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Research Progress Report - 1993

Note: this information is considered unpublished work and should not be used as final or finished results.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.

Introduction

Entomopathogenic nematodes can effectively control mint root borer larvae both pre-harvest and post-harvest. The advantage of a pre-harvest application is that it minimizes damage to the crop. In heavily infested fields, a pre-harvest application is almost necessary. Proper timing, however, is crucial in achieving good control. Because of the limited persistence of entomopathogenic nematodes in the soil combined with the prolonged emergence of mint root borer adults, an application may be applied too early. Similarly, a post-harvest application may be too late if hibernacula are already present since the hibernacula are resistant to both the nematodes and chlorpyrifos (Lorsban).

A narrow treatment window makes timing an important factor in successful mint root borer control. If successful, a split application of nematodes would alleviate some of the precision required in timing an application. Also, since one of the applications goes on pre-harvest, crop damage is minimized. A small plot experiment was conducted to test the feasibility of split applications of entomopathogenic nematodes.

Materials and Methods

A randomized complete block design was used with the following treatments:

1. untreated check
2. 3 applications of 0.5 billion IJs/acre on 7/24, 8/5 and 8/24
3. 2 applications of 1.0 billion IJs/acre on 8/5 and 8/24
4. 1 application of 2.0 billion IJs/acre on 8/5

The experiment, replicated five times, was conducted in small plots (8'x 40') with 2' borders surrounding each plot. The applications on 7/24 and 8/5 were pre-harvest; the application on 8/28 was post-harvest. The nematodes were applied with a CO2 backpack sprayer approximately l/2 hour into the irrigation set and followed by approximately one inch of irrigation. The plots were evaluated on 9-11 and 9-13 by taking eight 1 ft2 soil samples from each plot. The rhizomes were placed in Berlese funnels to extract mint root borer larvae. In addition, a visual search for hibernacula was made for each soil sample.

Results and Discussion

Compared to the untreated check, both split application treatments significantly reduced mint root borer numbers, as did the pre-harvest application of 2 billion IJs/acre. The split application consisting of two applications of a 1 billion IJs/acre rate resulted in a greater reduction of mint root borer than the split application consisting of three applications of a 0.5 billion IJs/acre rate (94.9 % reduction and 84.8 % reduction, respectively). Split applications look promising as the way to apply nematodes against mint root borers in the future.

Table 1: Control of mint root borer using split applications of entomopathogenic nematodes
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                                   Mean no¹(+ SEM)
Treatment                       MRB per ft2            % reduction
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Untreated                        2.0 ( 0.5) a                    ---
3 aps of 0.5 bil IJs/A       0.3 ( 0.1) b                   84.8
2 aps of 1.0 bil IJs/A       0.1 ( 0.1) b                   94.9
1 ap of 2.0 bil IJs/A        0.2 ( 0.1) b                   92.4
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¹ Means followed by the same letter are not significantly different. Separation of means by FPLSD, p = 0.05.