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Phil Stansly Integrated pest management (IPM) was a term coined in the 1950's to mean optimal combinations of chemical, biological and cultural control. The concept of cultural control rests on the assumption that any agricultural practice may tip the balance between crop and pest one way or the other. Unfortunately, the relationships are complex and it is not always clear which way the balance will tip. Nevertheless, investigation and/or grower experience has led us to some general conclusions about a number of common practices. The subject of mulches was taken up in a previous article (reference). Here I deal with crop placement in space and time. Crop-Free Periods The annual cropping system creates a simplified environment dominated by a single plant species that often makes an easy target for pests. Insecticidal control may provide temporary relief, but can make things worse in the long run by eliminating beneficial insects that could help maintain control. The result is a race with the pest to the harvest. Given this scenario, the size of the initial pest population can be critical and an infested field next door may spell disaster. Most pests are mobile in at least one growth stage and can move in huge numbers from old crops into new plantings. After 1 or 2 crop cycles the pest pressure becomes too great to deal with. Its time for a break! Silverleaf whitefly gave us a good lesson in southwest Florida of the importance of crop-free periods. The pest first appeared in 1986 on poinsettias but quickly jumped to a broad range of vegetable crops. Less than 2 years passed before clouds of whiteflies were seen in tomatoes and squash and with them a new plant disorder, irregular ripening caused by feeding of the nymphs. Then in the fall of 1989 a whitefly-borne tomato mottle geminivirus (ToMoV) devasted many fields, only to "miraculously" disappear in the spring crop after a Christmas freeze. The hard-earned lesson was taken to heart and growers responded to the call for rapid clean-up after the spring harvest. The result was almost a complete elimination of ToMoV in the next fall crop although there was still enough there to damage the spring (1991) crop. Another crop-free window was needed between the fall and spring, although this requirement has largely been eliminated by new systemic insecticides. However, at least one break a year is still needed. Crop-free period is also considered a necessity for a number of other important vegetable pests such as pepper weevil, tomato pinworm, and Thrips palmi and is recommended for management of all vegetable pests. Crop-free periods are most effective when practiced areawide, so cooperation among growers is important. Crop Rotation Most growers would rather not grow a single crop on the same piece of ground year after year, although they might think it necessary for economic reasons. Pest and disease problems build up over time, especially if the same chemical regimes are used season after season. The ideal rotation crop is one that is most distantly related to the previous crop and would therefore share the fewest pests and diseases. An example might be sweet corn and almost any other vegetable. However, pest and disease organisms often evolve host-specific strains that may be less virulent on even closely related crops, so any rotation is better than none. Host range can often be used as a guide to using rotation to manage some specific pest problems. For instance, Thrips palmi is a pest on most fruiting vegetables as well as potatoes but not on tomato. Tomato pinworm also attacks eggplant and potato but not pepper. Pepper weevil has not other crop hosts of any significance. Fall armyworm is essentially a pest of grasses and therefore corn although they will occasionally attack other crops such as pepper. Beet armyworm has a wide host range that includes most broad leaf crops but prefers pepper to tomato while preferences of the southern armyworm are reversed on these two crops. Again, an areawide approach is most affective for managing mobile pests through rotation. Companion Crops Sometimes two or more crops may be grown together to derive some benefit for one or both. A companion crop would hopefully be managed in a similar way as the main crop and would produce a salable product. However, management of two crops simultaneously in the same field can be a daunting task. Companion crops may have various purposes: (1) as refuges for natural enemies, (2) to repel pests, or (3) to act as a trap by attracting pests away from a more valuable or vulnerable crop. An ideal refuge crop would provide resources for beneficial insects such as nectar or alternative prey without itself becoming a source of pests for the main crop. Potential pests should be closely enough related to share key natural enemies but not the same host plant. Therefore, the correct choice for a refuge crop may require either a lot of knowledge of predator prey relationships or a lot of luck. Marigolds have often been used as pest repellents although their effectiveness may be a matter of opinion. A good trap crop would presumably be more long lived and attractive to the target pest than the main crop. By these criteria, eggplant might be a good choice to serve as a trap crop to attract whiteflies away from tomato. However, we have consistently seen whiteflies increase on tomatoes next to eggplant compared to tomatoes next to tomatoes unless the eggplant has been protected with imidacloprid (Admire®) as shown in the accompanying figure. Of course the grower’s resources would be better spent treating the tomato itself rather than the trap so that the use of attractive trap crops appears not to be a viable management option for silverleaf whitefly. |
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