Resource Information
Field Evaluation of Spray Adjuvants

By: Ronald F. L. Mau, Laura R. Gusukuma-Minuto, and Robin S. Shimabuku

University of Hawai at Manoa
College of Tropical Agriculture and Human Resources
Department of Entomology and Maui County Cooperative Extension Service


OBJECTIVE
Cole crops, onions and certain other crops have waxy leaves that present considerable difficulty in wetting when sprays are applied. In the case of head cabbage, the control of diamondback moth and other caterpillar pests is directly correlated with the timing and coverage of spray applications. Our objective was to evaluate the performance of different spray adjuvants on wetting waxy leaf surface of mature head cabbage.

MATERIALS and METHODS
The field study was conducted at the Kula Branch Station fields at the Maui Agricultural Park on December 8, 1994. Twelve spray adjuvants were evaluated on mature head cabbage for spray coverage on three critical locations of the plant. Each adjuvant was mixed with water and applied with a tractor equipped with a simple horizontal spray boom. The spray pump was PTO driven. The 30 ft. spray boom set with three banks of hollow cone (TX-26) nozzles (Spraying Systems). The boom was positioned approximately 3 inches above the plants when spraying. Sprays were applied at 150 GPA and 100 psi.

Spray adjuvant were applied at either a prevailing grower-use rate or at approximately the mid-point of the product’s labeled range with insecticides. A spray pattern indicator dye, Turf Mark, was added at a rate of 3 oz./A to assist in visualizing the spray coverage. Three gallons of spray was prepared for each test material, and the tank mix was agitated with a tank mounted by hydraulic jet. Sprays were applied to a three parallel rows of mature cabbage. Each test plot was 70 ft. in length.

Evaluations were made immediately after spray application. Twenty plants were randomly selected and rated for the extent of spray coverage using the following categories:
· base of head
· under leaf curl on top of head
· under wrapper leaves
· overall coverage


For each plant evaluated, a rank-value was given in each category. The ranking value ranged from 0 to 5. The ranks were defined as:
1 = no coverage
2 = very few drops present
3 = few drops present, ~ 25% of surface covered
4 = moderate coverage, ~ 50% of surface covered
5 = many drops on surface, almost all surface covered
6 = complete coverage, entire surface wet with a layer of wetness

The results were subjected to analysis of variance in a completely randomize design (SAS Institute, version 6.04). Mean ranks were separated using Tukey's studentized range test.

RESULTS
The results are presented in Table 1. When all results are considered, Silwett L-77 provided the best performance. It wet the entire surface of the cabbage. The Silwett L-77 treatment provided complete coverage by what appeared to be a thin layer of spray. There were no spray droplets on the leaf surface as in other treatments. This spray dried rapidly, within 5-10 minutes after application. The other silicon product tested, Sylgard, also provided excellent coverage, but it did not provide as a completed coverage of the leaf surface when compared with the Silwett L-77 treatment.
Several other spray adjuvants performed fairly well. These included Activator 90, Excel 90, and R 11. All of these products provided wetting of the cabbage leaves that included complete coverage with a thin layer of spray on certain areas of the leaf surfaces and with flattened beads of water on other areas.
There was a third group of spray adjuvants that did not perform as well as previously mentioned ones. These adjuvants generally provided good coverage only on the top of the cabbage head, and poor coverage at the other evaluation sites. We observed some surfactant action, such as the slightly flattened shape of spray droplets as compared with the rounded droplets in the water check plot. In some cases, the water droplets were smaller (i.e. Bivert and Latron). As a group, these spray adjuvants did not provide uniform wetting of the leaf surfaces.
The results provided a relative comparison of some of the spray adjuvants that are commercially available. It should be understood that good spray coverage is just one of several factors that is important in insect control. The next step is to evaluate whether the adjuvants provide adequate insecticide concentrations on the plant surface to provide effective control.
Since growers must make cost benefit decisions, we have compared the relative application costs of the spray adjuvants that were evaluated in this study. This information is provided in Table 2.

Table 1. Evaluation of spray adjuvants on head cabbage. Ranked values were based on the following scale: 0 = no coverage, 1 = very few drops present, 2 = few drops present, ~ 25% of surface covered, 3 = moderate coverage, ~ 50% of surface covered, 4 = many drops on surface, almost all surface covered, 5 = complete coverage, entire surface wet with a layer of wetness. Kula, Maui, HI. December 1994.

Means within a column are significantly different (Tukey's studentized range test P< 0.0001).
1Bivert was applied with a wettable powder pesticide as per label directions.


Table 2. 1994 Surfactant Trial. Evaluation of surfactants on head cabbage. Cost per test application. Price current as of 12-27-94. Kula, Maui, HI. December 1994.

PRECAUTIONARY STATEMENT
Use pesticides safely. Follow the pesticide label. Consult with the Cooperative Extension Service or the Hawai'i State Department of Agriculture for authorized special local need registrations or additional information. The user is responsible for the proper use, application, storage, and disposal of pesticides.

DISCLAIMER
Reference to a company or product name does not imply approval or recommendation of the product by the College of Tropical Agriculture and Human Resources, Cooperative Extension Service University of Hawaii, or the United State Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable. All materials should be used in accordance with label instructions or manufacturer's directions.