Hand pollination, also known as mechanical pollination is a technique that can be used to pollinate plants when natural or open pollination is either undesirable or insufficient. This method of pollination is done by manually transferring pollen from the stamen of one plant to the pistil of another. This is often done with a cotton swab or small brush, but can also be done by removing the petals from a male flower and brushing it against the stigmas of female flowers, or by simply shaking flowers in the case of bisexual flowers, such as tomatoes. Common reasons for choosing this method include the lack of pollinators, keeping control of cross-pollination between varieties grown together, and creating specific hybrids. Examples of this are vanilla plants, which are transported to areas where its natural pollinator doesn't exist, or plants grown in greenhouses, urban areas, or with a cover to control pests, where natural pollinators cannot reach them. Pollinator decline and the concentrated pollination needs of monoculture can also be a factor. However, these are not the only reasons, and variable techniques for hand-pollination have arisen for many specialty crops. For instance, hand-pollination is used with date palms to avoid wasting space and energy growing sufficient male plants for adequate natural pollination. Because of the level of labor involved, hand-pollination is only an option on a small scale, used chiefly by small market gardeners and owners of individual plants. On large-scale operations, such as field crops, orchards, or commercial seed production, honeybees or other pollinators are a more efficient approach to pollination management. Despite this, hand-pollination is a fairly widespread practice. Pears grown in Hanyuan County, China have been hand-pollinated since the 1980s, because they can't be pollinated with other varieties that have different flowering times; also, lice infestation requires the use of many insecticide sprays, which causes local beekeepers to refuse to lend beehives.