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Diversity and Refugia Make Biological Pest Control Work


Diversity and Refugia Make Biological Pest Control Work

by Jan Dietrick

Published as The Insect Side, in Acres USA, September 1995, p 32–33


Once upon a time, alfalfa hay and pastures provided the permanent refuge areas that were home to a thousand or more species of beneficial insects. They migrated from the non–irrigated foothills and provided biological control in all vegetable and field crops. Alfalfa hay was the one irrigated crop that was never sprayed. No one sprayed alfalfa because the price was too low to sustain the cost of spraying.


Another source of diversity was cover crops that were planted for soil improvement, pollination and green manure. When production fields and orchards were sprayed from time to time, the natural enemies which were plentiful in the havens of alfalfa hay and cover crops migrated into the sprayed areas. At hay cutting time, insects and their beneficials were driven from cut fields restoring biological control in the previously sprayed market crops.


Resistance to pesticides did not develop in that arrangement, because pests not killed by the poisons were controlled by predators and parasites immigrating from the hay fields where pesticides were never used. Alfalfa was grown for crop rotation, soil improvement, and animal forage and had no economic threshold for pest damage. When resident weeds overtook the hay stand, the fields were rotated to market crops or were replanted. There were plenty of irrigated, unsprayed fields where predators, parasites, pollinators, and diseases of insects that served as antagonists to pest development worked together like they do in all diverse natural ecosystems.


The picture changed dramatically when the price of alfalfa increased to the point where it became worth spraying. This was followed by herbicides replacing cover crops. Cattle and sheep grazing on the foothills without irrigation destroyed even more insect havens. There was almost nothing growing or irrigated anymore that was not a market crop with an economic threshold for pesticide use. The increasing cost of land and water forced farmers to focus on the bottom–line by maximizing production using total chemical pesticide management. Increasing labor costs, development of effective but costly farm machinery using more energy, the marketing of more and more powerful synthetic pesticides, concentration on cheap petroleum-based fertilizer, all contributed to decreasing the diversity of plant, animal and microbial life in the soil and crop.


Large, single–crop farms replaced small family farms that had been successful be-cause they produced several crops. In addition, maximizing individual crop yields replaced the goal of optimizing total farm production costs. Breeding for higher yields and precision planting of more uniform plants eliminated even more natural diversity within a single crop, thereby increasing the chances of pest outbreaks.Into this uniform, monoculture type of production, we started introducing more exotic pests from foreign lands, because there is much more international commerce and travel than before. Government funding for foreign exploration to research natural enemies to immigrant pests, slowed to a virtual standstill. Less and less research was done to colonize new beneficial species to attack exotic pests, even though we have more immigrant pests that are difficult to keep out.


It evolved this way so that now, when a new pest invades a monoculture environment, farmers have no biological options. They have to spray. The poison destroys all existing beneficial organisms that provide biological control of all other pests. Minor secondary pests become major problems and lead to a dependence on the pesticide treadmill. Total chemical pest management breeds resistance to all pesticides and disastrous consequences of "super pests" such as resistant sweet potato whitefly, leafminers, aphids, etc. Resistant medfly may become the next disaster when eradication fails and this pest becomes firmly established.


Individual farmers can overcome pesticide resistance using the principle of diversity. By diversifying crop selection, cover cropping and providing permanent refuges for natural enemies, immigrating pests and their sets of beneficial natural enemies, including pollinators, can be kept on their farms. Relatively small areas on the farm can be cost–effectively planted and managed to provide a favorable environment for beneficial insects and their hosts. Edges and corners of fields can be utilized along with limited use of border strips of diverse species of cover crops. These unsprayed cover crop plantings, along with resident weeds, are more than just shade and ground cover. They provide nectar, pollen, and plant material that a tracts many diverse sets of plant-feeding insects which are the prey for many, many more sets of predators and parasites.


Today, managing this resource on your farm has actually become a necessity. Augmentation of insectary-reared beneficials in market crops are more cost-effective on farms with diversity. You can manage with lower numbers of these key species when you are supplementing the resources of hundreds of species of natural enemies for which there is no other source except natural diversity on the farm.




In a world where all irrigated crops are grown for market, there is no nonsprayed, permanent sanctuary for insects anymore. The majority of all crops are grown for market and therefore managed totally with chemicals. Since there are no permanent unsprayed irrigated crops to sustain natural biological control, when chemically resistant and immigrant pests come in, farmers have no choice but to keep spraying.


Farmers interested in a transition from conventional pesticides to alternative biological control must create some diversity and provide refugia that keep beneficials on the land. Planting refugia is necessary for biological control on farms coming out of monoculture cropping and reliance on chemical pesticides. Refugia can be used to protect individual fields as well as the whole farm.


Refugia help keep migrating insects out of the market crops, generating larger numbers of predators at no cost (or damage) to market produce quality. Refugia can buffer the impact of insect migrations from the foothills when native plants mature and dry up. Refugia are not market crops and therefore no limits are placed upon heavy populations of plant-feeding insect pests feeding on those plants. The plant-feeding insects are valuable prey for growing predators and parasites, such as ladybugs, lacewings, insect eating spiders, etc. These predators will forage beyond the refugia, searching for pests in the market crops thus preventing or stopping pest invasion when it first starts


Many of the interferences to biological control such as pesticide drift, honeydew-seeking ants, dust and adverse climate effects can be managed cost-effectively by planting border strips of cover crop plants that affect temperature and humidity, initiating micro-climates favorable to the survival of natural enemies. Increased numbers and diversity of plants can shade out ever-expanding ant nesting sites and provide a complexity of insect life that increases populations of natural enemies.


Alternate prey are needed as food for natural enemies. This important resource of a complexity of plant-feeding prey living on resident weeds and cover crop plants attracts and feeds predators and parasites of pests and keeps them on the farm. Most carnivores must be supplied with enough prey or alternate food sources; otherwise, unless they go into diapause, they will leave in search of food. It is better to increase prey in sacrificial refugia on the farm rather than sustain them by taking damage on the market crops.


It is important to consider the pest-inducing effects of ants, dust and radiation, especially at the interface of irrigated farms with dusty field roads and open wild areas. Dust builds up on plants in between infrequent rainfall. Overhead sprinkler irrigation applied when needed can clean plants of dust. Conventionally sprayed plants look more beautiful, especially immediately following sprays. An alternative to poisons is water washing that may be advised in some situations to satisfy aesthetic needs. (It makes more sense than using pesticides to clean the plants!)


The intensity and duration of solar radiation on outside rows of market crops is implicated in physiological susceptibility of plants to pest populations. Spider-mite populations increase on the hot outside row of plants due to exposure to radiation effects. Cover crop planting and mulching along such borders reduces these pest-inducing radiation effects sufficiently to control many pests and improve fruit quality on these outside rows.


At the same time as cover crops provide prey for increased numbers of natural enemies, the decomposition from mulching of cover crop mowings (sheet composting) forms other biological systems that breakdown plant and animal life into food chains of alternate prey for soil pests and plant diseases. For example, ephedrid fly maggots that decompose plant material are prey for rover and ground beetles that may contribute to biological control of minor insect pests, such as fleabeetle rootworms, wireworoms, black vine weevils and other pests that over-winter in the soil. These systems are part of the complexity of natural enemies that we see working on healthy organic farms that regulate excessive pest population increases.


Trapping affects increases in such plant-feeding pests which are food for diverse populations of spiders and other predators. The overall effect of managed refugia for enhancement of biological control increases field insectary production of thousands of species of natural organisms interacting to improve efficiency of biological control in the whole agroecosystem. These smart choices of creating refugia can be applied around greenhouses just as well as on production farms and orchards.


Augmentation with some of the key beneficial organisms that are grown commercially becomes very cost-effective on farms where diversity is already present. The refugia can be managed and used at very low cost (compared to the rising cost of failing conventional total chemical management) attracting and trapping migrating pests and providing ideal hospitality to a well-balanced natural enemy system of sustainable biological control.


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