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Note: this information is considered unpublished work and should not be used as final or finished results. It has been included in IPMP 3.0 because it may not be available from other sources, and in some cases may include information that may not reach final publication.

Research Progress Report - 1991
Prepared by Joyce Takeyasu

Preliminary studies in 1990 indicated that a post-harvest application (September) of the entomopathogenic nematode, Steinernema carpocapsae, effectively controlled late instar mint root borer (MRB) larvae. Unfortunately, by this time, crop damage has already occurred. One way to minimize crop damage would be to apply parasitic nematodes earlier. In addition, lowering the rate would make the nematodes more affordable to growers. These two factors were investigated during the 1991 season along with developmental studies to provide more information on MRB biology. This developmental information may prove to be useful in better timing nematode and chemical applications.

Three rates of parasitic nematodes were compared on three different occasions during the 1991 season. There were two pre-harvest applications, one on July 18 and another on August 13, followed by a post harvest application on September 10. On each date, three nematode rates were compared: 1 ) the standard rate of 3 billion infective juveniles (IJs)/acre, 2) a half rate of 1.5 billion IJs/acre and 3) a quarter rate of 0.75 billion IJs/acre. Due to an error, the nematode rates on the second application date (August 13) were approximately 1) 4 billion IJs/acre, 2) 2 billion IJs/acre or a 2/3 rate, and 3) 1 billion IJs/acre or a 1/3 rate. An untreated check served as the fourth treatment. A randomized complete block design was used with each treatment replicated five times. Nematodes donated by Biosys (Palo Alto, CA) were applied to 160 sq. ft. plots using a carbon dioxide driven backpack sprayer delivering approximately 30 gallons/acre spray solution at 30 psi. The plots were irrigated prior to treatment to moisten the ground, then sprinkler irrigated immediately after application with approximately one inch of water. Each plot was divided into six equal sections and a 1/2 sq. ft. soil sample randomly taken from each section. Samples were taken approximately one month after nematode application for the pre-harvest treatments-and one week after nematode application for the post-harvest treatment. Rhizomes were placed in Berlese funnels to extract live MRB larvae.

In the two pre-harvest applications, all three nematode rates significantly reduced the MRB population (p < 0.05) from that of the untreated checks (Table 1). Percent reduction varied from 63.5% to 96.1 %. In the post-harvest application, both 1 .5 billion IJs/acre and 3 billion IJs/acre significantly reduced the MRB population (p < 0.05) by 51.2% and 75.6% respectively. However, no differences were detected between the quarter rate (0.75 billion IJs/acre) and the untreated check. On ll three treatment dates, there was a positive correlation between nematode rate and percent reduction.

Table 1. Results of parasitic nematode efficacy against MRB larvae, 1991.
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                                                      Average Number of
Date of Nematode                                 MRBs/3 sq. ft.
Application             Trts                       (mean + SEM)     % Reduction
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7-18-91                      1                    17.0 + 3.2    a                 --
                                 2                     6.2_ 1.6    b                63.5
                                 3                      6.2 + 2.5    b               63.5
                                 4                     2.4 + 1.1    b               85.9

8-13-91                      1                   10.2 + 1.8    a                 --
                                 2                     2.6 + 0.6    b               74.5
                                 3                     0.4 + 0.4    b               96.1
                                 4                     1.4 + 0.6    b               86.3

9-10-91                      1                     8.2 +    1.6    a             --
                                 2                      5.8 +    1.2    ab         29.3
                                 3                      4.0 +    1.0    bc         51.2
                                 4                      2.0 +    1.2    c           75.6
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a Treatments:     1: untreated check, 2: quarter rate (0.75 billion IJs/acre), 3: half rate (1.5 billion IJs/acre), 4: standard rate (3.0 billion IJs/acre)

NOTE: Due to an error, the rates for the August 13 application are approximately 1 billion IJs/acre (trt 2), 2 billion IJs/acre (trt 3), and 4 billion IJs/acre (trt 4).

Entomopathogenic nematodes, applied earlier in the season and at reduced rates, show promise in controlling the mint root borer. Although there are some data to support the use of the lowest rate, extreme caution must be exercised in light of the results of the post-harvest application. In this instance, there was no significant difference between the lowest rate and the untreated check. Also, since there is an increasing trend in percent reduction as rate increases, more studies are necessary before the lowest rate can be recommended.

It should be noted that percent reduction in general is lower in these experiments, especially the post-harvest application, than what was observed previously in 1990. This could be due to the fact that the nematodes were applied with a boom sprayer instead of being chemigated. The presence of foliage may have resulted in non-uniform coverage in the pre-harvest applications. In the case of the post-harvest application, there may not have been sufficient time to see the full effect of the nematode treatments since these plots were sampled one week after application.

Results suggest that a rate of 1.5 billion IJs/acre is not sufficient, 63.5% mortality pre-harvest and 51.2% mortality post-harvest. The 96.1% reduction from the August 13 application represents a rate of 2.0 billion IJs/acre suggesting that this is the minimum effective rate for MRB. However, more work is needed to pinpoint the lowest rate possible that will provide adequate control.

One concern about pre-harvest application is that the nematodes may not persist long enough to provide adequate control if they are applied too early. According to the pheromone trap count data, the application on July 18 corresponded to the time of peak male moth flight. Using developmental information gathered from the laboratory studies as a guide, eggs were probably being deposited on the foliage for at least one week after nematode application and eggs may not have hatched for another week. The nematodes would have had to persist in the soil for two weeks to provide adequate control. A second sampling of these plots (four 1/2 sq. ft./treatment) were inconclusive. The results indicated no differences between the treatments at the p = 0.05 level but showed an increasing trend in percent reduction as rate increased. Again, further studies are warranted.