The Alfalfa Crown Borer, AKA Clover Root Curculio,
Sitona hispidulus (Fabricus)
A New Feeding Strategy for an Old Pest??
In the spring of 2005, TRA Extension received calls regarding “worms” boring inside alfalfa stems. Borers were found in about 10 fields in Mesa, Delta and Montrose Counties. We initially identified the insect as an unknown weevil and referred to as the alfalfa crown borer.
The plants in the lighter areas of the field pictured above were infested with larvae of a weevil which was boring into stems at crown level. A damaged stem with the weevil larva is shown in the left picture. The alfalfa had grown to about 6" in April when infested growth died back to ground level. The plants resprouted and grew back, but yield loss in the affected areas was significant.
The borer was killing plants in some fields. Spring growth was dying and shoots resprouting in other fields. Alfalfa crown borers were observed once in the early 1990’s then not seen again until 2005. They were widespread in 2005, then disappeared until 2008, when they were found in two fields. One 2-year old alfalfa field was a complete loss in 2005. The dying alfalfa had been replaced by wild lettuce. Borers could be found in the crown of every dead alfalfa plant. S. hispidulus adults were abundant in the field in June.
Damaged areas in a 2008 infested field were easily located by the presence of dead spring growth residue. Spring growth had been killed, then regrowth had occurred, covering it up pictured above. This is the most common scenario for finding crown borer damage.
Borer larvae are legless white colored grubs with a distinct brownish head capsule. Grubs are found inside stems at crown level. Mature grubs are about 1/4 inch long (scale at leftis mm). The larvae pictured at right has tunneled up the center of the stem and left a trail of frass where it had fed.
Borer damage was evident throughout affected areas of the field. There were usually several borer larvae present in damaged plants. This picture shows the extent of the damage on severely affected plants.
Identifying the culprit
In 2005, all attempts to rear adults for identification failed. In 2008, borers were seen in 2 Delta Co. fields and adult weevils were successfully reared from larvae. Adults emerged in lateJune & early July. Weevils were identified as S. hispidulus, based on characteristics of ♂ genitalia & ♀ 8th sternite in addition to plumage patterns on the elytra (Bright, 1994). Larval mandible morphology matches that of S. hispidulus pictured in Manglitz (1963).
A new feeding behavior?
The literature on Sitona hispidulus is extensive. All descriptions of larval feeding speak of root and nodule feeding. There is no mention of larvae boring within stems. S. hispidulus has developed a new feeding behavior! Given our observations and ID, we have no choice but to accept this fact. We will continue research to determine the extent and management of this new feeding behavior.
Bright, D.E., 1994. Revision of the genus Sitona (Coleoptera: Curculionidae) of North America.Ann. Entomol. Soc. Am. 87:277-306.
Manglitz, G.R., D.M. Anderson, & H.J. Gorz. 1963. Observations on the larval feeding habits of two species of Sitona in sweetclover fields. Annals Antomol. Soc. Am. 56:831-835
This page was updated on April 26, 2014