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H. vitripennis is a stem feeder and leaves no visible symptoms of its feeding other than a white, powdery, dried excrement on plant surfaces.
Feeding causes no visible signs of damage, even though the insect consumes hundreds of times its body weight per day in xylem fluid. Most non-xylem-feeding leafhoppers produce a sugary or particulate excrement, but the excrement of xylem feeders is watery, high in ammonia and dries to a fine, whitish powder which can cover the stems, foliage and fruit when the insects are abundant (Phillips, 1998). High densities of feeding sharpshooters excrete enough waste product to cause a 'rain', which falls from the trees; this rain can easily be seen on sunny days and can be felt on the skin. This phenomenon is particularly acute in Tahiti where puddles form on roads and side walks as result of sharpshooter rain.
Egg masses are usually laid into recently expanded foliage. Older foliage will contain the distinctive scars left after the eggs have hatched. When populations are more abundant, egg masses can be laid into the rind of immature fruits of crops such as citrus and melon. Old hatched egg masses appear as grey or tan scars on surface of the rind (Blua et al., 1999).
Phytosanitary measures can be used to slow dispersal and range expansion of H. vitripennis.
Chemical control practices are not generally necessary as suppression of the vector rarely leads to a significant reduction in disease incidence. Chemical control is only recommended in instances where the objective is to slow the dispersal of the vector to new ranges. There are no prescribed control programmes within the native range of H. vitripennis in the south-eastern USA. The exception is in citrus in southern California, where newly planted orchards, up to 3 years of age, can sustain vigour/growth losses due to high populations (50-100 insects/plant) during the hot, dry summer months. Evaporative demand coupled with xylem fluid losses resulting from insect feeding can result in wilt symptoms at midday. Sustained high populations can also result in losses in yield and fruit quality. In such cases, chlorpyrifos can be used, with limited residual activity, to kill active stages. The application of imidicloprid as a systemic insecticide can suppress the population by 95% for up to 5 months. Field trials in California suggest that pyrethroids and synthetic neonicotinoids may be the most suitable for use in integrated pest management programmes, and combinations and rotations of products may be necessary to minimize resistance development (Akey et al., 2001; Bethke et al., 2001).
Biological control from indigenous mymarid egg parasitoids can account for 20-40% of mortality in the spring generations in California. The second generation of egg masses in summer experiences >95% parasitism and ~100% of eggs in discovered egg masses are parasitized. The mass production and release of parasites is currently under investigation as a control tactic. However, given the difficulty of mass rearing GWSS year round it is unlikely that seasonal augmentative releases of mass-reared mymarid parasitoids will be economically feasible, or that the parasitoid would be produced in high enough numbers to have an impact.
Sweep net sampling and yellow, sticky cards or ribbons can be used for field monitoring of populations or new infestations.
In economic terms, grapes are worth US $3.2 billion and associated economic activity exceeds US $33 billion in California. In addition, crops such as almonds and stone fruit have been valued at US $897 million and US $905 million, respectively, and are at risk from strains of X. fastidiosa, which cause disease in these host plants (CDFA, 2003). The H. vitripennis-Xylella fastidiosa combination could potentially have a severe and adverse affect on California's agricultural industries and subsequently on the state's economy. In 2002, the state of California and primary producers incurred additional economic costs resulting from spread containment activities such as inspections of nursery stock being moved out of southern California, and shipments of bulk grapes and citrus from H. vitripennis-infested counties (CDFA, 2003). There are currently in excess of 70 research programmes on H. vitripennis or X. fastidiosa. Because of the serious nature of this problem and the vast sums of money at stake, the National Academy of Sciences (NAS) of the USA has subjected these programmes to evaluation and assessment (CDFA, 2003). The NAS has a mandate requiring it to advise the Federal Government on scientific and technical matters.