When broad-spectrum pesticides are used for management of primary pests, such as codling moth, secondary or minor pests can become problematic because natural controls are reduced or eliminated. Flint and van den Bosch (1981) said that the post-World War II pesticide revolution “ushered in a whole new spectrum of previously unknown pests.” Croft (1982) noted that several apple arthropod pests, including San Jose scale, oystershell scale, European red mite, two-spotted spider mite, apple rust mite, rosy apple aphid, wooly apple aphid, white apple leafhopper, and red-banded leafroller had reached seriously problematic levels because their natural enemies had been killed or adversely affected by pesticides. Additionally, many studies have shown it possible to enhance biological control and stabilize insect communities in agroecosystems through the design and construction of habitats that support populations of natural enemies.
Our pesticide-based apple pest management program has no doubt created or exacerbated problems with some pests. The advent of new codling moth management technologies, such as mating disruption, new bio-based and reduced-risk insecticides, and granulovirus, present unprecedented opportunities to reconfigure apple pest management programs to substantially reduce reliance on broad-spectrum insecticides. Changes like the elimination of broad-spectrum insecticides and altering vegetation in a complex biological system will result in changes in the pest complex. We do not have a good understanding of what might happen over time in apple orchards as we implement more selective management programs and use new covers as natural enemy habitat. Therefore, we conducted a study evaluating population changes of selected arthropods in an apple orchard grown with either a grass or alfalfa alley cover and managed without the use of insecticides during the growing season. We were particularly interested in leafrollers, the new “key pest,” in orchards using codling moth mating disruption.
This study began in 1999 and extended through four growing seasons. Contiguous, one-acre plots, three with grass and three with alfalfa cover, were established in a 5th leaf, bearing Fuji (BC.2/ M.9) apple block. Mating disruption had been used for codling moth management since 1994 and was continued throughout the project. Other than a delayed-dormant oil application, no other insecticide treatments were applied. We monitored populations of selected arthropod species over four years.
Pandemis pyrusana, the pandemis leafroller, population in both cover treatments was very low in year one, increased slightly in year two, and exploded in year three (41 percent of tight cluster buds infested). Then in year four, leafroller populations fell precipitously with only about 5 percent of buds infested, and the summer shoot infestations level was less than in year one in both cover treatments.
A pandemis granulovirus epidemic (Pfannenstiel et al., 2004) likely was responsible for the population collapse. Confirmation that larvae were infected with this virus was made via symptom evaluation. Parasitism also contributed. We reared several parasite species from pandemis leafroller larvae. Additionally, it was common to find spiders and earwigs, both generalist predators, in leafroller retreats by year four.
Limb tap samples were conducted in spring to assess populations of campylomma, lygus, western flower thrips, and stinkbug. At no time did campylomma populations cause a significant level of fruit damage. Lygus counts were, overall, highest in alfalfa, a preferred lygus host plant; however, no discernible fruit injury was observed. Alternate row mowing, separated by a week or more, may have discouraged lygus movement from the alfalfa cover to tree canopies. Populations of western flower thrips vacillated greatly between cover crop treatments and years throughout the study. In no year did thrips achieve pest status. Stinkbugs were absent in the spring, for both cover crop treatments in all years of the study.
White apple leafhopper nymph populations were highest in year two for both cover treatments, 1.1 nymphs per leaf in alfalfa and 1.0 nymphs per leaf in grass. Over the four years, white apple leafhopper populations did not exceed levels that caused damage. Populations of the western tentiform leafminer were very low throughout the four-year study in both covers, in part due to high levels of parasitism. Apple grain and green apple aphid populations never became problematic in either cover treatment. Rosy apple aphid infestations were greatest in year four but not at levels warranting treatment. All colonies of apple grain aphid, green apple aphid, and rosy apple aphid disappeared as summers progressed. Various aphid predators were observed in the colonies. Ladybird beetles and larvae were most common. Their presence was greatest the last year of the study. It is interesting to note that all species, except Deraeocoris, were more prevalent in alfalfa cover plots in the final year.
European red mite, McDaniel spider mite, two-spotted spider mite, and apple rust mite populations all declined, in both covers, over the course of the study, but at no times were mite populations problematic. Predator mite populations followed relative levels of prey populations.
Codling moth was still the key pest in this orchard. Adult codling moth trap catches generally increased during the study with no differences between cover treatments. The increase and resultant substantial fruit damage is attributable to a failure of mating disruption as a stand-alone management tool. It was the first time such a failure had occurred in this orchard. We attribute the outbreak to several factors, including low pheromone treatment rate (half rates), suboptimal emitter placement, and immigration of mated females from nearby abandoned orchards and a bin storage area.
Fruit cull analysis showed a general increase in codling moth injury over the course of the study. In the final year, an estimated seven to eight bins of fruit per acre were destroyed by codling moth. Fruit damage by leafrollers increased each of the first three years of the study, but in the last year, fruit damage approached or was less than year-one levels. The fruit damage reflected observed leafroller larval populations. In the last year of the study, we estimated the percentage of culled fruit that had leafroller damage exclusively. For the alfalfa treatment, it was estimated that 8.6 percent of culled fruit suffered leafroller damage exclusively and for the grass treatment 9.0 percent.
In the absence of insecticide treatments, only codling moth and leafroller (especially in year three) caused significant fruit injury. None of the other pests reached population levels that caused direct economic damage to the fruit. This suggests they may be primarily pesticide-induced pests, and biological controls, if not disrupted, are sufficient to maintain their populations below damaging levels. Alfalfa, as a cover crop, seemed to have only a slight effect on the decrease of pest species or increase in beneficial species.
Our data certainly suggest that in the absence of a broad-spectrum insecticide treatment, pandemis leafroller populations can dramatically increase over a three-year period before a reduction occurs. Whether lowered densities observed in year four represented a new, lowered equilibrium cannot be established since our study stopped after four years. Though leafroller populations, even in year four, were higher than would typically be tolerated in a commercial orchard, our rudimentary analysis suggested these were not of economic significance. Codling moth is subject to few natural controls in commercial orchards, and therefore, aggressive management is required. The challenge is to evoke an integrated management strategy not disruptive of biological control of secondary pest species. Leafroller densities might be managed by judicious use of Bt (Bacillus thuringiensis) while biological controls develop.
We have continued our highly restricted pest management program on three of the original seven experimental acres and now entering our eighth season, we still have not experienced economic injury from any secondary pests, including leafrollers (they are all but absent).
We have continued to struggle with codling moth populations and resultant fruit injury. Through a combination of mating disruption, granulovirus treatment, and aggressive orchard sanitation, we have gained economically satisfactory control and the ability to manage the arthropod pest complex biologically. And as Lewis, et al. (1997), suggested, we anticipate only the occasional need to use pesticides as a system back-up. Certainly, it would be valuable to determine if our experiences can be replicated in other orchards.