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Research Progress Report - 1994
Prepared by Joyce Takeyasu

Note: this information is considered unpublished work and should not be used as final or finished results.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

The entomopathogenic nematode, Steinernema carpocapsae (Weiser) All strain, is capable of controlling the mint root borer (MRB), Fumibotys fumalis (Guenee), both pre- and post-harvest. Since the pre-harvest nematode application targets earlier instars, crop damage is minimized. The nematode, however, can be applied too early. Despite being univoltine, the MRB has a prolonged adult emergence which spreads this single generation over a two month period (Berry, 1974). This, in combination with the short persistence of S. carpocapsae in the soil, makes timing an important factor in achieving good control. To benefit from a pre-harvest application, the nematode should be applied as early as possible to minimize crop damage, yet late enough to ensure control of larvae resulting from later emerging adults.

Just as a pre-harvest application can be applied too early, a post-harvest application can be applied too late. Prior to entering the prepupal stage, late instar MRBs construct a silk-lined earthen cell called a hibemaculum in which to overwinter (Berry and Fisher, 1993; Pike et al., 1988). Once it enters the prepupal stage, the MRB is no longer susceptible to the nematode; therefore, fields must be treated before hibernacula form. Since nematode applications must be accompanied by irrigation to move the nematode into the soil and to ensure nematode survival (Shetlar et al., 1988; Zimmerman and Cranshaw, 1991), proper timing of a post-harvest application is hampered by the lack of irrigation immediately before and after harvest. Also interfering with post-harvest timing is the time-consuming nature of diagnosing and treating fields. Processing soil samples either by hand or with Berlese funnels is a slow, labor-intensive process. If an infestation is found, growers typically need up to a week to treat a field, depending on how long it takes to cover a field with irrigation.

The discovery of variability in MRB development between fields has created further difficulties in properly tirning a nematode application. For exarnple, soil samples taken from two fields in mid-September 1992 revealed 48.3% hibernacula forrnation in one field but only 3.5% hibemacula formation in the other field. At the same point in time' while it is essentially too late to treat one field, another field may still be treatable. Variability in MRB development only serves to narrow an already narrow treatment window. Faster developing fields run a higher risk of having too many hibernacula forrned by the time larvae can be treated post-harvest; slower developing fields run a higher risk of having a pre-harvest treatrnent applied too early.

Extensive sampling is the key to properly timing a nematode application. Given the timeconsuming nature of MRB sampling, however, there may be some merit to treating more than once with a lower nematode rate instead of a single application at a higher rate. Initiating a split application with a preharvest treatment not only contributes to minimizing crop damage, but also provides assurances that the control measure will be properly timed. Promising results from a small plot split application trial conducted in 1993 led to a large scale experirnent in 1994 to further investigate the potential of applying the nematodes in a split application.

MATERIALS AND METHODS

The following treatments were compared using the length of an irrigation line as an experimental unit:

1) untreated control
2) 2 applications of 0.5 billion IJs/acre on July 26-29 and August 15-20
3) 2 applications of 0.75 billion IJs/acre on July 26-29 and August 15-20
4) 1 application of 1.5 billion IJs/ha on August 15-20

The applications on July 26-29 were pre-harvest; the applications on August 15-20 were post-harvest. On the date of application, the irrigation was started to wet the ground for at least one hour prior to applying the nematodes. The nematode was injected into the irrigation line over at least a 30 minute period and immediately followed with approximately 2 inches of additional irrigation. The first evaluation was made on August 11 and 15, sampling two blocks on each date by taking eight 1/2 ft² samples to a depth of 21/2 inches along each irrigation line. Because the single application of 1.5 billion IJs/acre had yet to be applied, MRB numbers were determined in the untreated control and the pre-harvest applications of the split application treatments only. The second and final evaluation was made on August 25 for the untreated control, and September l for the nematode treatments. The untreated areas were sampled first to allow the grower to treat those areas as soon as possible. Along each irrigation line, ten l/2 ft² soil samples were taken. MRB nurnbers were determined by separating the rhizomes from the soil and placing them in Berlese funnels equipped with 75 watt bulbs. The extraction process took four days or until the rhizomes were completely dry. The soil was sifted and a visual search was made for larvae and hibernacula.

RESULTS

There were significant differences detected among all the treatments on the first evaluation. An average of 12.3 MRBs per ft² found in the untreated areas was reduced 28.9% with the pre-harvest application of 0.5 billion IJs/acre. The 0.75 billion IJs/acre rate was even more effective, resulting in a 60.9% reduction in MRB numbers. Neither rate, however, was successful at lowering MRB numbers below the treatment threshold of 2-3 MRBs per ft². On the second evaluation, all nematode treatments significantly reduced MRB numbers compared with the untreated control. In addition, all nematode treatments reduced MRB numbers below the treatment threshold. The post-harvest application of 0.5 billion IJs/acre resulted in an additional 85.1% reduction for an overall percent reduction of 88.9%. The 0.75 billion IJs/acre rate resulted in an additional 85.5% reduction for an overall percent reduction of 94.3%. Overall percent reduction for the single application of 1.5 billion IJs/acre was 97.9%. Although significant differences were not detected among any of the nematode treatments, best control was seen with the single post-harvest application. Between the split application treatments, the higher application rate gave better control.

DISCUSSION

In addition to pre-harvest and post-harvest applications, a split application of S. carpocapsae is yet another option for MRB control. The split application treatments reduced MRB numbers to a level comparable to that of the single application at the higher rate. Keeping in mind, however, that a significant increase in MRB control was not observed and that implementation would require additional input of labor, split applications are not a substitute for a properly timed single application. Only in certain situations should a split application be considered.

A split application should be considered if a field is scheduled to be harvested late or a delay is anticipated in restoring the irrigation after harvest. In the past, MRB control typically occurred in September. Taking into consideration the resistant nature of the prepupal stage and the variability in MRB development, inconsistent MRB control may be attributed in part to too many hibernacula forming in the field before treatment. In hindsight, efficacy may have been higher had control measures been initiated sooner. The single post-harvest application in this experiment was properly timed in mid-August; however, attempts to treat fields earlier will increase the probability that harvest will conflict with the application. In such cases, a split application will ensure proper timing.     Heavily infested fields also are candidates for a split application. The higher the infestation, the more critical is the need to initiate control pre-harvest because waiting until after harvest can result in severe crop damage. A single pre-harvest application, however, is risky since the consequences of an improperly timed treatment are magnified in heavily infested fields and can easily result in a problem the following year. A split application starting with a pre-harvest treatment provides assurances that proper timing will occur with minimal crop damage. Differences were not detected in control efficacy between the single and split applications; however, the results suggest less control with a split application. Additional replication may have been helpful in determining whether differences exist between the single and split applications. Nevertheless, it is a disturbing trend that warrants further investigation.

    Better post-harvest control with S. carpocapsae is associated with higher infestation levels, even when lower nematode rates are used (unpublished data). In contrast, at lower MRB densities, control efficacy appears to be positively correlated with nematode rate. This provides a possible explanation for the results since the field used in this experiment was heavily infested with an average of 12.3 MRBs per ft². With the pre-harvest treatment, the split applications may have lowered MRB density to a level that adversely affected the effectiveness of the post-harvest treatment. The single post-harvest application which resulted in better control was directed at a higher MRB density. Studies on population dynamics are needed to determine the density below which a field would not require treatment the following year. Without knowing what this level is, it is difficult to know whether the decrease in control seen with the split application is cause for concern.

Although some control appears to be sacrificed with a split application, it may be more important to initiate control earlier in heavily infested fields than to suffer severe crop damage. A crop damage index was not conducted; however, the 60.9% reduction in MRB numbers obtained with the pre-harvest application of 0.75 billion IJs/acre may have prevented substantial damage to the crop. Another potential advantage of using a split application is the possibility of obtaining adequate control with the first application. Although none of the split application treatments provided adequate control with the first application, further studies are needed to evaluate different nematode rate combinations. For example, a higher pre-harvest rate may be desirable to further minimize crop damage early in the season, followed by a lower post-harvest rate, if necessary. This combination may yield better control than a split application using the same rate.