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Coastal California Row Crop IPM - A Review
By Phil A. Phillips, PhD, BCE, CCA
A healthy plant is the baseline defense against insects, mites, nematodes, and pathogens. Chemical applications are just a part of a total system that needs to be managed fwith plant health in mind: other components include irrigation and nutrition. When pesticide intervention is necessary, the use of selective as opposed to the more traditional broad-spectrum pesticides is a very important part of any IPM program. Classes of pesticides such as the botanicals, soaps and oils, Bacillus products (BTÃs), and the newer generation fermentation byproducts are generally considered selective and more environmentally benign than the organophosphate, carbamate, and pyrethroid classes of pesticides. The more selective pesticides can be used with less concern for disruption of secondary pests such as aphids, whiteflies, leafminers, and mites than when the more broad-spectrum pesticides are used. This is primarily because beneficial predators and parasites which keep secondary pests under control are conserved with the use of selective pesticides. Additionally, with many of the insecticides within the broad-spectrum classes of pesticides there can occur the phenomenon known as hormolygosis. With hormolygosis, after a pesticide treatment, the treated plant responds as if it has been wounded by producing more free amino-acids in cell sap. This in tern provides the herbivorous pests with a more nutritious food source which allows them to potentially produce more and stronger progeny. The net result is a more prolific pest population than prior to the pesticide application. Many of the problems associated with the use of broad-spectrum pesticides can be overcome when these materials are used as spot treatments of relatively small delineated areas. This allows reservoirs of undisturbed beneficials to remain around the treated area. The use of these same materials formulated as a bait rather than a spray tends to serve the same purpose by exposing only those target organisms to the toxicant which are attracted to the bait. With selective pesticides, the nature of the selectivity is often not so much toxicological as it is lack of residual toxicity. Since many of the selective pesticides only kill the target organism for a short period of time, timing the application of these products is generally crucial to their success. In order to determine the most appropriate time for pesticide application or for the release of beneficial organisms, it is necessary to carefully monitor both the pest and appropriate beneficial species. For row crop IPM programs there are various monitoring techniques: pheromone traps, sticky traps, timed searches, beating trays and replicated observations, to name a few. With very minimal time expenditure it is possible to produce a very clear picture of the status of any particular pest at any particular time by using one or more of these techniques. The success of any monitoring system is predicated on the consistency of the monitoring technique used and the frequency of its use. For instance, in monitoring for a diamondback moth larval hatch early in a crop of broccoli or stock (fresh cut flower), it is important to search either the same number of plants each time or to search for the same length of time (timed-search) at each location in the field during an inspection so that the information gathered is not only comparable from location to location within the same field, but is also comparable between fields or for the same field over several inspection dates. It is only in this way that an accurate assessment of the current population and its increase or decrease over time can be made. This sort of consistent , quality information is invaluable when an assessment of a recent pesticide treatmentÃs efficacy is desired or in planning for a subsequent pesticide application. Although the short term nature of row crops relative to permanent cropping systems such as tree crops (or even strawberries) often doesnÃt allow sufficient time for many beneficial species to build their populations in response to pest buildup, their are in fact quite a few opportunities for the grower or PCA to take advantage of regarding biological control in row crops. Leafminers, for instance, are generally under excellent biological control until they are released from their parasites by the application of organophosphate, carbamate, or pyrethriod insecticides. The parasite Diglyphus begini, one of several leafminer parasites, can average 75% leafminer parasitism several weeks after transplanting when selective products such as the BTÃs and avermectin are used as opposed to the carbamate methomyl and the pyrethriods. Beet armyworm can be heavily parasitized by Hyposoter exiguae in host crops where the more environmentally benign pesticides are used. Trichogramma parasites which attack the egg stage of many lepidopterous pests such as cabbage looper and corn earworm are extremely sensitive to the broader-spectrum pesticides. In fact, most of the parasitic hymenoptera (small stingless wasps) such as these are easily decimated by even pesticide drift from one field to another adjacent field down wind. A neighborÃs poor or inappropriate pest control program may be as much of a problem as the target pest organism at hand when attempting to develop a more biologically intensive IPM program. In order for growers to optimize the free biological control offered up by the myriad of beneficials which attack row crop pest insects and mites, it is best that large areas of several hundred acres or more be under a biologically intensive IPM program in which every effort is made to use only selective pesticides or broad-spectrum pesticides in bait form in combination with other control tactics such as cultural controls. Cultural control methods include manipulations of management operations such as the time of planting or irrigation scheduling. For example, in managing the seedcorn maggot, it is important to delay the spring planting of direct-seeded crops into fields containing large amounts of undecomposed organic residue from a previous winter crop until decomposition is nearly complete. This avoids serious crop loss from resident maggot populations. It is also recommended to use an additional cultural technique, that of dragging a chain directly over the seed row and behind the planting shank so as to cover the freshly exposed moist organic odors emanating from the soil and prevent adult seedcorn maggot flies from homing in and laying their eggs directly into the new seed row. In disease management it is often important to manipulate irrigation practices to mitigate severe disease pressure. For instance, many fungal pathogens require water on the plant surface before they can infect the plant. Botrytis grey mold is a good example of this. By scheduling irrigation runs, especially overhead sprinklers, during the night, when plant surfaces are likely to be wet anyway, and shutting them off early in the morning, plant surfaces are permitted to dry off during the day and are thus subject to a shorter period of wetness than if they had been irrigated during the daylight hours. Since disease severity or pressure is often worse with longer periods of plant surface wetness (leaf wetness), this sort of irrigation management can reduce disease pressure considerably. As well, merely shifting from sprinkler to drip or from surface drip to subsurface drip can significantly reduce periods of leaf wetness and thereby reduce disease pressure.