Conservation Tillage Systems in the Southeast

Changes in Insect Pest Status

Overview

Conservation tillage can change the population dynamics of various insect pests. This can result in the pest status changing or staying the same, meaning more damage to a crop, less damage or no affect on crop damage. The direction and magnitude of change, however, are highly variable and depend on the crops, pest species, geographical locations and cropping practices. Therefore, each pest and crop situation is different and must be considered separately [4]. Comprehensive reviews of the responses of various vertebrate and invertebrate pests to conservation tillage are available [4, 62]. The following sections briefly review recent studies that examine the effects of conservation tillage on the abundance and damage of important pest species of the Southeast. Table 13.1 summarizes the effect that conservation tillage has on these pests. Relevant studies from other regions are also discussed.

Cotton

Conservation tillage has become an important practice in cotton production, especially with the advent of transgenic herbicide-tolerant varieties. These varieties have been engineered using molecular techniques to tolerate applications of herbicide that would harm a conventional variety. Various thrip species and cotton leafhoppers are less abundant in conservation tillage fields than in conventional tillage fields [3, 45, 50].

Other insect pests increase with conservation tillage. For example, in cotton fields that use conventional tillage, the tillage and residue burial typically kill many of the soil-inhabiting insects present. Post-season tillage is likely to destroy most pupae of tobacco budworm and cotton bollworm (also called the corn earworm) overwintering in the field [59]. However, the burial of infested cotton fruits by conventional tillage increases survival of boll weevils by protecting them from heat and desiccation on the soil surface [27].

The absence of tillage can lead to increased damage by cutworms, delayed maturation of plants and decreased seed-cotton yield [42]. The abundance and activity of red imported fire ants increases in conservation tillage. This in turn can lead to an increased abundance of cotton aphids [43]. The fire ants collect honeydew from the aphids and protect them from their natural enemies.

Cover crops can play an important role in attracting natural enemies of pests and reducing the abundance of cotton pests. Heliothine caterpillars such as the cotton bollworm and tobacco budworm are more abundant in Georgia cotton plots under conventional tillage than in plots under conservation tillage with crimson clover or rye as cover crops [65]. Increased numbers of cotton bollworm and tobacco budworm predators under conservation tillage with cover crops may counteract the reduced mortality of their pupae in the soil. Similarly, the densities of thrips and associated damage were two to eight times higher in cotton plots without cover crops under conventional tillage relative to plots with cover crops under conservation tillage [49]. This illustrates the importance of cover crops in conservation tillage systems.

Corn

Studies on corn pests suggest that closely related insect species can have different responses to conservation tillage when compared to conventional tillage. Under reduced tillage, western corn rootworm populations increased [63], northern corn rootworm populations were unaffected [63] and southern corn rootworm populations decreased [9]. European corn borer abundance did not change under reduced tillage in Georgia [1], while it was reduced in Delaware [67]. Lesser cornstalk borer was more abundant in conservation tillage in Florida [41], but the opposite trend was observed in two studies in Georgia [1, 2]. Cutworm populations increased under conservation tillage [41, 63]. Fall armyworm does not respond [41] or is less abundant [1].

Populations of caterpillars are often associated with the availability of weed or cover crop hosts in the field. So, changes in the type or density of weeds or cover crops are expected to affect populations of these pests. Fall armyworm damage can be especially severe when herbicide applications kill a previous grassy cover crop or crop such as cereal rye, forcing the movement of caterpillars to the corn crop [4]. Corn becomes susceptible to fall armyworm infestations when the seedlings grow taller than the residue [4].

Soil-inhabiting beetles often become more abundant when plowing is reduced or eliminated. Southern corn billbugs are more abundant in reduced tillage [1, 4, 24, 56]. Increased southern corn billbug infestations are associated with the presence of grassy weeds that serve as host plants for billbug larvae [1]. The planting of small grains before corn also exacerbates the problem when small grains become larval hosts and/or overwintering habitat for the weevils [4]. Several wireworm species are favored with the cooler soil temperature, higher soil moisture and higher soil organic matter content under conservation tillage [4].

Peanuts

Researchers in the Southeast have only recently started studying peanut pests and their natural enemies under conservation tillage because of the historical notion that conservation tillage is incompatible with peanut production. In general, conservation tillage, most notably strip-tillage, reduces pest abundance and injury to peanuts [16]. Some of the pests that are less abundant and/or are less damaging to peanuts under conservation tillage include the corn earworm, granulate cutworm, velvetbean caterpillar, lesser cornstalk borer and various species of wireworms [16]. Similarly, thrips cause more damage to peanuts in fields with no residues than in fields with moderate to high amounts of rye residue [49]. Strip-tillage also reduces the incidence of tomato spotted wilt virus, a disease transmitted by thrips [16]. On the other hand, the three-cornered alfalfa hopper and the burrower bug complex are more abundant and damaging in peanuts in conservation tillage than in conventional tillage [15, 16].

Soybeans

The response of soybean pests to conservation tillage is inconsistent [31]. Studies in Kentucky and Louisiana suggested that eggs and larvae of green cloverworm are more abundant in no-till plots [61, 66]. In contrast, pre-plant tillage has little effect on the population densities of the foliage-feeding green cloverworm and the pod-feeding southern green stink bug, but it favors velvetbean caterpillar [23]. Southern green stink bugs and lesser cornstalk borers are more abundant in soybeans under conservation tillage but do not significantly increase damage to the seedlings [41].

Germinating soybean seedlings may be attacked by the seedcorn maggot. This is particularly true in no-till fields that incorporate manure, organic matter or cover crop residues [30, 32, 48]. High levels of organic matter in the soil are conducive to the egg laying and development of seedcorn maggots. In addition, the delayed germination of soybeans in the cooler soil of no-till plots allows prolonged exposure of seedlings to the maggots [4].

Wheat and Other Small Grains

Overall, the responses of small grain insect pests to reduced tillage are inconsistent. Hessian fly populations can increase in no-till wheat production systems [17, 20, 52, 69], perhaps due to an increase in infested surface residues and an increase in its survival rate [52]. However, a recent study in Idaho reported similar Hessian fly egg densities and variable pupae densities in conventionally tilled and no-till plots [13].

Additional surface residues in plots without pre-plant tillage may also increase the infestation of wheat and barley by cereal aphids [33]. In fields practicing a sorghum>cotton rotation and reduced tillage, the abundance and damage by corn earworms and rice stink bugs are lower [18].

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