No-till and Biological Control

Most agricultural systems are designed to be very simple the crop and not much else. This simple system often leads to pest outbreaks because plant-feeding pests have an almost unlimited supply of food with very few natural enemies present. The pests can invade and damage a field before most biological control agents can respond. A major obstacle to a successful biological control program is having sufficient biocontrol agents present to reduce the target pest species. What is needed is an agricultural system in which biological controls are present before a pest causes economic loss. However, there is no incentive for natural enemies to be present in most agricultural systems.

Ecological theory states that a complex system is more stable (less likely to have pest outbreaks) than a simpler or less complex system. While this is an over-simplification of a complicated subject, we will accept it as true. One relatively simple way to make an agricultural system more complex is by no-tilling or having a mulched layer of decaying organic matter present in the agricultural system. Although there are soil property changes in a no-till system, the largest effect on biocontrol agents is the result of the presence of the decaying organic matter.

Organic Matter and Its Effects. How can something as simple as a 3-6 inch layer of organic matter increase predatory arthropods (insects, spiders, centipedes, mites) in an agroecosystem? The mulch layer reduces soil temperature and increases soil moisture in the top 3-4 inches of the soil, creating a very favorable environment for fungi and bacteria to thrive and begin to break down the mulch. In addition, there are detritovores (organisms that feed on detritus) that help breakdown the mulch layer. Detritovores include millipedes, many different mite species, beetles, etc. One thing that impresses people when they first start using no-till is the incredible number of organisms they find under a mulched area. Some growers become alarmed when they see so many insects concentrated in one area, thinking that this cannot be a good thing. In fact, the opposite is true. Most of these arthropods are detritovores and are harmless to the crop. Detritovores break large pieces of organic matter into small pieces while they are feeding on the mulch. This shredding of the mulch and the mixing of the detritovores feces with the mulch makes it possible for a more rapid breakdown of the material by fungi and bacteria. Some of the most common detritovores found in no-till fields are oribatid mites and collembola (or springtails). These two groups of organisms can be found by the hundreds under the mulch in every square foot of a no-till field. They provide a constant food source for arthropod predators. These arthropods, in turn, are followed by still larger arthropods that feed on many different things, such as different insects, seeds, mites, each other, etc. Although they are primarily interested in what they find in the mulch, they also move out of the mulch layer and search surrounding plants and soil for prey. This type of agroecosystem is a more "stable" one for predators. The predators have alternative food sources (besides pest species) and a conducive environment in which to live. Therefore, they can be present in fairly large numbers before pests begin to build-up in a no-tillage field. No-tillage fields have from 50% to 5 times more predators than a similar conventional-tillage field. These predators are there regardless of the numbers of pests present.

The process of breaking down the organic matter can take place over a fairly long time depending on the type of organic matter present. Corn stalks, wheat or rye take longer to break down than legumes such as clover, soybean or hairy vetch.

Research in the Midwest has shown that the type of mulch in a no-till field can influence predator activity. Grass mulch, such as corn or rye, will be present longer than a legume mulch, such as soybean. However, the legume cover will attract a greater number of predators early in the season compared with the grass mulch. The predator population will peak early as the leguminous organic material decays rapidly, so that by July little usually remains of the organic matter and consequently, there are fewer predators. The grass mulch is slower in attracting predators in the earlier part of the season compared with the legume mulch, but because the organic matter is more resistant to decay it is present for most of the season. This results in a more even distribution of predators throughout the season and fewer peaks.

Yes, But What Can These Predators Do For Me? Now that these generalist predators are in our no-till field, what can they do for us as biological control agents? One of the most important groups of predators found in no-till systems are ground beetles. Adult ground beetles (family Carabidae) are usually dark colored and range in size from 1/16 - 1 1/2 inches long. They rarely fly, preferring to rapidly run away if disturbed. They are generalists, feeding on a wide variety of pests such as cutworms, armyworms, corn borers, Colorado potato beetles and rootworm. Studies in the Midwest have shown a 30-50% reduction of pest damage (black cutworm and rootworm) in no-till corn fields compared with conventional-tilled fields. Immature ground beetles are even more predatory than the adults and spend most of their time 1-2 inches below the soil surface. Ground beetle larvae feed on any insect or mite they come across. Ground beetle adults, and at times even the larvae, will climb plants and search for prey in the evening. Predation on large Colorado potato beetle larvae (which do most of the defoliation) reduced defoliation by half in mulched systems compared with non-mulched potato systems.

Other biocontrol agents found in the no-till systems include predatory mites, predatory bugs, spiders and centipedes. While each of these predator groups has little impact individually, as a whole they can significantly reduce pest problems in most no-till systems because of their greater numbers as compared with conventional-tilled systems.

If There Are So Many Predators Why Don't I See Them At Work? Most of the predators found in no-till like cool, dark places and are not active during the day. They are most active between 10:00 p.m. and 2:00 a.m. This activity includes moving above the surface of the mulch and climbing plants. I have found large (1 1/2 inches long) ground beetles foraging in the canopy of corn, soybeans, and potato plants. Because of the timing of their activity, their predatory acts usually go largely unnoticed by most growers and researchers. An easy way for most people to test for the presence of these predators in no-till fields during the day is to peel back the mulch layer very quickly and observe the insects that scurry away. Any medium-to-large sized (3/4 - 1 1/2 inches), dark colored beetle is almost certainly going to be the beneficial ground beetle.

As discussed in previous issues of MBCN, the generalist predator has both good and bad points. The bad points are that the predator may not feed enough on the pest and will eat other predators. The good points are that the generalist always will be present in a system as long as there is alternative prey and it can feed on several different life stages of a pest such as the eggs, small and large larvae, pupae, or adults. These last two factors are very important in making the predators in no-till systems effective biocontrol agents.

Individually, most of the groups would have a small impact on any pest population. However, the generalist predators found in most no-till systems work in a predatory-guild. A predatory-guild is a group of several types of predators that feed on a particular developmental stage of a pest. An example would be small ground beetles, centipedes, and predatory mites that feed on the eggs of rootworm; this would constitute the predatory-egg guild. Some predators from one guild also could feed in other guilds (e.g., centipedes also feed on small rootworm larvae). The presence of these guilds is very beneficial in a field. Environmental conditions change each year and of course, throughout a growing season. Some months it is cool and wet, others it is hot and dry, etc. No single predatory group is active throughout a season. There are some periods in which a predator is very active (catching a lot of prey), and periods when it is not. However, when a predatory-guild is present, there is usually at least one or two types of predators within that guild that are active to feed on the pest. These predatory-guilds result in a fairly constant and reliable biological control resource in no-till systems. Although there are some peak periods of predation, which usually coincides with the pest's peak activity, there is usually a minimal amount of predation that goes on even at the least active times of predation. Therefore, while there are drawbacks with generalist predators, there are also some very positive aspects of their behavior that makes them successful biological control agents.

Other Biological Control Agents. The predators found in no-tillage systems are by far the most important components of biological control. However, parasitoids can also be more active in no-till systems. It is difficult, though, to separate out the no-till effect and the effect that weeds have on this increase of parasitoids. Adult parasitoids generally are attracted to a sugar source such as flowers and the more flowering weeds that are present the better chance parasitoids will be present. But, weedy fields are not appreciated by most growers and the presence of the weeds can reduce yields and mask any benefit of biological control. So, while parasitoids do play a role in the biological control of pests in no-till systems their contribution is not well understood and is not as important as the predatory-guilds.

Allelopathy and Other Attributes of No-till. Another aspect of no-till and the presence of mulch is that as the organic matter decays chemicals are released from the plant material that affects other plants and their germinating seeds. The chemicals can interfere with weed-seed germination. This interaction is called allelopathy and although it does not affect insect pests (at least we don't think it does), the release of these chemicals can reduce certain weed species. The mulch also keeps light out that some weed seed species need to germinate. Thus, the mulch can be used to suppress serveral weed species. The absence of weeds could affect the insect population both negatively and positively, but the absence of weeds is usually beneficial to an agricultural system as a whole. The mulch also keeps the soil moist and cool, resulting in increased yields for crops like potato. In corn, the cooler, wetter soil slows its early growth, but the no-till corn quickly catches up, and at times can surpass conventional-tilled corn in yield. The presence of mulch can also affect some pests by reducing their colonization of the mulched area and thereby reducing their damage.

Are No-tillage Systems a Panacea? While I may have painted too rosy a picture of predators in a no-till system, like any other biological control agent, they will not stop every pest outbreak. Some pests, like black cutworm and armyworms, will be attracted to weedy no-till fields. In a few instances, pests are able to reproduce rapidly enough that the predators cannot keep pace with them. In addition, there are other pests that may invade a field that the predators are not very good at controlling. An example of this would be potato leafhopper in potato or soybean. No-tillage systems and the presence of a mulch layer will increase the number of alternative prey in the system, which in turn, will increase the population of predatory arthropods. Anytime you can increase predators in a system and have them available before pests start to increase rapidly, you are going to have a much more successful biological control program.

- Gerald E. Brust, Purdue University

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