|Adult whiteflies feeding on eggplant leaf.
PHOTO COURTESY GILLIAN FERGUSON
Infection method of EPN: Infective juveniles, the only free-living stage of EPN, enter the bodies of insects via the mouth, anus or spiracles (breathing openings) and then mechanically penetrate the body cavity. Once inside the insects, the EPN release bacteria that they carry in their gut. The bacteria then multiply and cause death by a sort of blood poisoning within 24 to 72 hours. The nematodes, in turn, feed on the multiplying bacteria and dead insect tissue, develop, and may reproduce, depending on suitability of the insect host.
Research on Steinernema feltiae against whiteflies: Much of the research against whiteflies has involved the sweet potato whitefly, Bemisia tabaci, and very few with the greenhouse whitefly, Trialeurodes vaporariorum, the species most often found in greenhouse vegetables.
Sweet potato whitefly studies: One study that demonstrated effectiveness of S. feltiae against sweet potato whitefly, using the commercially available formulation, Nemasys®, was done under both laboratory and glasshouse conditions in the U.K. and reported in 2007. The lab study examined the ability of S. feltiae to reduce whitefly numbers on tomato, cucumber, verbena, poinsettia and chrysanthemum. However, the glasshouse study only included tomato and verbena.
Suspensions of the nematodes with Agral™ mixed in at 0.02 per cent were applied at 10,000 (normal recommended rate), 5,000, 2,500, 1,000, and 1,000 infective juveniles (IJ) per ml, followed by another application of 1,000 IJ per ml 24 hours later.
Plants were sprayed with the nematode suspension during the late evening on a dull, overcast day, kept at 20ºC and a high relative humidity (over 85 per cent). The second larval stage of the whitefly’s life cycle was the target of the spray applications because it is the most susceptible one to nematode infection. Larval mortality was assessed after 72 hours.
The lab study resulted in almost 100 per cent kill of the whiteflies on tomato leaves, and in more than 90 per cent kill on cucumber and the three ornamental species.
However, in the glasshouse study, mortality was about 20 per cent lower than that of the lab study on both tomato and verbena. The glasshouse study also showed that the highest rate of application produced the highest mortality in verbena, but in tomato, mortality using 10,000 IJ per ml was not different from that obtained with 5,000 IJ per ml. These results differ from those of another study done in Egypt that indicated that increased concentrations correlated with increased mortality.
The Egyptian study, reported in 2008, investigated the effects of concentrations of S. feltiae ranging from 5,000 to 15,000 IJ per metre on cucumber, hibiscus and collard. The nematode suspensions were applied to the undersurfaces of the leaves until runoff and the plants were then held in a glasshouse kept at 90-95 per cent RH for the first 72 hours and then 70-80 per cent RH for the next four days.
The percentage of infected whitefly larvae was checked seven days following treatment. Observations indicated that S. feltiae significantly increased whitefly mortality as the nematode concentration increased from 5,000 to 15,000 IJ per ml on all the host plants tested. The highest mortality achieved in cucumber was about 35 per cent.
Greenhouse whitefly study: A greenhouse study, done in Slovenia on cucumber during 2007 and 2008, compared the effectiveness of S. feltiae (Entonem®) with the pesticide thiamethoxam (Actara®) in suppressing greenhouse whitefly.
Similarly to the studies mentioned previously, a surfactant was added to the suspension of nematodes, which was applied at a relatively low rate of 2,500 IJ per ml to the leaves on a weekly basis. Greenhouse conditions were quite variable, ranging from 26.0-45.5ºC during the day to 14.0-25ºC at night.
Sprays were applied in the early evening with a backpack sprayer. Control was evaluated using only numbers of adult greenhouse whiteflies on leaves of randomly selected plants. Observations indicated that both nematode and pesticide treatments had significantly lower numbers than the control plants that were sprayed with just water.
Suppression of the whiteflies in the nematode treatment was comparable with that in the pesticide treatment during both years. Indeed, during the 2008 trial, whitefly numbers in the nematode treatment was lower than that in the pesticide treatment.
IMPROVING THE SUCCESSFUL SUPPRESSION OF WHITEFLIES
Generally, details of the above studies and others indicate that the following factors improve chances of successful suppression of whiteflies using S. feltiae:
- Nematode applications should target the second larval stage of sweet potato whitefly, and this may also apply to greenhouse whitefly.
- Use of an adjuvant significantly increases whitefly mortality.
- Cool, humid conditions favour activity of the nematodes on the leaf surfaces because EPN need a thin film of water on the leaf surface to reach their insect hosts. Research also indicates that for optimal use and effectiveness of S. feltiae, cool, moist conditions must be maintained for up to six to eight hours following application.
The main reservation with this biocontrol is that the environment required to maximize its performance is similar to those that favour establishment of common diseases in vegetable crops.
But, perhaps careful manipulation of the environment, and consideration of the various factors that favour activity of the nematodes, can all be combined to achieve some measure of reduction in whitefly populations without compromising the crop’s susceptibility to disease.