Dragan ŽNIDARČIČ and Stanislav TRDAN
Agronomy Department, Biotechnical Faculty, University of Ljubljana, Jamnikarjeva 101, 1000 Ljubljana, Slovenia (contact e-mail: dragan.znidarcic@bf.uni-lj.si)

Introduction

The cabbage whitefly (Aleyrodes proletella L.) have been considered one of the most important group of insect predator in Brassica agroecosystem in Slovenia. Growers currently spray insecticides to control these insect on a one- to two-week preventive schedule. Use of large amounts of chemical can be expensive and damaging to beneficial pollinators and the environment. Therefore, use of yellow sticky traps have been promising in reducing whitefly populations on Brussels sprouts (Brassica oleracea L. convar. oleracea L. var. gemmifera DC.) compared to other insecticide methods.

Material and methods

The study was conducted at the Biotechnical Faculty in Ljubljana, for two consecutive growing season. 'Boxer F1' sedlings were transplanted into black plastic on May 03 2000, and on May 17 2001.

The experimental design was a completely randomized design of four tretments in four replicates. In the natural infestation trial, the four treatments were as follows: spraying with a systemic insecticide imidacloprid (Confidor SL 200), spraying with a contact insecticide pirimiphos-methyl (Actellic-50), the yellow sticky traps and the control (no agents whatsoever were used). Each experimental unit (parcel) had two rows, each with 6 plants. On June 01 2000, and on June 08 2001 three sticky traps were placed on each parcel in the treatment 'yellow sticky traps' for the first time. The traps were removed in 10 to 14 days long intervals and replaced by new ones.

The first spraying against cabbage whitefly (Aleyrodes proletella L.) in the first year of the experiment took place in the beginning of August (August 02 2000), while in the year 2001 it was performed on September 06. 50 mg a. i./m² of pirimiphos-methyl and 20 mg a. i./m² of imidacloprid were used. In the year 2000 the same method was used on September 08, while on October 05 Brussels sprouts was sprayed only (for the third time) with the contact insecticide. Second (i.e. the last spraying) in the year 2001 was performed on October 23.

The preliminary tests on the efficacy of the control methods in the experiment were performed on September 05 and September 29 in 2000, and on September 19 and November 15 in 2001 respectively. The cabbage whitefly larvae were counted on one leaf from the upper third of plant, on each of the 12 plants in all 16 experimental units. At harvest time (on December 04 2000, and on December 28 2001) all Brussels sprouts were harvested and their weights recorded. The data were subjected to the analysis of variance and Newman-Keuls' methods (5 % level).

Results and discussion

In the year 2000 the first spraying took place approximately a month after the flying forms of the cabbage whitefly were first recorded (more than 2 adults were caught on a trap per day), while in the year 2001 the first spraying was performed a month later. The reason was a considerably lower pest population in the second year of the experiment. The second spraying in the year 2000 took place when the number of adults was approximately 40-times higher, however the abundance of the cabbage whitefly in the second year of the experiment before the second spraying was smaller due to already mentioned unsuitable climatic conditions. The insecticide pirimiphos-methyl can be used up to three times in the growing season of vegetables, therefore it was also used for the third time in the year 2000. By then, the number of the cabbage whitefly adults caught daily on each trap reached 130.

In both years and all terms of evaluations the lowest number of the cabbage whitefly larvae (Aleyrodes proletella L.) was found on the plants sprayed with pirimiphos-methyl. In the year 2000 this number was only from 41 to 59 %, compared to the plants given other three treatments, and it was statistically significantly lower than the number of the larvae found in the other three treatments. In the year 2001 statistically significant differences among all four treatments was not confirmed, the number of larvae on plants sprayed with the pirimiphos-methyl however was from 56 up to 77 % of those that were found on the plants from the other three treatments.

Figure 1: The average number of the cabbage whitefly (Aleyrodes proletella L.) larvae on randomly chosen leaf per plant (A) and average weight of Brussels sprouts per plant (B). Values followed by different letters differ significantly at the 5% level (Newman-Keuls' test)

Among the most important factors that had influence on the mentioned number we expose a relatively late term of the second evaluation in the year 2001 (minimum temperature during November was below 0^oC and this factor had the influence on increasing of pest mortality), this fact should not make us underestimate the role of some other factors, above all the lack of vitality (result of a dry growing season that diminished the quality of the pest host) of a major part of the plants in the experiment.

In the first year of experiment the highest average weight (780.4 g/plant) showed a statistically significant difference compared to the weight in the other treatments; this being the case on Brussels sprouts sprayed with the contact insecticide pirimiphos-methyl. The average weight in this treatment was from 20 up to 31 % higher than the weight in the other treatments. This fact can indicate either a direct impact of the cabbage whitefly on the weight of Brussels sprouts or a positive effect of the chosen insecticide on reduction of the pest number below the damage threshold.

In the plants grown under other three treatments in the year 2000 as well as in the plants that were grown in all four treatments in the year 2001 no such differences in the average weight were found. The considerable insufficiency of soil moisture in the second year of the experiment influenced smaller weight compared with the year 2000.

Conclusion

The results of our experiment show that the most effective control method against the cabbage whitefly on Brussels sprouts is the use of the contact insecticide pirimiphos-methyl, while the yellow sticky traps have proven to be less successful for mass trapping. The systemic insecticide imidacloprid has proven to be less efficient than expected.

Based on the results of our two-year experiments we conclude that the cabbage whitefly could cause the weight reduction only in the growing seasons that enable its mass occurrence. This fact is closely connected with the environmental factors that have direct (heavy rainfall can significantly reduce the number of adults etc.) or indirect influence (water deficiency can prevent normal growth of the whitefly host plants that affect the quality of its food etc.) on the populations of this pest.

References

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