Conservation Tillage Systems in the Southeast

Management of Insect Pests

Overview

While control practices for insect pests are similar in reduced-tillage and conventional tillage systems, some practices may play a greater role with reduced tillage. An example is increased biological control caused by increased predation in no-till systems. Integrated pest management stresses the balanced use of biological, insecticidal, cultural and host-plant resistance tactics to manage insect pests, particularly over the long term [44]. The following section reviews selected practices for insect control in reduced-tillage systems.

Biological Control

Biological control is the reduction of pest populations by natural enemies such as predators, parasitoids and pathogens. In reduced-tillage systems, several factors influence the abundance of natural enemies. These include the crop, cover crop, timing of crop production, amount of residue left on the field, availability of alternate hosts/prey and the range of insects available for natural enemies to feed on.

Not surprisingly, natural enemies that live in or on the soil are generally more affected by conservation tillage and increased surface residue. Reduced tillage can increase the abundance and sometimes diversity of ground beetles [12, 14, 19, 36, 37, 53], although some species are more abundant with conventional tillage [12, 14]. The role of many ground beetle species in agricultural systems is still poorly understood, but some beetle species are valuable predators of important pests. The value of ground beetles as destroyers of weed seeds is becoming increasingly apparent [34]. Other ground beetles are omnivores [40]. As a result, producers can benefit by conserving these beetles since they consume pest insects.

Some species of ants are favorably affected by tillage while some are adversely affected [51]. In the Southeast, one of the most common ant species is the red imported fire ant, an active predator of numerous species. Conservation tillage typically increases the fire ant’s abundance [46, 57, 65]. However, fire ants also consume the sweet honeydew produced by aphids and whiteflies. They will often protect these pests from their natural enemies in order to obtain this sugary food [39]. As a result, fire ants can exacerbate problems with aphids and whiteflies in a variety of crop systems. When these honeydew sources are not present or abundant, the ants will generally focus on their predatory activities that can be quite impressive against insect pests, particularly caterpillars.

Tillage can also dramatically affect the diversity and abundance of spiders. There is a tendency for the number of spiders to increase as ground cover increases [58]. Spiders are increasingly being recognized as significant natural enemies of insect pests in crop systems [55]. Practices that encourage their activity will presumably foster biological control and reduce pest infestations.

Reduced tillage can also affect the numbers of insect natural enemies above the soil. For example, conservation tillage systems increased the abundance of big eyed bug predators relative to conventional tillage systems [65]. Overall, however, such changes are quite variable and are very difficult to predict.

Tillage practices can also affect entomopathogens, organisms that cause a disease in an insect. In one study, spores of the pathogenic fungus Beauveria bassiana were more abundant under conservation tillage than under conventionally tilled corn [8]. This is not surprising given the more stable temperatures and more moist microhabitat that surface residues foster. But the surface residue also prevented spores from splashing up onto plants so that infections of insects in the plant canopy were reduced in conservation tillage [8]. Similarly, vegetable soils in conservation tillage had a greater abundance and activity of fungal insect pathogens than did conventionally tilled soils [38]. However, this was not the case for the entomopathogenic nematode Steinernema carpocapsae, which was not suppressed by tillage [38].

Although a number of studies have evaluated the diversity and abundance of natural enemies in conservation tillage systems, only a few have directly assessed the function of these natural enemies, and the results are variable. It is generally assumed that biological control is enhanced because of the increased diversity of the system [6], but most studies are based on correlations. This means that in conservation tillage systems, natural enemy populations were observed to be higher and some pest populations were observed to be lower. Unfortunately, few of these studies provide mechanistic explanations for these patterns,for example more spiders eating insect pests. Studies have shown that predation of European corn borer eggs by chewing predators was more common in conventionally tilled corn than in no-till corn, but predation by sucking predators (most notably green lacewings) was higher in no-till corn [7]. Similarly, the number of southern corn rootworm eggs was greater in no-till than in conventionally tilled corn, but increased predation on immature rootworm stages by predatory mites, beetles, centipedes and ants in no-till corn led to reduced pest pressure and increased yield [9]. Natural enemies in annual cropping systems tend to be generalists that can switch among several prey or host species. Conservation tillage systems can favor generalist activity by providing alternate prey and hosts as well as the modified habitat [10].

Chemical Control

Insecticides remain the dominant tool for pest suppression. There are numerous problems associated with insecticides, including the development of resistance, impacts on non-target organisms, outbreaks of secondary insect pests and hazards to applicators, as well as other human and environmental health issues. However, properly timed applications of insecticides can be part of a safe and effective pest management program. Only use pesticides as described on the label. To minimize unwarranted applications, economic thresholds are recommended. Economic threshold refers to the density of a pest population above which insecticide treatment is justified.

This chapter does not include guidelines for insecticide use because of the large number of chemicals available and the variability in pest-specific recommendations within the Southeast. In general, the same chemical recommendations are used in conservation tillage as in conventional tillage [4]. Refer to local production guides for appropriate insecticide recommendations.

Seeds are increasingly treated with insecticides such as neonicotinoids before planting [47]. For some crops such as corn it is becoming increasingly difficult to obtain seed without such treatments. Some pests are more problematic with conservation tillage, and seed treatments provide an easy and effective way to resolve these issues. Early-season pests can be major issues in no-till as the corn grows slower than in conventional tillage. High rates of the neonicotinoid insecticides clothianidin or thiamethoxam as corn seed treatments suppress insects such as cutworms, billbugs and grubs [11] that can have increased incidence in conservation tillage.

Cultural Control

Cultural control manipulates the environment to manage pests. Examples of cultural practices that can help manage pests include crop rotation, planting/harvest dates, cover crops, plant density, fertility rates, variety selection and irrigation.

Cover Crops

Cover crops can play an important role in controlling certain pests in conservation tillage. Combined with conservation tillage, cover crops can help reduce the disturbance of natural enemies by preserving their habitat. Leguminous cover crops can also provide increased levels of nitrogen by fixation and rapid decomposition early in the growing season. This promotes plant growth when plants are susceptible to many seedling pests [60].

Fire ants in particular are often enhanced by the use of cover crops [57]. Cotton research in Georgia showed that crimson clover and rye reduced infestations of tobacco budworms and cotton bollworms through increased predation by fire ants and big eyed bugs [65]. In a separate study, the number of thrips in cotton fields and the damage due to adult and immature thrips were two to eight times higher in plots without a winter cover crop than in plots with winter covers [49]. Conservation tillage also reduced thrips numbers and injury to cotton.

Planting into living cover crops may increase the risks from certain pests such as cutworms. Most recommendations suggest that cover crops or weeds be killed well in advance of cash crop planting to reduce pest problems and to minimize water and nutrient competition. Also, leguminous cover crops tend to exhibit a greater cutworm risk than do grass cover crops [25].

Planting Dates

Injury from many insects can be avoided by planting early in both conventional and conservation tillage systems. This is the case with the corn earworm and fall armyworm in corn [4]. In some cases, delayed planting may help to increase the growth rate of seedlings, thus avoiding some pests such as the seedcorn maggot in corn [4]. Planting dates can also affect the impact of reduced tillage on beneficial insects [21]. For instance, early planting of conventional soybeans provides predators with habitat and promotes early colonization, thus reducing the destructive effect of plowing on beneficial insects [21].

Physical Control

Tillage has historically been considered a mortality factor in controlling pests that pupate in the soil, such as the corn earworm [54]. In the absence of tillage, it was assumed that many such pest species would become more serious problems. This has generally not been the case for reasons not entirely understood. Increases in certain predator populations, such as ants, in conservation tillage systems have had some effect. A research review showed that 43 percent of the surveyed studies reported a decrease in insect damage under reduced tillage and 29 percent reported no impact [62]. For example, the absence of tillage can reduce the incidence of the lesser cornstalk borer in corn, sorghum and soybeans following a small grain [4].

Some insects are attracted to weeds or cover crops that occur in reduced-tillage fields. When insect infestations are heavy, consider tillage as an alternative to the use of insecticides. Tillage will bury the insects and prevent or reduce migration of insects to the following crop. For example, armyworms are frequent pests of corn when double-cropped with a small grain, as the larvae can move to corn from the small grain crop after an herbicide is applied in the spring [4]. Also, as noted above, reduced tillage can lead to increased ant abundance. The corn root aphid is strongly associated with fire ants, which use the aphids for honeydew. When this is the case, as in newly planted corn, infestations of aphids can be avoided by tillage to disrupt ant populations prior to planting [4].

Crop Rotation

Many insects require crop residues throughout the year. This happens, for example, with continuous corn production. Crop rotation can break the cycle and reduce pest damage. In continuous corn, the absence of tillage can lead to the buildup of soil insects. Rotation with a non-host crop can be effective in reducing pest infestations. For example, the southern corn billbug is a pest of corn but not a pest of soybeans. A rotation of corn and soybeans can help to reduce infestations in corn [4]. Soybeans also have reduced damage from lesser cornstalk borers in double-crop systems relative to corn or sorghum [4]. Therefore, soybeans are recommended in areas with historically high infestations. In addition to tillage, rotation has been suggested to reduce infestations of Dectes stem borer in soybeans [28].

Crop rotation effects may be different for different insect pests. For example, a rotation of sorghum with cotton can reduce the abundance of bollworm but can enhance populations of the rice stink bug [18]. Not all soil-associated insects are affected by rotation. For example, Colorado potato beetle numbers were the same whether or not tomatoes were rotated with other vegetable crops in both conventional tillage and no-till systems [68].

Varietal Selection

Advances in molecular biology allow plant breeders to insert genes that code for insecticides into plants. The plants then produce the insecticides themselves. Such plants are referred to as transgenic. The bacterium Bacillus thuringiensis (Bt) is commonly found in soils and produces toxins that can kill certain insects when ingested. Transgenic Bt crops have been engineered to express at least one Bt toxin. Transgenic crops expressing Bt toxins offer the same benefits in both conventional and conservation tillage systems. However, the value of such crops can be further enhanced in conservation tillage when late planting is necessary and late season insect pressure is likely. In corn, Bt hybrids can provide control of European corn borers, stalk borers and corn rootworms as well as suppression of cutworms, corn earworms and armyworms. Differences in efficacy among available transgenic traits underline the importance of choosing appropriate hybrids for the anticipated insect pests. In cotton, Bt varieties provide good control of bollworms, tobacco budworms and other caterpillar pests. These can be more abundant under no-till because of the absence of plowing, an important mortality factor for their pupae in the soil [35]. When planting a Bt crop, federal regulations require that a portion of the field be planted with a non-transgenic variety to act as a “refuge,” or a nearby area where the targeted pest species can avoid contact with the Bt toxin. The purpose is to lower the risk of resistance developing among the insect population. Refuge requirements vary by region and crop variety.

Fertilization

Seedling pests and other pests can often be avoided or their impact reduced by using fertilizer to encourage rapid crop growth and good plant health [5]. Given the tendency for slow seedling growth in no-till or reduced-tillage systems, modifications in fertility regimens may be beneficial. However, in a tillage study, applications of different rates of fertilizer had no effect on pest densities in sorghum under either conventional or reduced tillage [18].

Weed Control

Many insect pests are attracted to weeds that frequently grow in or around fields. Proper management of weeds can help to reduce many insects, including cutworms, armyworms, billbugs, stalk borers (late season weed control) and grasshoppers. Controlling johnsongrass in sorghum can help to reduce sorghum midge infestations [4]. However, herbicidal control of johnsongrass can be difficult. Herbicide use can also lead to increased insect damage to the crop by removing alternate hosts and driving pests towards the crop [64].

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