Biocontrol Agents Are Working Hard in Utah
Classical biological control involves searching for and introducing organisms (usually insects or pathogens) that target a specific pest (invasive plants or insects). Typically the target organism is one that has co-evolved with the pest in question in their native habitat. With biocontrol, eradication is not the goal; rather, it is reducing populations to a tolerable level. Many years of research are required before releasing one non-native organism to target another.
|Tamarisk leaf beetles are voracious feeders and offer hope to reclaiming invaded riparian areas.|
Tamarisk is a non-native invasive plant that now covers more than 2 million acres of riparian land. It increases soil salinity, displaces native plants, and lowers the water table. According to the Nature Conservancy, each mature plant “mines” nearly 200 gallons of water per day from the water table, amounting to a loss of 2 to 4.5 million acre-feet of water per year. It is also highly flammable, posing a fire hazard in public sites. Control of the extensive invasion of saltcedar by cutting or by herbicide is ineffective and impractical.
The tamarisk leaf beetle, Diorhabda elongata, native to central Asia where saltcedar also originates, was considered a viable biocontrol option. Adults and larvae of the tamarisk beetle feed on the foliage, resulting in reduced photosynthesis and poor plant vigor. In 2001, after 20 years of research by USDA-Agriculture Research Service (ARS), the beetle was released in Utah (in collaboration with USDA-APHIS and BLM) and several other western states. Three years later, a Utah program was enacted allowing beetle collection from Delta for release on state and private lands. As a result, beetles are now successfully established on thousands of riparian acres.
Worries that the beetle will kill native vegetation are unfounded. USDA-ARS scientists found that although larvae did feed on plants in the genus Frankenia (seaheath), they preferred feeding and egg-laying on tamarisk.
Because the beetles feed only on the foliage, plants are not killed immediately. Observations in Delta show that repeated defoliation by the larvae can result in plant death in three to five years. As the tamarisk population declines, so will the beetle population.
Three species of knapweed—squarrose, diffuse, and spotted—are all considered noxious weeds in Utah, and have destroyed millions of acres of rangeland. They are Mediterranean natives, and spread through much of the west by seed dispersal on migrating cattle and sheep and on vehicle tires. Removal by mechanical means is difficult; roots can reach as deep as 5 feet. They also produce allelopathic toxins from all plant parts, inhibiting germination of native vegetation.
At least eight insect species have been released in Utah in the past several decades for biological control of knapweeds and several are widely established, including Larinus weevils, Urophora gall flies, and a root-boring beetle, Sphenoptera jugoslavica. A majority of these insects feed on the flower heads, and most of the remaining feed on roots and stems. None of these insects kill plants outright, but in conjunction, they have been shown to significantly reduce density and vigor of plants. Studies have shown that gall flies plus weevils caused a 50% reduction in seed production in Montana, and a 92% reduction in British Columbia. It is estimated that a over a period of 5-15 years, knapweed populations in Utah should start to noticeably diminish.
-Marion Murray, IPM Project Leader