“No matter what we spray or how often, resistance to pesticides is something we need to watch for and manage to prevent it as much as possible.”
David Haviland, Kern County farm advisor, cited that axiom again for vegetable growers and PCAs gathered at an entomology seminar in Salinas, Calif.
Signs of insect pests developing resistance to products include failure in control, need for higher rates, need for more complete coverage, and reduced residual effects, all translating to more costs for the grower.
Along the way, the grower may also be forced to resort to more expensive but less effective materials.
From the scientist’s viewpoint, he said, resistance is a shift in the LD50 performance of a compound, or the lethal dose required to kill 50 percent of a pest population.
In practical terms, it takes more and more of the product to control the population, and it becomes only a matter of time until insects are immune to it.
Resistance not only means having to spray more often, but it can also have impact on worker safety practices and beneficial species.
Haviland emphasized that insects, like humans, have individual traits with broad variations. “Even with insecticides having a new chemistry, a certain portion of the insect population is resistant for whatever reason.”
Problems emerge when a product at a certain rate kills all the susceptible bugs, but not the resistant ones. When some susceptibles are left to mate with a few resistant types, resistance is avoided or at least diluted. However, when only resistant individuals are left to mate with other resistant survivors in surrounding fields, the results can be disastrous.
Haviland listed several points that growers and PCAs should remember. “First, know all the available options. Biological control is a great force against resistance. A beneficial insect doesn’t care whether its food source has a resistant or a susceptible gene. It tastes just the same.”
Cultural controls, such as planting winter crops, when possible, to avoid insect problems can be another choice. Practices of fertilization and irrigation, proximity of crops to sources of biological controls, and resistant varieties are other options to consider.
As a guide in selection of insecticides and acaricides, the Insecticide Resistance Action Committee (IRAC) provides periodically updated data with tables arranged by modes of action and resistance management strategies. Similar tables have been developed for herbicides and fungicides.
“IRAC is composed of chemical manufacturers and researchers who get together to analyze new chemicals and put together categories for the products. Get accustomed to the grouping numbers. Products with the same numbers have the same mode of action to make it all simplified,” Haviland said.
Learn the potential for resistance in the particular pest you are dealing with. “Does it have four generations during the crop year, or does it mate just once a year? And how many offspring does it produce in a generation?”
Haviland said the landscape and weather around cropland also has much to do with resistance potential and needs to be taken into account. Some pests migrate to crops from foothill slopes, while others ride in on prevailing winds.
Where compatible with cropping patterns, IPM plans can be developed to help head off resistance. These range from establishing thresholds for treatment and biological and cultural controls to rotation of pesticide chemistries by their modes of action and use of oils, which kill insects, regardless of any resistance they may carry. Neonicotinoids, including imidacloprid, acetamiprid, thiamethoxam, and others, Haviland noted, have become some of the most widely used insecticides in California since the category was introduced in the late 1980s. They work against the nervous systems of insects.
Their expanded use results from being effective on target species, reducing impact on natural enemies, and adaptability to many crops and situations.
Neonicotinoids can be used either as soil, foliar, or seed treatments for control of whiteflies, aphids, leafhoppers, scales, and other pests. Imidacloprid, for example, is available in more than 20 labels under multiple trade names.
The “trick” with using them, he said, is to be familiar with all the different names before selection to avoid overuse of products having the same mode of action and resulting potential for cross-resistance.
Haviland added that neonicotinoids, when used as soil treatments, can perform differently in different parts of the state, depending on how they are applied and the soil texture of the specific site.
In another presentation during the seminar, Eric Brennan, research scientist with USDA in Salinas, described his multi-year observations with sweet alyssum as a beneficial insect habitat in organic romaine and broccoli.
In recent years, organic growers in the Salinas Valley have shown interest in sweet alyssum and other flowering plants as insectaries for syrphid flies and other beneficials, whose larval stages feed on aphid pests of crops.
The sweet alyssum in Brennan’s trials was interplanted with the crop. “This,” he said, “provides a temporary habitat while the hedgerows on the edge of the field provide a more permanent place for the beneficial insects to live.”
It is important to have both, he added, because when the crop is harvested, the beneficials can survive in the hedgerows.
“The idea,” he continued, “is to suppress the aphid population to achieve greater marketable yield from the crop.”
The question has been how much space in the field to devote to the insectary plants so that syrphids will have adequate nectar and pollen to maintain sufficient populations to deal with the aphids.
Brennan learned that by planting alyssum on the leeward side of the bed, size of alyssum plants was maximized and competition with the marketable crop was minimized. This encouraged the alyssum growth to extend into the furrow rather than clustering around crop plants.
In romaine, the greatest crop area occurred when alyssum was planted between romaine at regular spacing, to provide 100 percent romaine and 100 percent alyssum.
For broccoli, a seedline was devoted to alyssum for 50 percent broccoli and 50 percent alyssum.
Brennan also found that alyssum interplanted with the less competitive romaine may delay crop maturity, although it may hasten maturity of the more competitive broccoli.