by Lee Townsend, Extension Entomologist
J. D. Green, Extension Weed Control Specialist
Nodding thistle (Carduus nutans L.), also known as musk thistle, is a European weed that was accidentally introduced into the United States in the mid- to late-1800s. Now spread over much of North America, it has become a significant problem weed in many areas. The weed's primary economic impact is in pastures where it interferes with grazing. In addition, infested roadways and railroad rights-of-way, fence rows and wastelands provide a source of seeds for further spread. Efforts to control the weed in these sites result in a significant expenditure by state and county governments each year. Although the musk thistle is found all across Kentucky, it has the greatest potential impact on agriculture in the central region of the state.
The weed is spread through seeds blown by wind or transported in contaminated hay or farm equipment. It has generally been controlled by cultural and mechanical practices or the use of herbicides.
Biological Weed ControlA method of biological control to reduce nodding thistle infestation in Kentucky involves establishing a weevil, the Rhinocyllus conicus thistle-head weevil, in areas where it does not yet occur.
The successful introduction of the thistle-head weevil has resulted, over time, in a 50 to 95 percent reduction in numbers of thistles in an area.
The thistle weevil, a beetle native to Europe, was approved for release in the United States after careful study to ensure that it would not attack economically important plants. Since its release in the mid-1970s, it has thrived and multiplied in central Kentucky.
- The biological control approach described in this publication has the following advantages:
- - It is inexpensive, especially for rights-of-way and areas where other practices are not practical.
- - It poses no threat to non-target organisms or crops.
- - It allows the weevils, once established, to disperse to adjacent areas. They can be collected and moved to additional sites in a county.
- - It can become part of a total thistle management approach, incorporating properly timed cultural and mechanical practices and use of herbicides.
The method is not without disadvantages, however. The area chosen for release of the weevils should not be mowed or sprayed for two years. Additionally, an average of five to seven years is required for the weevil population to build up to the point of providing thistle control.
Once the weevil is well established, early spring or fall herbicide sprays can be applied without reducing insect numbers. Properly timed mowing also will have a minimum impact on the insect. See the Kentucky Cooperative Extension Service's publication AGR-20,"Nodding Thistle and Its Control in Grass Pastures," for recommended cultural and chemical control practices.
Biology of the Thistle-head Weevil
The dark brown-black, 1/4" long beetle has a short, broad snout and patches of small golden scales on its wing covers.
The larval stage of the insect, a legless white grub, feeds on seeds in the receptacle of the developing thistle flower.
Feeding does not harm the plant but reduces the numbers of seeds produced by an infested flowerhead. The more grubs present in a head, the fewer the number of viable seeds.
The weevils overwinter as adults under ground litter. In late spring, the adults feed on the leaves of musk thistle rosettes and mate.
Near the time musk thistles begin to bolt, each female lays approximately 100 eggs on the bracts of developing flowers and covers them with a secretion of chewed plant material. This material gives the eggs a dirty, scale-like appearance.
Eggs hatch in six to eight days. The small grubs tunnel into the receptacle, or swollen base of the thistle flower, where they feed for about 25-30 days. When mature, the larvae enter a resting stage, called the pupa, which lasts another 8 to 14 days. During this time the insect transforms into an adult.
When 15 or more grubs feed within a single head, the thistle plant does not form seeds. Numbers of larvae per head tend to decrease as more flowers develop. Their feeding causes some flowerheads to turn brown prematurely because of damage to the head or to the stem just below the receptacle. Incompletely filled flowerheads with dead plant tissue in the center can be a sign that thistle weevils are present.
One generation of the weevil occurs each year. Adults emerge in July and seek overwintering sites under new musk thistle rosettes, ground litter and wooded areas where they will remain dormant until the following year.