The effectiveness of crop rotation as a tool for insect management depends on the life cycle of the target insect. For crop rotation to control an insect pest well, the insect must spend the period from the end of one crop to the beginning of the next in a stage with low mobility and must have a restricted range of host plants. Not many insect pests fit this pattern. Most have a period during the adult stage when they can travel easily across at least a single farm. Often this highly mobile stage comes when insects are emerging from their overwintering stage in the spring, so crop rotation from one year to the next will not affect them. Many pests, including corn earworms, cabbage loopers, and potato leafhoppers, do not even overwinter in the northeast US. Rather, they travel hundreds of miles to re-infest host crops each year.
However, some key pests can be managed through crop rotation (table 3.5). Western corn rootworm and northern corn rootworm are important examples. The adult beetles feed on corn silks (and on many flowers, including cucurbit flowers) in August and September and lay their eggs in soil at the base of corn plants. The eggs overwinter, and then newly hatched larvae feed on corn roots in the spring. They do not survive on small grains, sorghum, or broadleaf crops or weeds, and survive only at a very low level on grassy weeds. Thus, these two beetle species have relied on widespread planting of continuous corn for their survival and became major pests only when continuous planting of corn became a common practice in the Midwest.
In the same way, Colorado potato beetle became a major pest of potatoes because of the practice of continuous potato planting, particularly in the northeast US. The beetles emerge in late summer (August in Massachusetts) and spend the winter as adults, mainly along the field edges but also in the soil, in the same field where they developed. When they emerge from overwintering the following spring, they prefer to walk to hosts, if any are in the area, and there is a delay before they can regenerate their flight muscles. Spring infestations can be delayed or reduced by moving potatoes to a new field distant from the previous year’s potatoes, and by using various kinds of barriers (plastic-lined trenches, dense plantings of grain crops, etc.) to interfere with their ability to walk to the new field. The life cycle includes a long-distance flying stage—in first-generation adults, which emerge in mid-summer—that can hit fields far from the previous year’s potatoes, but rotation is an effective way to avoid early-season damage.
Wireworms (larvae of click beetles, Elateridae) and Phyllophaga grubs tend to be localized pests that can build up in a field following sod or small grains and then remain for multiple years, because the larvae are long lived. When monitoring indicates a problem, root crops and corn should not be planted in the field.
No-Till Farming, Early-Season Weeds, and Crop Residue Effects on Insect Pests
As explained in chapter 2, crop rotation planning must account for the whole range of field requirements and operations. Field management practices can influence insect pest problems, and the overall rotation plan should take this into account. The scientific literature, mostly written about corn and other field crops, indicates that no-till farming has a mixed effect on various species of insect pests within a single field. Some pests with low mobility, notably black cutworms, army-worms, slugs, white grubs, and wireworms, are favored by the presence of crop residue, cooler and wetter soil conditions, and the lack of soil disturbance in no-till farming. However, many predators of soil pests, including ground beetles, spiders, and predatory mites, are also favored by the same conditions within a field. In landscapes consisting of large fields tilled at least annually, strips of undisturbed vegetation around field edges are critical refuges for these predators, and also for the overwintering stages of parasitoids of insect pests such as European corn borer, true armyworm, and cereal leaf beetle.
Mobile insect pests may overwinter in crop residue in the field, and thus plowing and breaking down this residue will reduce their survival. European corn borer, squash vine borer, onion maggot, and spinach leaf miner are examples of insects whose overwintering stages, and the crop residues in which they reside, can be destroyed by plowing. Because these species are all mobile adults when they emerge in the spring, the effect of increased mortality may not be apparent in a single field or farm, but timely plowing across a region could reduce the size of the local population.
Plowing also destroys winter annual or biennial weeds that can have a role in drawing insect pests to a field at the beginning of the season. Black cutworm moths lay their eggs early in the spring and prefer low-lying, poorly drained, weedy fields. The caterpillars begin development on the weeds or on a cover crop. When those host plants are plowed and replaced with a cash crop, the caterpillars attack the stems of the new crop, unless enough time (10–14 days) has been allowed to starve them out before planting.
Cruciferous weeds, especially yellow rocket, also attract crucifer flea beetles (Phyllotreta cruciferae) into the field early in the season, before other host plants are present. The flea beetles overwinter as adults and are active and feeding from the first warm days in early spring.
Freshly plowed residues, still breaking down at the time of planting, attract seedcorn maggot flies to lay eggs. The larvae hatch out and feed on germinating corn, bean, or cucurbit seeds or tunnel into stems of new transplants.
Planting Time and Insect Pests
Some pests, including corn flea beetles, crucifer flea beetles, striped flea beetles, and cabbage maggots, do most of their damage early in the season on small seedlings. Damage can be avoided by delaying planting or protecting early plantings with row covers. A rule developed from research in New York State to avoid cabbage maggot damage is to wait to transplant seedlings of cole crops until after the peak bloom of yellow rocket. Timing to avoid flea beetle damage in cole crops may push the planting even later—until mid-June in Connecticut.
Other pests, such as Mexican bean beetle, come out of their overwintering sites later (late June in Connecticut) but prefer to lay eggs on the largest and most vigorous plantings, which are often the earliest plantings. Biological control programs using the parasite Pediobius foveolatus, a small beneficial wasp, have taken advantage of this preference by planting early trap crops of attractive host plants (snap beans) and releasing the wasps in these heavily infested early plantings.
Movement Between Crops During the Growing Season
A good crop rotation plan includes not only the crop-to-crop sequences but also how the crops will be laid out on the farm over the entire season. For many specialist pests with multiple generations per year (such as crucifer flea beetles), successive plantings of the same crop within a field increase the opportunity for building up high populations. While adult insects generally have a dispersal stage that easily travels long distances, having multiple plantings adjacent to each other helps the insects to find new food plants even in life stages that normally travel only a few yards.
“For many specialist pests with multiple generations per year (such as crucifer flea beetles), successive plantings of the same crop within a field increase the opportunity for building up high populations.”
Other pest insects feed on many different host plants and may build up in large numbers on one host, then move to another. An awareness of these possibilities can help a grower avoid a potential problem. For example, tarnished plant bugs feed on hundreds of different species of broadleaf host plants, preferring to feed on flowers and developing seeds. The nymphs may develop on cover crops such as hairy vetch, weeds such as pigweed or lambsquarters, or roadside vegetation. The new adults then move out when the plants are mowed or when the seeds become more mature and less suitable. Strawberry growers trying to reduce damage can substitute straight rye (a poor host for tarnished plant bug) for a mixture of rye with hairy vetch (a favorable host) adjacent to and upwind from strawberry fields. Strawberry growers who include alfalfa or clovers in their crop rotations can also avoid mowing these legumes during the time the strawberries are flowering and developing fruit to avoid mass movement of tarnished plant bug adults.
A few major insect pests can be managed directly by crop rotation on a single farm, but many more are influenced by farm management choices of whether and when to till, when to plant, how to manage spring weeds, and whether to put successive plantings of a crop in the same field. Because insects are so diverse in their overwintering, dispersal, and host-finding strategies, it is hard to identify cultural practices that will prevent problems across a range of crops and pests. The best approach is to identify the most important pests that have historically damaged major crops on a particular farm, and find cultural methods that interrupt the life cycle, interfere with movement, or otherwise limit the numbers of pests or damage from those pests.
For further reading, see References 33, 34, 47, 50, 55, 57, 71, 86, 93, 101, 103, 104, 122, and 127.