutrients and producing chemicals that stimulate plant Growth.
Among the vital functions of soil organisms are:
A healthy, biodiverse soil will support high levels of potentially benefi- cial soil organisms and low levels of potentially harmful ones. A soil rich in fresh residues — sometimes called particulate or light fraction organic matter — can feed huge numbers of organisms and foster abundant bio- logical activity.
A soil’s physical condition — its degree of compaction, capacity for water storage and ease of drain- age — is also critical to soil and plant health. Good soil tilth promotes rain- fall infiltration, thereby reducing run- off and allowing moisture to be stored for later plant use. It also encourages proper root development.
When aeration and water availabil- ity are ideal, plant health and growth benefit. For example, crops growing in friable soils with adequate aeration are less adversely affected by both wet and dry conditions than those growing in compacted soils. Soils with good physical structure remain sufficiently aerated during wet periods, and — in contrast to compacted soils — they are less likely to become physical barriers to root growth as conditions become very dry. Organic matter im- proves aeration by promoting the ag- gregation of soil particles. Secretions of mycorrhizal fungi, which flourish in organic matter, also improve a soil’s physical properties.
Among the important chemical determinants of a soil’s health are its pH, salt content and levels of avail- able nutrients. Low quantities of nutrients, high levels of such toxic elements as aluminum and high concentrations of salts can adverse- ly affect the growth of your crops. Healthy soils have adequate — but not excessive — nutrients. Excessive available nitrogen can make plants more attractive or susceptible to insects, and overabundant nitrogen and phosphorus can pollute surface and groundwater. Well-decomposed organ- ic matter helps healthy soils hold onto calcium, magnesium and potassium, keeping these nutrients in the plants’ root zone.
The biological, physical and chemical aspects of soils all interact with and affect one another. For example, if your soil is very compact, it will have few large pores and thus will be less hospitable to such organisms as springtails, mites and earthworms. In addition, its lower levels of oxygen may influence both the forms of nutrients that are present and their avail- ability; under anaerobic conditions, for instance, significant quantities of nitrate may be converted to gaseous nitrogen and lost to the atmosphere.
Managing Pests With Healthy Soils
Healthier soils produce crops that are less damaged by pests. Some soil- management practices boost plant-defense mechanisms, making plants more resistant and/or less attractive to pests. Other practices — or the favorable conditions they produce — restrict the severity of pest damage by decreasing pest numbers or building beneficials. Using multiple tactics — rather than one major tactic like a single pesticide — lessens pest dam- age through a third strategy: it diminishes the odds that a pest will adapt to the ecological pest management measures.
Practices that promote soil health constitute one of the fundamental pillars of ecological pest management. When stress is alleviated, a plant can better express its inherent abilities to resist pests (Figure 2). Ecological pest management emphasizes preventative strategies that enhance the “immunity” of the agroecosystem. Farmers should be cautious of using reactive management practices that may hinder the crop’s im- munity. Healthier soils also harbor more diverse and active populations of the soil organisms that compete with, antagonize and ultimately curb soil-borne pests. Some of those organisms — such as springtails — serve as alternate food for beneficials when pests are scarce, thus maintaining viable populations of beneficials in the field. You can favor beneficial organisms by using crop rotations, cover crops, animal manures and composts to supply them with additional food.
In southern Georgia, cotton and peanut growers who planted rotation crops and annual high-residue winter cover crops, then virtually eliminated tillage, no longer have problems with thrips, bollworms, budworms, aphids, fall armyworms, beet armyworms and white flies. The farmers report that the insect pests declined after three years of rotations and cover crops. They now pay $50–$100 less per acre for more environmentally benign insect control materials such as Bacillus thuringiensis (Bt), pyrethroids and/or insect growth regulators.
In their no-till research plots with cover crops and long rotations, Uni- versity of Georgia scientists haven’t needed fungicides for nine years in peanuts, insecticides for 11 years in cotton, and insecticides, nematicides or fungicides for 17 years in vegetables. They also are helping growers of cucumbers, squash, peppers, eggplant, cabbage peanuts, soybeans and cotton reduce their pesticide applications to two or fewer while harvest- ing profitable crops. This system is described in greater detail in Managing Cover Crops Profitably, 2nd Edition (Resources, p. 104).
As many as 120 species of beneficial arthropods have been found in southern Georgia soils when cotton residues were left on the surface and insecticides were not applied. In just one vegetable-growing season, 13 known beneficial insects were associated with cover crops. When eggplant was transplanted into crimson clover at 9 a.m., assassin bugs destroyed Colorado potato beetles on the eggplant by evening. Similarly, other ben- eficials killed cucumber beetles on cucumber plants within a day.
Underlying those benefits, accord- ing to the Georgia researchers, was the soil-improving combination of cover crops with conservation tillage: soil organic matter increased from less than 1 percent to 3 to 8 percent in most of their plots, and a majority of growers saw similar improvements in soils and pest management.
Impacts of Fertilizers on Insect Pests
By modifying the nutrient composition of crops, fertilizer practices can in- fluence plant defenses. A review of 50 years of research identified 135 stud- ies showing more plant damage and/or greater numbers of leaf-chewing insects or mites in nitrogen-fertilized crops, while fewer than 50 studies reported less pest damage. Researchers have demonstrated that high nitro- gen levels in plant tissue can decrease resistance and increase susceptibility to pest attacks (Table 2). Although more research is needed to clarify the relationships between crop nutrition and pests, most studies assessing the response of aphids and mites to nitrogen fertilizer have documented dra- matic expansion in pest numbers with increases in fertilizer rates.
Crops could be expected, therefore, to be less prone to insect pests and diseases where organic soil amendments are used, since these amendments usually result in lower concentrations of soluble nitrogen in plant tissue. Indeed, most studies documenting fewer insect pests in organic systems have attributed these reductions in part to lower nitrogen content in the crop tissues:
smaller infestations of flea beetles and cabbage aphids than con-
ventionally fertilized broccoli. Researchers attributed those reduced
infestations to lower levels of free nitrogen in plant foliage, further
supporting the view that farmers can influence insect pest preferences
with the types and amounts of fertilizers they use.
Implications for Fertilizer Practices
Conventional synthetic fertilizers can dramatically affect the balance of nu- tritional elements in plants. When farmers use them excessively, these fer- tilizers likely create nutritional imbalances with their large pulses of avail- able nitrogen, which in turn compromise crops’ resistance to insect pests.
In contrast, most organic farming practices lead to increased organic matter and microbial activity in soils and the gradual release of plant nu- trients; in theory, this should provide more balanced nutrition to plants. While the amount of nitrogen that is immediately available to the crop may be lower when farmers use organic inputs, their crops’ overall nutritional status appears to improve. By releasing nitrogen slowly, over the course of several years, organic sources may help render plants less attractive to pests. Organic soil fertility practices also can supply secondary and trace elements, such as boron, zinc, manganese and sulfur, which are occasion- ally lacking in conventional farming systems that rely primarily on syn- thetic sources of nitrogen, phosphorus and potassium.
If, indeed, biochemical or mineral-nutrient differences in organically grown crops enhance resistance, this may explain — at least in part — why lower pest levels have been reported in organic farming systems. Observa- tions of these lower levels support the view that long-term management of soil organic matter leads to better plant resistance to insect pests.
At the USDA Beltsville Agricultural Research Center, researchers discov- ered a molecular basis for delayed leaf senescence and tolerance to diseases in tomato plants grown in a hairy vetch mulch, compared to the same crop grown on black plastic. The finding is an important step toward a scientific rationale for alternative soil management practices.
Probably due to regulated release of carbon and nitrogen metabolites from hairy vetch decomposition, the cover-cropped tomato plants showed a distinct expression of selected genes, which would lead to a more ef- ficient utilization and mobilization of C and N, promote defense against disease, and enhance crop longevity. These results confirm that in intensive conventional tomato production, the use of legume cover crops offers ad- vantages as a biological alternative to commercial fertilizer, in addition to minimizing soil erosion and loss of nutrients, enhancing water infiltration, reducing runoff, and creating a “natural” pest-predator relationship.
Traditionally considered in isolation from one another, aboveground and belowground components of ecosystems are now thought to be closely linked. The (crop) plant seems to function as an integrator of the above ground and below ground components of agroecosystems. This holistic approach is enhancing our understanding of the role of biodiversity at a global level. In agriculture, such close ecological linkages between above- ground and belowground biota constitute a key concept on which a truly innovative ecologically based pest management strategy can be built.