The buzz

The water that you use to grow greenhouse crops may also produce mosquitoes. While these insects don’t harm plants, they may transmit pathogens that cause West Nile virus, malaria and encephalitis. Mosquitoes also are vectors for diseases that afflict animals. lf high populations of mosquitoes are on your property, and health issues arise, you could be financially liable.

Standing water is essential for all life stages of mosquitoes, except for the adults. However, water that is stagnant more than 72 hours in vegetated buffers and impoundment structures such as recycling ponds provides habitats suitable for mosquitoes. The nutrients and organic matter in runoff from greenhouse operations are also conducive to the development of mosquitoes.

Practices to suppress mosquito breeding and mitigate human health risks must be integrated into plans for managing greenhouse runoff. Mosquito control is most effective when breeding areas are managed and larvae are targeted. This is best accomplished using a multifaceted IPM program that combines biological, physical and chemical control techniques to eliminate as many mosquitoes as possible before becoming biting adults. Regular monitoring for immature stages of mosquitoes is critical where open stagnant water is unavoidable. Monitoring is also important for evaluating control methods. Information on managing mosquitoes is at http://ipm.ucdavis.edu/PMG/PESTNOTES/mosquitoes.html.
 

Biological control

Natural mosquito predators can significantly reduce mosquito populations, although their effects are typically insufficient to preclude treatment. Predators that eat mosquito larvae, such as fish, are the most effective for biological control. The mosquitofish, Gambusia affinis, has been used worldwide for mosquito control. These fish are most effective in man-made bodies of water that don’t connect with natural waters (in California it is illegal to introduce mosquitofish into natural waters because they can potentially disrupt populations of indigenous species).

Emergent vegetation provides mosquito larvae with refuge from predators, interferes with the ability of predators to move throughout vegetated areas, increases nutrient availability and impedes monitoring efforts. It is best to use concrete or liners in shallow areas to discourage unwanted plant growth and periodically remove vegetation conducive to mosquito production from waterways and impoundment structures. Maintain water in recycling ponds and constructed wetlands at depths deeper than 4 feet to limit invasive emergent vegetation such as cattails.
 

Physical control

Practices for making habitats less suitable for mosquito production by reducing standing water and blocking egg-laying females from water include:

• Keep water flowing—utilize pumps or aerators. Avoid barriers or diversions such as riprap that retain standing water. Design runoff structures so they do not hold standing water for more than 72 hours, or seal structures to prevent entry of adult mosquitoes.
• Maintain filters and screens to reduce clogging.
• Design impoundment structures with appropriate equipment so that water can be easily discharged.
   

Chemical control

Insecticides for controlling mosquitoes include larvicides such as surface oils to suffocate larvae, growth regulators to inhibit larvae development, and adulticides. But chemical control can damage aquatic ecosystems. One insecticide that does not, is Bacillus thuringiensis subspecies israelensis (Bti). This biopesticide forms spores that kill mosquito larvae when they eat the insecticide. Bti is not effective against pupae, since they don’t feed.

Julie Newman is an environmental horticulture advisor at the University of California Cooperative Extension.

Have a question? You can write Julie at jpnewman@ucdavis.edu.