Effect of Reduced-Risk and Other Biorational Pesticides on the Control of Spider Mites (Tetranychus urticae)

Steven A. Tjosvold
University of California Cooperative Extension
Santa Cruz and Monterey Counties
1432 Freedom Blvd.
Watsonville, CA  95076
satjosvold@ucdavis.edu

Pesticides that control spider mites often account for a large portion of the total pesticide use in many ornamental crops. With the neccessity to frequently enter the fields and greenhouses to harvest many of these crops, there is a need to use miticides with minimal re-entry intervals.

Reduced risk pesticides, as defined by the Environmental Protection Agency, and other biorational pesticides usually have minimum re-entry intervals. Some are 4 hours or less, and thus are good candidates for use in crops that require frequent harvesting. In addition, reduced risk pesticides will become more available in the near future because they receive special "fast track" registration by the EPA .

The goal of this experiment was to evaluate the effectiveness of various registered and experimental "reduced-risk" pesticides (and other pesticides that have minimum re-entry intervals) for control of two-spotted spider mites, Tetranychus urticae.

Materials and Methods

Spider mites (Tetranychus urticae) were collected from a commercial rose greenhouse with a history of having difficulty in controlling spider mites with conventional registered pesticides. In fact the nursery was actively transitioning to use predatory mites for spider mite control. The collected spider mites were raised in a greenhouse on a flat of young lima bean plants for about 4 weeks in Salinas California. Concurrently with the developing mite population, recently germinated lima bean plants were grown for about 2 weeks in 4 inch pots. From each plant, a fully expanded leaf was selected and all other leaves were removed. Fifteen adult female spider mites (Tetranychus urticae) were aspirated onto a collection disk and transferred onto the selected leaf. The mites were allowed to develop on the selected leaf for about 2 weeks. A complete cohort of eggs, nymphs and adults existed at the time of treatment application. There were 5 replications of each treatment and the pots were laid out in a randomized complete block experimental design on the greenhouse bench. Water "moats" around each experimental plant insured that mites would not move from one plant to another. The average greenhouse temperature during the experiment was 69.2 F.

Treatments were applied on September 10, 2001 at rates according to the manufacturers’ recommendations. See table 1 and 2. Treatments were sprayed with pressurized hand sprayers so that the leaves and spider mites were thoroughly covered.

Spider mites were counted directly on the leaves with the aid of a dissecting microscope 3, 7, 14, and 21 days after the treatments were made (DAT). Spider mite counts were made for nymphs (Table 1) and adults(Table 2).

Results and Discussion

The nymph data in Table 1 best represents the overall effectiveness of the treatments. The adult data in Table 2 represents relatively low counts and therefore are not as useful to understanding treatment efficacy. However, E-RASE (IJO Products, 0.5 and 1.0%) at the 1.0% rate was relatively more effective on adults than nymphs for comparable periods and treatments.

Avid (Syngenta, 4 floz) had very good effectiveness even though the spider mites were removed from greenhouses that only two years ago apparently contained Avid-resistant mites. For over a year, spider mites in these greeenhouses were being controlled by the predator Phytoseiulus persimilis. It appears that these spider mites may have reverted back to an Avid- susceptible state. Ultiflora (Gowan, 8 and 16 fl oz) had very good effectivness. The active ingredient, milbemectin, is a close chemical relative to avermectin (Avid) so there was concern about the possibility of cross resistance. In this experiment and other experiments, this has not proven that to be the case.

Of the insecticidal surfactants, Mpede (Mycogen, 1%), DR A034 (Ecosmart, 1 and 2%), E-RASE (IJO Products, 0.5 and 1.0%) and LQ-215 (Safe Science, 0.2 and 0.4%) only DR-A034 and E-RASE (1%) were moderately effective. The unexpected lack of significant effectiveness with some of these products might have been due to the relatively warm temperatures and dry conditions on the day of the treatment. A longer wetting period might have increased the activity of these contact compounds.

The ovicides, Hexygon (Gowan, 2 oz) and Ovation (Scotts, 2 fl oz), had very good control. Maximum activity was seen after 14 days. TetraSan (Valent, 2 and 4 oz) a mite growth regulator had very good control, and activity was seen in only 7 days. There was limited "knock down" for these treatments as evidenced by the the relatively high numbers of nymphs in the 3 DAT observations.

Acari (Sepro, 12 and 16 floz) treatments had good mite control. Pylon (Olympic, 2.4 floz) had very good mite control. Floramite SC (UniRoyal Chem., 4 and 8 floz), a new formulation, and Floramite WP (UniRoyal Chem., 4 oz) had very good control. With the SC formulation, the higher rate had significantly better control in the first 7 days.

Phyton 27 (Source Technology Biologicals, 13 and 26 floz) had no significant activity on spider mites.

(Accompanying tables available only in print Bulletin.)

(Report dated September 2001.)