The Newsletter of Pheromone-based Orchard Pest ManagementVol. 2, No. 4 -- April 1, 1997
Cooperative Extension programs and employment are available to all without discrimination.
These 74 growers farmed a total of 25,293 acres. Yakima Valley growers (15 in number) represented 43% of the total acreage, 22% of the acreage (10 growers) was in the Columbia Basin and 35% (53 growers) was from North Central Washington (NCW). Some growers had blocks in more than one area.
Total acreage under MD in 1996 for these growers was 6,536 acres. (An estimated 22,000 acres were under MD in Washington that year, based upon information from the dispenser manufacturers). The MD acreage in the survey was in apples except for 284 acres of pears. Among the growers surveyed, MD use is more widespread in NCW. NCW growers had 36% of their acreage under MD, compared with 30% in the Basin and 14% in Yakima. The NCW growers also had more small orchards using the practice. Many different sizes of orchards were represented in the survey. The proportion of the total acreage under MD decreased as the acreage of the grower increased (see below).
|Grower Size||No. of Growers||% of Orchard in MD|
Pheromone dispensers: In 1996, Isomate-C+ was used by 61 growers on 79% of the treated acres. Checkmate-CM was used by 14 growers on 18% of the acres, and Disrupt-CM was used by 3 growers on 0.5% of the total MD acreage. Ten growers who used a different product in 1996 plan to use Isomate-C+ in 1997, increasing its proportion of the MD acreage represented by this survey to 97%. Five growers plan to continue Checkmate-CM use, reducing its proportion of the total MD acreage to 1%, and two growers will use Disrupt-CM again.
Dispenser rates: Isomate-C+ was used at many rates, from 200 to 400 per acre. The full rate (400 dispensers per acre) was used exclusively by 55% of the Isomate-C+ users, with the remaining 45% treating all or part of their acreage at lower rates. Lower rates were used most commonly in block interiors and after several years of full rate use. Checkmate-CM users applied dispensers twice, as indicated by the label, generally at rates of 120 to 160 per acre. Several indicated that they hung a third application in 1996. Disrupt-CM, first registered last year, was applied by the three users per label instructions (200 per acre).
Supplemental sprays: Eighty-three percent of those surveyed applied at least one spray for CM control in addition to using MD. Guthion (azinphosmethyl) was the most common material, used by 77% of all growers. The number of applications, and the amount of the orchard treated, varied greatly. Many growers indicated that the treatments depended upon CM pressure and history. Some applied from 0 to 3 covers within one orchard. Of those using Guthion, 15% spot treated or sprayed borders only, 45% applied one cover, 25% put on two covers, and 10% put on three or more.
Many other materials were used, each by less than 7% of the growers. The other materials listed for CM control included, by frequency of use: summer oil, Imidan, Confirm, Penncap-M, Lorsban, Ryania, Bt, Thiodan, Sevin and CM granulosis virus.
Many concerns were also noted:
Bt is a rod-shaped, spore-forming bacterium that occurs naturally throughout the world in many different environments. It has the distinctive quality of being toxic to a limited number of insects while being completely harmless to other insects and animals, including all vertebrates. Susceptible insects, depending upon the Bt strain, include most lepidoptera larvae (including leafrollers, codling moth and cutworms), some flies (mosquitoes and black flies) and a few beetles (particularly Colorado potato beetle). Close to 20 different strains of Bt have been identified, but only a few have been developed for pest control purposes. The original strain identified was B. t. kurstaki (Btk). Btk and B. t. aizawi (Bta), are the two used in Bt formulations targeted at lepidoptera larvae, although Btk is more active against leafrollers.
Bt must be eaten to be effective, having no contact activity. The Bt organism contains a spore and a crystalline protein endotoxin. When eaten, the Bt enters the insect gut where this endotoxin is solubilized and the toxin is formed. This protein toxin then binds with specific receptors on the insect's gut wall. Feeding usually stops within minutes, and death generally follows within one to five days.
There are many different Bt formulations available. Some contain Btk only (e.g., Javelin [Sandoz/Novartis], DiPel [Abbott], and Biobit [Dupont]), others Bta only (e.g., XenTari [Abbott]) and still others contain Btk and Bta (e.g., Agree [CIBA/Novartis], Cutlass [Ecogen], and Condor [Ecogen]). Bt formulations have been developed using recombinant gene technology, including CRYMAX (Ecogen) and MVP (Mycogen). In the latter, the gene that produces the Bt crystal protein endotoxin has been inserted into another bacterium, Pseudomonas fluorescens. These bacteria are propagated, then killed, creating an encapsulated Bt toxin.
Bt use in tree fruit pest management programs has been mostly focused on the control of leafrollers, such as the pandemis (PLR) and obliquebanded (OBLR) leafrollers. Codling moth control with Bt sprays is poor, largely because the newly-hatched larvae consume only a very small portion of the fruit skin before entering the fruit, where they are no longer exposed to Bt residues. Cutworms can be controlled with Bt, provided they are actively feeding on the treated plants.
There are two key points to remember with regard to Bt products that affect success greatly:
Therefore, for successful leafroller control, coverage of the foliage is probably the most critical factor. Applications need to be timed for the presence of actively feeding larvae. Both OBLR and PLR begin feeding by the half-inch green stage of apple bud development. Bt applications made in the period from pink until several weeks after bloom are often the most effective, as good coverage is easier to obtain then before there is extensive shoot growth. To thoroughly cover all foliage, make sure that tractor speed is low enough to allow the spray to reach throughout the tree. Dilute applications are not a requirement for good control. In fact, a concentrate application may be better IF coverage is good, because the Bt toxin concentration is higher and leafroller larval kill can be more complete.
In the spring, leafroller development is also fairly compact, allowing applications when all the population is in the susceptible stage (larvae). To apply during periods of active feeding, daytime high temperatures should be at least 65ƒF in the day(s) following application. Good control has been obtained even if the first Bt application has been delayed until after bloom, in order to treat during a warm period. Many growers have successfully reduced a problem leafroller population in one season by use of a delayed dormant oil/Lorsban spray together with spring Bt applications, or with Bt applications alone. In each case, proper coverage and timing were critical components of their success.
Several spray adjuvants have been considered for use with Bts to increase their effectiveness. The use of wetting agents (spreaders) may improve control if their use results in more thorough coverage. Trials with organosilicone surfactants in particular have shown improved control with their use. The addition of feeding stimulants, like Coax, to Bt applications has also improved leafroller control in some field trials. In many cases, a more cost-effective improvement in control may be had by simply increasing the rate of Bt product used.
If leafroller larvae consume only a small dose of Bt they may be sickened and stop feeding for a period, but not die. These larvae later resume development, but well behind their untreated cohorts. This sublethal effect results in reduced kill and an extended period of development, complicating the timing of controls aimed at the next generation. The sublethal effect also has a bearing on the timing of repeat applications. Following a Bt application, if many of the leafroller larvae that survive do not feed for a week or more, then the next Bt spray should be delayed for over seven days in order to kill most of these survivors. The sublethal effect is minimized by attention to the same factors that provide the best control:
The Bts are an important component of pheromone-based pest management programs. Within the next two years several new materials will probably become available (the insect growth regulators Confirm and Comply, and the spinosad Success). These materials can provide good to excellent leafroller control, and in most cases fit well with pheromone-based programs, but in many instances the use of a Bt may remain the most cost-effective option. Pay attention to the factors critical to successful control with Bt, and your investment of time and money can pay off.
This table contains basic information about each site:
|W. Parker Heights||WA||469||469||7|
Ted Alway, Editor
Phone: (509) 664-5540
Fax: (509) 664-5561
Partial Funding provided by: Washington State Tree Fruit Research Commission, U.S. Department of Agriculture-Agricultural Research Service.
AREAWIDE IPM UPDATE
WSU Cooperative Extension, Chelan County
400 Washington St.
Wenatchee, WA 98801