Testing the Efficacy of Mycotrol ES, Beauveria bassiana, on Tarnished Plant Bugs, Lygus lineolaris, in New York Strawberries 1997

Joe Kovach and Greg English-Loeb
IPM Program
Department of Entomology
Cornell University, Geneva, NY

Introduction

In New York, there are two major pests of strawberries: the disease, Botrytis fruit rot, and the insect, tarnished plant bug (TPB), Lygus lineolaris. Results from recent IPM research projects lead us to believe that strawberries can be grown in NY with fewer fungicides and insecticides than are currently used. Broad spectrum fungicides use on strawberries can be reduced or eliminated by using cultural techniques and the biological control agent Trichoderma harzianum , to control Botrytis fruit rot. As the need for synthetic fungicides are reduced, the possibility of using fungal biocontrol agents to control insects such as tarnished plant bug can become a reality. One insect pathogen that shows promise against the TPB is Beauveria bassiana. This fungus acts as a contact insecticide. Spores attach to the insect, germinate and grow directly through the insect cuticle. The fungus grows rapidly within the insect, killing it in 3-7 days. Speed of kill depends on the number of spores contacting the insect, insect age, and ambient temperature. Particular strains of this fungus have proved to be effective against heteropteran pests such as whiteflies and aphids.

Results from tests conducted in 1995 at the New York State Agricultural Experiment Station (NYSAES) showed that the Mycotech formulation of Beauveria bassiana (Mycotrol WP, 2 x 10 13 spores per pound) in combination with the wetting agent Silwet, applied either as two prophylactic sprays or as one spray at tarnished plant bug (TPB), Lygus lineolaris, threshold levels, reduced TPB injury in strawberries to commercially acceptable levels.

In 1996, two formulations of Mycotrol (WP and ES) were tested, but no effect on TPB populations was observed, primarily because individual plot sizes were smaller in 1996 than in 1995 and migration of TPB between treatments may have occurred. Because more strawberries were available for use at the NYSAES in 1997, plot sizes for this test were larger and the design was more similar to 1995 trials.

The objective of this study was to test the efficacy of two rates of Mycotrol ES (1 qt/A and 1 pt/A) applied at two timings (calendar and threshold) on TPB population in NY strawberries.

Methods

This experiment was conducted at the New York State Agricultural Experiment Station in Geneva, NY on strawberries planted in 1996. Trials were conducted on a June bearing cultivar, `Earliglow' and the day neutral cultivars, `Tribute' and `Tristar'. All of these cultivars are susceptible to TPB damage. There were six treatments replicated 4 times each on the June bearing and day-neutral varieties. Mycotrol ES was applied at two rates (1pt/A and 1 qt./A) and two timings (calendar based and threshold). Malathion 8F (1 qt/A) was applied as a grower standard and an untreated check was also used in this study. Treatments were randomly assigned and consisted of a row of strawberries 10-15 meters long with appropriate buffers between treatments. Sprays were applied at a rate equivalent to 50 gallons per acre using a backpack sprayer. Calendar based sprays were applied on 4 Jun, 9 Jun, 13 Jun, and 23 Jun. Threshold sprays were applied on 9 Jun, 13 Jun, and 23 Jun. The one malathion spray was applied on 4 June.

Throughout the bloom and fruit set season, TPB nymphs were monitored using the cluster tapping method on 30 May, 3 Jun, 6 Jun, 10 Jun, 12 Jun, 16 Jun, and 19 Jun. At least 6 fruit or flower clusters per replicate were sampled to determine TPB densities. After counting, TPB nymphs were returned to their respective replicate. Fruit was harvested when ripe from one meter sections in each plot on 19 Jun, 23 Jun, 26 Jun, 2 Jul, and 10 Jul. At each harvest session berries were counted, weighed, and percent TPB damage was calculated. Harvest and population density data were analyzed separately and pooled by variety, berry weight, and % TPB damage. Log (X+1) transformations were performed before analysis where appropriate. Differences between treatment means were determined using standard statistical methods (SuperANOVA, Abacus Concepts Inc.).

Results and Discussion

Figure 1 shows the mean number of TPB nymphs/cluster at the seven sampling dates of the four different Mycotrol treatments compared to the untreated check and malathion treatments. Generally throughout the season, all Mycotrol treatments reduced TPB populations to lower levels than that found in the check but population densities in these treatments were not as low as in the malathion treatment.

Table 1 shows the percent TPB damaged berries at the 5 sampling dates between the June bearing cultivar "Earliglow' and the day neutral cultivars. Although there were differences in the mean percent damaged fruit between date, cultivar, and treatment, none these differences were not significant (P>0.05).

Results from this experiment are summarized in Table 2 and it shows the mean number of TPB nymphs/cluster for the different timings and rates of Mycotrol and the percent of strawberries that were damaged by TPB for each treatment. There was no significant difference between pre treatment population levels of TPB however, the Mycotrol calendar/pt treatment did have lower initial numbers (0.15). All plots treated with Mycotrol at any of the rates or timings did significantly reduce TPB nymphs compared to the check however these treatments were not as good as the malathion treatment. The four applications of Mycotrol at the rate of one pint per acre was the best treatment and reduced the TPB damaged berries by nearly 50% compared to the check but this may have been due to the slightly lower pre treatment TPB densities. Again, however even this treatment was not as effective in preventing damage as the one spray of malathion. It is estimated that the cost of this one spray of malathion is about $5.00 per acre.

Because Mycotrol is a biological pesticide and it is somewhat slower acting than a chemical such as malathion, it would probably be more effective if it were applied early in the development of a rising population. In this experiment, even initial sprays of the calendar treatment were applied somewhat late. As shown in the pre treatment counts in Table 2, TPB densities were already established before the first applications were made and some damage to the berries had probably already occurred.

In summary, the Mycotrol treatments did have some effect on suppressing TPB populations in this experiment. The best treatment was the four applications of Mycotrol at a rate of one pint per acre which resulted in a nearly 50% reduction of TPB damage fruit compared to the untreated check. However, this treatment was not nearly as good as the grower standard of one spray of malathion. Because this biological pesticide is slower acting than our conventional insecticides, it probably needs to be applied early in pest population development. Therefore, we believe that the threshold timing is not appropriate for this material and applications should be timed to other population development parameters such as percent egg hatch rather than the number of nymphs/cluster. Although this has not been tested it should improve control. In addition, we believe that Mycotrol may work best in settings that have moderate TPB populations such as in strawberry plantings that include cultivars that are more tolerant to TPB damage such as `Honeoye'. Finally, we feel that if an organic formulation of Mycotrol could be developed, this would be a useful product for the organic strawberry grower.

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