Challenges of managing Lygus in strawberry plantations on the central coast of California
Department of Entomology, University of California, Davis, CA, USA
Correspondence: gzalom@ucdavis.edu
Lygus bugs are a serious pest of strawberries grown in California's Central Coast area and Santa Maria Valley where fruit are produced through the summer months. Lygus are rarely pests in southern California and the Central Valley where harvest is generally completed by the end of June. However, Lygus damage may occur on second-year plantings and when berry plants remain in the field through the Summer and into Fall. The main species present is Lygus hesperus, but a second species, Lygus elisus, can also be found in mixed populations. Adult Lygus bugs typically overwinter from September to January on flowering plants and legumes outside of strawberry plantations. Cover crops including clover, fava beans and vetch are good Lygus hosts. Some Lygus overwinter on summer planted strawberries which become infested during the Fall migration and on second-year strawberries which had become infested during the first year. Three seasonal populations of nymphs occur on central coast strawberries, the first in May or early June, the second in late June or early July, and the third in late July or August, depending on location and temperature conditions. Adults from the third generation overwinter, and will lay eggs in the Spring, contributing to the next season's problem. Feeding by the second hatch of nymphs on strawberries usually causes the greatest damage if not controlled, however monitoring and management of all nymphal hatches is important. Lygus nymphs and adults feed on strawberry seeds and both are capable of damaging fruit. When a seed is damaged, the flesh behind it does not develop properly creating a characteristic discoloration and malformation called “cat-facing”. Damaged strawberries will not recover, so it is necessary to suppress Lygus before the damage occurs. Successful Lygus management requires an understanding of its biology and life cycle, and is best achieved through a combination of control approaches and careful monitoring both within strawberry plantations and on its weed hosts. A Lygus phenology model has been adapted from the cotton safflower system of California's San Joaquin Valley which can be used to predict when Lygus nymphs will be present in weeds and in strawberries. Controlling Lygus nymphs is preferred because they can't migrate from weed hosts or trap crops that are destroyed when they are present, and nymphs are also more susceptible to control using insecticides. Treatment thresholds for Lygus in strawberries is very low because of the value of the strawberry crop. The gross return for a hectare of California strawberries approaches $125,000. Treatment is recommended when 1 Lygus is found in 10 plants when sampling by suction machine, or at 1 Lygus per 20 plants when sampling by beating tray. Therefore, insecticides are widely used for controlling Lygus, but there are few registered products that are effective. Pesticide registrations are difficult to obtain on strawberries because of the relatively limited area on which they are planted (about 10,500 hectares), the frequency of harvest, and frequent worker entry into the plantations. Those products that are registered must therefore be used wisely to maintain their efficacy. Vegetation management in areas nearby strawberry plantations is important to reduce local Lygus populations that can serve as sources of infestation. Weed abatement and trap crops are both employed by some growers, but there are challenges in implementing both approaches to prevent these alternate Lygus hosts from instead becoming sources of strawberry damage. Tractor mounted suction machines, or “bug vacs” are also used by some growers, but their efficiency can vary considerably depending on machine.
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