Can Dicyphus hesperus control whiteflies in greenhouse tomatoes?

Table of Contents

1. Introduction
2. Details of Trials Setup
3. Monitoring Populations of Whiteflies & Dicyphus
4. Results
   • Whitefly population levels
   • Mullein as a Banker plant for D. hesperus
5. Dicyphus/Mullein Plant in "hot spots"
   • Population of D. hesperus and whiteflies
   • Location of D. hesperus on tomato plants
   • Dispersal by D. hesperus
6. Conclusions
   • Acknowledgements
7. For more information...

1. Can mullein plants contribute to establishment of the predatory bug, Dicyphus hesperus, under commercial conditions?
2. Can D. hesperus populations on mullein banker plants suppress whitefly populations in commercial greenhouse tomatoes?

During the winter of 2002, trials were established at two commercial greenhouse tomato operations that consisted of several physically separated houses. Such separation facilitated allocation of one of three treatments to a separate house. Treatments included (1) Dicyphus + mullein, (2) Dicyphus + no mullein, and (3) no Dicyphus.

Figure 1. Light green recently moulted Dicyphus hesperus.

Figure 2. Mature darker adult Dicyphus hesperus.

| Top of Page |

Details of Trials Setup

Mullein plants, with a diameter of 20-25 cm and planted in 20-cm pots or hanging baskets, were placed in the greenhouses during the last week of January, 2002, approximately 1-2 weeks after the tomato transplants were planted on rockwool slabs. The initial number of mullein plants was 21 per 1000 m2 in both operations. All mullein plants were initially located in one area for convenience of care for both the plants and the predatory bugs. All mullein plants were watered via the drip irrigation system provided for the tomato crop. At both operations, release of D. hesperus (supplied by Biobest Canada Inc.) onto the mullein plants began on January 28 and continued weekly until the end of February to provide a total of 1.3 bugs/m2. Ephestia eggs were provided as a protein source for the predatory bugs until late June. Observations of all mullein plants for adult and nymphal D. hesperus began on February 7 in both operations, and continued weekly until the end of July. Release of Encarsia formosa and Eretmocerus eremicus proceeded in all houses according to normal practice.

Figure 3. Mullein plants in holding area prior to moving to "hot spots" in crop.

Figure 4. Mullein plant hanging within tomato crop.

| Top of Page |

Monitoring Populations of Whiteflies & Dicyphus

Whitefly populations in all treatments were monitored by means of sticky cards and crop inspection. When a "hot spot" (1-3 plants with >10-20 adults at the growing point) was observed, one mullein basket was taken from the initial holding area and hung from the support wire in the "hot spot." Once the mullein plants were placed at the "hot spots," no further Ephestia eggs were placed on those plants. Subsequently, four entire tomato plants, two on either side of the mullein, were examined for adults and immature stages of both whiteflies and D. hesperus. Estimated populations for both insects were recorded.

Results

Whitefly population levels

Whitefly incidence was generally low for the duration of the trial particularly during the first three months. Because the low whitefly populations did not justify release of D. hesperus in the Dicyphus/no mullein treatment, only two treatments resulted. The population trend in all three houses in each operation was similar, with a peak in the population occurring towards the end of May followed by a reduction in the population.

Mullein as a Banker plant for D. hesperus

Mullein plants plus Ephestia eggs demonstrated potential as a banker plant system for D. hesperus. Adults and nymphs were observed on the mullein plants for 19 weeks following the last release of the predator which occurred at the end of February. Population growth of D. hesperus on mullein differed in the two operations.

In operation I, numbers of both adults and nymphs fluctuated throughout the season with an increasing trend until the end of July when numbers decreased.This reduction in population may have been because provision of Ephestia eggs had ceased about a month previously, causing the predators to emigrate from the mullein onto the neighbouring tomato plants. In this operation, the highest average number of adults estimated per mullein plant in one week was 40, and that for nymphs was 28 (see Figure 5).

In operation II, adult populations were at lower levels than in operation I, but nymphal levels were generally similar except for a much higher peak occurring in March. This early high population of nymphs was likely due to the healthier condition of the plants in this operation. The decline of nymphs in the last few weeks may have also been due to cessation of the Ephestia egg supply a few weeks earlier. For this operation, the highest average number of adults estimated per mullein plant in one week was 15, and that for nymphs was 50 (see Figure 5).

| Top of Page |

Operation I

Operation II

Figure 5. Average number of adult and nymphal Dicyphus hesperus per mullein plant (total of 19 mullein plants in Trial I; total of 12 in Trial II) maintained under commercial conditions, Leamington, February - July, 2002.

Text equivalent of image

| Top of Page |

Dicyphus/Mullein Plant in "hot spots"

Population of D. hesperus and whiteflies

Observations at several "hot spots" indicated that D. hesperus moved from the hanging mullein onto the adjacent tomato plants. The presence of very young nymphs on the tomato plants suggested that D. hesperus had proceeded to establish themselves on those plants. Generally, the population of D. hesperus appeared to respond to that of the whiteflies, and therefore, to contribute to suppression of the whitefly population on those plants. In all the "hot spot" areas, the population of whitefly nymphs declined from peaks of over 150 per plant to very low levels after 4-6 weeks. It must also be noted that there was a high level of parasitism by both Encarsia formosa and Eretmocerus eremicus on plants within the "hot spots." It was therefore difficult to determine to what extent D. hesperus assisted in suppressing the whitefly populations in those areas.

Location of D. hesperus on tomato plants

Although the predator was often observed on lower leaves, adults and nymphs were also observed at the growing point, and on mid-canopy leaves. This distribution could have been influenced by the position of the mullein plant which was adjacent to the upper third of the plants. Individuals were often observed on, or close to, the mid-rib on upper and lower leaf surfaces.

Dispersal by D. hesperus

Observations of neighbouring plants revealed the presence of this predator on plants a couple of meters away from the mullein plant along the length of the row, as well as on plants directly across the row. However, earliest observation of their presence on plants outside of the "hot spot" was about four weeks after the mullein banker plant was in place. General observations indicate that this predator may not disperse readily as long as a food source is within the immediate vicinity. If this is indeed the case, then precise placement of banker plants would be necessary for effective suppression of whitefly populations.

| Top of Page |

Conclusions

1. Mullein plants together with Ephestia eggs as a protein source provide a good potential system for establishing D. hesperus under commercial conditions. However, the mullein plants need to be healthy and robust for optimum development of D. hesperus. Mullein plants require optimum watering, good drainage, and open, sunny locations.
2. Use of mullein banker plants/D. hesperus may contribute to suppression of whitefly populations and are likely to work best when placed directly in "hot spots" (as defined in this article) that are just developing.
3. Further work on use of mullein plus D. hesperus in a single long-season crop is necessary to determine its ability to contribute to management of summer populations of whiteflies which are almost invariably very high.

Acknowledgements

The cooperation of the commercial operations, Igino Ingratta & Son and Southshore Greenhouses, and the technical assistance provided by Lorne King and Tom MacDonald of MGS Horticultural Inc., are gratefully acknowledged. I also thank Kelly Devaeres of MGS Hort. Inc. and Dave Gillespie for kindly reviewing this article. 

| Top of Page |