Use of Bacillus thuringiensis israelensis and Methoprene to Control the Asian Tiger Mosquito, Aedes albopictus (Skuse) (Diptera: Culicidae), Larvae and Pupae Populations in Non-circulating Hydroponics Tanks

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S.C. Furutani & L. Arita-Tsutsumi, College of Agriculture, University of Hawaii at Hilo, 200 W. Kawili Street, Hilo, HI 96720-4091, USA

Abstract

Bacillus thuringiensis israelensis, (Bti) and methoprene (Isopropyl (2E-4E)-11-trimethyl-2,4-dodecadienoate) were tested for control of the Asian tiger mosquito, Aedes albopictus (Skuse) in non-circulating hydroponics tanks of lettuce. The results showed that Bti and methoprene formulations reduced mosquito larvae and pupae population for the duration of the lettuce crop (4 to 5 wk) compared to the non-treated control. Bti and methoprene treatments, however, displayed reduced lettuce head weight and root growth. Leaf lettuce cultivars Red Sails' and Green Ice' were more tolerant to Bti treatment compared to Manoa'. Lowering the Bti (VectoBac G) application rate from 1.04 to 0.54 g/m² increased head weight and root growth for Manoa lettuce. The tank treatments presented in this study were not replicated due to the use of large, commercial-scale tnks and limited greenhouse space.

Introduction

In recent years, there has been great interest in stagnant water (non-circulating) hydroponic systems since it is cheaper to setup and operate and does not require electrical power compared to traditional circulating hydroponic systems (Kratky 1990). At present, there are several large commercial non-circulating hydroponic operations in Hawaii that have recently been established which are proving to be very successful. However, as non-circulating hydroponic operations expand and increase in number and size, growers are facing the problem of mosquitoes breeding in the stagnant water of the hydroponic tanks. The mosquitoes breeding within these hydroponic tanks pose no problem to crop production, but can be extremely irritating to farm workers.

The mosquito commonly found breeding in non-circulating hydroponic tanks in Hawaii is the Asian tiger mosquito, Aedes albopictus (Skuse). The A. albopictus life cycle starts with eggs hatching in water where they soon pass through the larval and pupal stages. The adults emerge with females searching out a blood meal to initiate egg production. Nutrient-rich, stagnant water, such as those used in non-circulating hydroponics, provides an ideal site for larval and pupal development. Furthermore, upon adult emergence, the farm workers tending to hydroponic plants provide the blood meal which perpetuates the breeding of these mosquitoes.

Mosquito larvae and pupae can easily be controlled with today's chemical insecticides (Ali et al. 1995), however, none are cleared for use in hydroponics. In addition, the use of these insecticides is not an option for hydroponic growers in Hawaii since these products are typically grown and sold under a "pesticide-free" label. This label precludes the application of traditional pesticides to all plant parts including the nutrient solution.

Bacillus thuringiensis israelensis, (Bti) (Ali et al. 1995; Becnel et al. 1996), and the insect growth hormone, methoprene (Ali et al. 1995; Basci et al. 1994; Becnel et al. 1996; Fargal et al. 1988; Sulaiman et al. 1994; and Toma et al. 1990), have been shown to be very effective in controlling A. albopictus in the laboratory and field and are not considered as pesticides from the stand point of qualifying for the pesticide-free label. This study was conducted to investigate the effectiveness of Bti and methoprene to control A. albopictus larvae and pupae populations in non-circulating hydroponic tanks and their effect on lettuce, Lactuca sativa L., yield.

Materials and Methods

General methods

The Bti and methoprene formulations used in the experiments were Altosid (methoprene pellet, Sandoz Crop Protection Corporation, 1300 E. Touhy Ave., Des Plaines, IL), Mosquito Dunk (Bti floating donut, Summit Chemical Company, 7657 Canton Center Drive, Baltimore, MD.) and VectoBac G (Bti organic granules, Abbott Laboratory, Chemical and Agricultural Products Division, 14 th Sheridan Road, North Chicago, IL). Altosid is a chemical growth regulator which disrupts the development of mosquito larvae and pupae preventing their development into adults. Altosid is formulated either as a liquid or a pellet, of which the pellet has longer residual activity. Since the recommended rate of application for the Altosid pellet formulation is between 210 and 280 g/ha, 280 g/ha (0.028 g/m²) was selected for testing in the hydroponic tanks. VectoBac G uses an organic vermiculite carrier to which Bti is impregnated. The recommended rate of application (note: commercial use on lettuce is not consistent with the pesticide label and therefore a violation of federal law if used commercially) for the VectoBac G formulation is 2.8 to 11.3 kg/ha. The 11.3 kg/ha rate (1.08 g/m²) was chosen for our experiment. The Mosquito Dunk is a donut-shaped Bti formulation designed to float. Its recommended dosage, which was used, is one dunk (15 g) / 9.29 m² of surface water (1.61 g/m²)

In order to duplicate typical hydroponic farming conditions, four tanks measuring 2.44 m long, 1.22 m wide by 19 cm deep were constructed with 1.90 cm thick exterior grade (2.44 x 1.22 m) plywood. The sides were constructed with 5.10 x 20.30 cm lumber. The inner surface of the tank was lined with 3 layers of 6 mil thick black polyethylene sheeting. A plywood sheet (2.44 m long, 1.22 m wide by 1.30 cm thick) was used for the tank cover and also served as the support for the forestry tubes (3.80 cm diameter x 20.30 cm long plastic tubes), in which the lettuce seedlings were planted. To support the forestry tubes, the plywood cover was drilled with holes (3.80 cm in diameter) spaced 22.90 x 22.90 cm on center (60 holes per tank). Half of the holes were used for planting and the remaining holes were left open to allow mosquitoes to freely lay eggs within the hydroponic tank. The upper surface of the plywood was painted white to reduce surface temperature.

Lettuce seedlings of cvs. Manoa', Red Sails', and Green Ice' were prepared by first sowing seeds 0.25 cm deep into seedling trays filled with a standard potting mix (Pro-mix BX, Premier Horticulture Inc., Red Hill, PA). After 2 wk, the lettuce seedlings were transplanted into foresty tubes containing a 1:1:1 mixture of peat (ProMix BX): horticultural perlite (medium grade): vermiculite (no. 2 grade). The seedlings were grown under greenhouse conditions for approx. 2 wk before transplanting.

The nutrient solution was prepared by adding 227 liters of water into each hydroponic tank, as described by Kratky 1993, which submerged the bottom of the forestry tube 2.50 cm. The following nutrients were added to each tank; 72 g of 4-18-38 hydroponic mix (Chem-gro, Hydro-gardens, Inc., Colorado Springs, CO 80932), 72 g CaNO3, 15 g KNO3, and 43 g MgSO4. No additional fertilizers were added to the tanks. Electrical conductivity (1.5 ±0.5 milli-siemens, mS) and pH (6.5 ±1.0) of the nutrient solution was maintained throughout the growing period.

Mosquito larvae and pupae populations within each tank were estimated by visually counting live mosquito larvae and pupae within a 0.75 m² area of the 3 m² tank and then multiplying the count by 4. The entire tank could not be counted since movement of water or disturbance of the lettuce plants would result in lower yields when grown under non-circulating hydroponic methods (Kratky, 1993).

Lettuce heads were harvested and weighed when market size was obtained. Root growth measurements and/or ratings were also recorded at time of harvest.

The tanks used in the above experiments were not replicated. The experimental tanks were built to duplicate tanks that are currently used in commercial hydroponic lettuce farms in Hawaii. Due to the size of these tanks and the limited greenhouse space available for this study, the hydroponic tanks were not replicated. The researchers did not want to compromise the size of the hydroponic tanks since it would more closely reflect plant growth and mosquito control in commercial operations.

Mosquito Dunk and Altosid treatments on Manoa lettuce

Manoa' lettuce was grown as described above. Manoa' was selected since it is one of the most widely grown lettuce cultivars among non-circulating hydroponic growers in Hawaii. Manoa' lettuce seeds were started by sowing seeds into seedling trays on 2 July 1996 and transplanted into forestry tubes 16 July 1996. Forty transplanted seedlings were used for each treatment. Three tanks were prepared as above and treated with either Mosquito Dunk, Altosid or left non-treated (control) with application rates as described above. Salinity and pH readings and mosquito larvae and pupae counts were recorded weekly. Lettuce head weight and root length measurements (length and weight of roots pertruding from the forestry tubes) were recorded on 15 August 1996.

VectoBac G and Mosquito Dunk treatments on lettuce

Lettuce cultivars Manoa', Red Sails', and Green Ice' were seeded 23 August 1996. Ten transplants from each cultivar were planted into separate rows within each tank. The tanks were prepared as described above and treated with VectoBac G, Mosquito Dunk, or left non-treated (control). The plants were 10-days-old at time of transplanting. All lettuce cultivars were harvested 1 October 1996. Mosquito larvae and pupae counts; lettuce head weight and rooting data were also collected as described above at time of harvest.

VectoBac G application rate treatment on Manoa' lettuce

To observe the influence of application rate on Manoa' lettuce yield, plants were grown as described above and treated with 3 rates of VectoBac G. The application rates were 0.54 g/m² (0.5 x), 1.08 g/m² (1.0 x), and a 0 g rate which served as the control. The lettuce seeds were sown 20 Sept 1996, transplanted 1 Oct 1996 and harvested 12 Nov 1996. Mosquito larvae and pupae counts; lettuce head weight and rooting data were collected as described above.

Results and Discussion

Mosquito Dunk and Altosid treatment on Manoa lettuce.

There were no differences in average lettuce head weight between the control, Mosquito Dunk, and Altosid treatments (Table 1.). Weight and length of roots, however, were significantly lighter and shorter compared to the control. The Altosid treated plants had 64% reduction in root weight and 43% reduction in root length. Plants treated with Mosquito Dunk had 39% reduction in root weight and 42% reduction in root length. The reduced root weight and length may be a result of the petroleum based ingredients used in the formulation of pellets and floating dunks. Plants grown in non-circulating hydroponic culture are very sensitive to these chemicals as they often leave an oily film on the surface of the nutrient solution affecting gas exchange which may in turn inhibit root growth. Circulating hydroponic systems do not display such sensitivity since these films tend to disperse with moving water.

Altosid and Mosquito Dunk treated tanks had lower numbers of larvae and pupae compared to the nontreated control for the duration of the lettuce crop (Fig. 1). While the Mosquito Dunk treated tanks did have some larvae after 3 wk, the larvae were small and no pupae were observed. Both larvae and pupae were observed in the Altosid treated tank one wk following treatment. Although larvae and pupae were seen in the Altosid treated tank, they probably did not develop into adults. Altosid is a juvenile hormone which is not designed to immediately kill the larvae and pupae. Instead, the hormone blocks the development of the larvae and pupae into adults. Hence, 2 wk following treatment, most of the individuals observed within the Altosid treated tanks were either unusually large larvae or pupae. After 5 wk, no larvae or pupae were observed in the Altosid treated tank, 8 larvae and pupae were observed in the Mosquito Dunk treated tank compared with 76 larvae and pupae for the control tank.

Table 1. Effect of Mosquito Dunk (1.61 g/m²) and Altosid (0.028 g/m²) treatments on mean head and root weight (g), and root length (cm) ± SEM of Manoa' lettuce when grown in non-circulating hydroponic culture.
Treatment * Head wt (g) Root wt (g) Root Length (cm)
Control 109.0 ± 8.5 6.4 ± 0.9 10.1 ± 1.0
Mosquito Dunk 106.6 ± 6.1 3.9 ± 0.5 5.9 ± 0.4
Altosid 103.6 ± 8.0 2.3 ± 0.4 5.8 ± 0.7

* N=40 for each treatment.

VectoBac G and Mosquito Dunk treatments on lettuce

The results of head and root weight and root length for all cultivars treated with Mosquito Dunk and VectoBac G are presented in Table 2. Red Sails' had head weights that were not significantly different from the controls after treatment with VectoBac G, but Green Ice' had a lower head weight. Manoa' had a 42.6% reduction in head weight. There was a significant reduction of root weight and length for Green Ice' using VectoBac G. Compared to the control, Manoa' root weight and length was significantly lowered by 54.1% and 16.9%, respectively.

The Mosquito Dunk affected the head weights of all 3 cultivars. Head weight was reduced by as much as 50.8%. Similiar significant reductions in root weight and length were also observed.

Table 2. Effect of Mosquito Dunk (1.61 g/m²) and VectoBac G (1.08 g/m²) treatment on mean head and root wt (g), and root length (cm) ± SEM of 3 lettuce cultivars grown in non-circulating hydroponic tanks.
Cultivar Treatment * Head wt (g) Root wt (g) Root Length (cm)
Manoa' Control 120.7 ± 15.7 8.5 ± 1.3 7.7 ± 0.6
Mosquito Dunk 81.0 ±7.6 3.2 ± 1.2 4.6 ± 0.5
VectoBac G 69.3 ± 17.2 3.9 ± 0.8 6.4 ± 1.1
Red Sails' Control 105.9 ± 13.4 5.6 ± 1.3 7.4 ± 0.8
Mosquito Dunk 52.1 ± 13.3 3.0 ± 1.2 3.4 ± 0.9
VectoBac G 124.9 ± 13.1 8.6 ± 1.6 8.2 ± 1.0
Green Ice' Control 144.4 ± 11.0 12.6 ± 1.3 9.3 ± 0.6
Mosquito Dunk 76.8 ± 14.6 3.9 ± 1.4 5.0 + 1.2
VectoBac G 114.6 ± 19.b 5.6 ± 1.6 6.9 ± 1.2

* N=10 for each treatment.

Apparently, Mosquito Dunk and VectoBac G greatly reduced root growth in Manoa' lettuce. The reduced root growth may have been directly related to the lighter head weights observed for Manoa' lettuce. Green Ice' and Red Sails' had similiar reductions in head weight and root growth when treated with Mosquito Dunk.

There were no mosquito larvae and pupae developing within the tanks during the cropping period. The absence of larvae and pupae may have been due to the hot and dry weather during this period.

VectoBac G application rate treatment on Manoa' lettuce.

Manoa' lettuce treated with 0.5x rate of VectoBac G had similiar head weight and root length compared to the control (Table 3). Root weight for 0.5x VectoBac treated plants, however, were 39.0% lighter than the control. Plants treated with VectoBac G at 1.0 x rate displayed 69.0% decreased head weight, 92.7% reduction in root weight and 85.4% root length compared to the control.

Table 3. Effect of VectoBac G application rate on the mean head and root wt (g), and root length (cm) ± SEM of Manoa' lettuce grown in non-circulating hydroponic tank culture.
Treatment * Head wt. (g) Root wt. (g) Root length (cm)
Control 65.5 ± 12.6 4.1 ± 0.8 4.1 ± 0.5
VectoBac G (0.54 g/m²) 72.7 ± 5.9 2.5 ± 0.5 4.4 ± 0.5
VectoBac G (1.08 g/m²) 20.3 ± 4.2 0.3 ± 0.1 0.6 ± 0.3

* N=40 for each treatment.

Larvae and pupae number were consistently high for the last 3 wk in the control tank with as much as 72 larvae and pupae at the fourth wk (Fig. 2). Tanks treated at the 0.5x and 1.0x rate had no larvae and pupae for the entire cropping period, except for the fourth wk in the 0.5x treated tank where only 4 small larvae were observed.

The recommended rate for VectoBac G application is 2.79 to 11.29 kg/ha. The highest rate of application (11.29 kg/ha or 1.08 g/m²) was used for these experiments. Reduced yield and inhibited root growth was observed at this rate. At the 0.5 x application rates 0.54 g/m², yields similiar to the control were obtained, however, a 40% reduction in root weight was observed. Mosquito larvae and pupae were adequately reduced at the 0.5x rate, although 100% control was not observed. Thus, a split-application of the 0.5x rate (2 applications of 0.25x administered 2 wk apart), may have a longer lasting control and be safer for the plants compared to a one-time 0.5x application.

In conclusion, Bti and methoprene formulations tested were very effective in controlling A. albopictus mosquito larvae and pupae within the nutrient solution of non-circulating hydroponic tanks. These formulations, however, reduced head weight and root growth of lettuce. Of the 3 lettuce cultivars tested, Manoa' lettuce is the most susceptible to yield and root growth reduction. However, lowering the application rate of VectoBac G from 1.0x to 0.5x on Manoa' lettuce resulted in the production of heads similar to that of the control. While this rate of application did not control the mosquito larvae for the full 4 wk cropping period, the larvae numbers recorded at the 4 wk period was very low. Split-applications at lower dosages are currently being investigated.

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