Clover Root Curculio: A Pest of Alfalfa and Clover
Sitona hispidula (Fabricus)
The clover root curculio is a common insect in Pennsylvania alfalfa and clover fields. It is believed to be of European origin, and was originally reported from North America in New Jersey in 1875. It is distributed from coast to coast in the U.S. and also exists in Canada and eastern Mexico. Under most growing conditions it's injury goes unnoticed, but when conditions are too dry or too moist and therefore stressing plants, the plants may die or have reduced vigor. Following extreme conditions in the southern and southeastern regions of the state in 2001, some alfalfa fields experienced significant stand losses.
The Problem in Pennsylvania
The clover root curculio is typically not considered a major pest of alfalfa, but it is probably of more economic importance than given credit. Little work has been done on it's economic impact in alfalfa, but it is known to reduce plant vigor and cause death to plants when a field or areas of a field experience stress conditions. Damage from the clover root curculio became more apparent when insecticide sprays against alfalfa weevil were discontinued due to successful implementation of biological control, in particular the establishment of parasitic wasps. Injured roots can be found in most fields across the state, but it is rare to see vast stand losses from the pest. Injury is typically more subtle, going largely unnoticed as plants succumbing to this pest are scattered randomly across a field. Shortened stand longevity is considered it's major impact and is usually due to the clover root curculio and its interaction with several root pathogens including Fusarium oxysporum which causes root rot and/or plant wilt.
The clover root curculio passes the winter primarily in the egg and adult stages. Eggs hatch in the spring and these newly developing larvae feed on the roots and crowns of alfalfa and clover plants. Around late May thru early June the larvae complete development and pupate. Adult weevils emerge during late June thru early July and can be found in alfalfa and clover fields with populations greatest in white clover fields. These adults feed actively for two to four weeks but damage is typically only notching of leaf edges. During the middle of the summer adults become less active, but increase activity again in early fall when they feed and mate. Females begin depositing their eggs near the crowns of alfalfa and clover plants. Eggs are laid over many weeks, depending on ambient temperatures, sometimes into early November. Adults can become active and lay eggs on warm winter days (@ temperatures of 50°F [10°C] and above). Mating and egg laying resume in early spring as temperatures moderate. Newly hatched larvae move to the root nodules and feed internally in one or more nodules until they are too large for the chamber. As they become larger (3rd thru 5th instar) they feed externally on the fibrous, lateral, and tap roots
Description of Life Stages
Adult clover root curculio.
Clover root curculio larva.
The adults are brownish to black weevils with a robust body and a blunt short snout. They can be distinguished from alfalfa weevils, which may be present at the same time, by the short snout and the lack of a medium brown strip down the back. The egg are seldom seen, but they are small oval and whitish when newly laid then turn black in a day or two. The newly hatched larvae are small plump white and grub like. Larvae pass through five instars before pupating. The larger late instars are about 1/10 inch long and cream colored with a brown head. Unlike the alfalfa weevil, clover root curculio larvae have no legs behind the head capsule.
Type of Damage
Clover root curculio larvae injure the plant by severing the fibrous roots and scarring and tunneling the lateral and taproots. Most of the scarring, on the lateral roots and taproot, penetrates to the cambium layer and into the vascular tissue, hence disrupting nutrient and water transport. When populations are extremely high, the pest may nearly girdle the plant and within two to three years scar 30% to 60% of the taproot surface. Plants with surface scarring become more readily infected by several soil borne fungal pathogens: verticillum wilt, fusarium wilt, and bacterial wilt. However, more than 30 different root pathogens have been identified from these feeding sites on alfalfa.
The clover root curculio also physically transports pathogens to uninfected sites on the roots of host plants. Clover root curculio injury can cause losses in forage yield, forage quality (primarily crue protein), and reduce nitrogen fixation. The combination of direct feeding and the introduction of these pathogens can cause premature stand decline in alfalfa fields, even under normal growing conditions. Stand decline is a result of direct plant mortality and winter heaving because of a weakened root system. It is difficult to separate the stresses imposed by clover root curculio and root pathogens since they commonly occur together on infested plants. Thus, damage associated with the clover root curculio is most likely a combination of insect and pathogen injury. Of these stresses possibly premature stand decline (2-3 years) is the most significant. Besides these effects on established stands, clover root curculio has also been known to kill alfalfa seedlings when a field is newly seeded next to an older stand that already harbored a large population of this pest. Adults are foliar feeders, but rarely cause significant damage to alfalfa.
There are no effective insecticide management options for this pest. Targeting the adult stage could possibly help, but applying a spray at this time would kill the beneficial wasps that help regulate alfalfa weevil populations. This would lead to future outbreaks of alfalfa weevil and possibly a return to the 1960's and 1970's level of insecticide use, when 80 to 100% of alfalfa fields required an insecticide application to control alfalfa weevil. Researchers have investigated the use of entomopathogenic nematodes and parasitic wasps, but neither has proven to provide consistent and acceptable levels of control. Good production practices may help reduce the pest's impact. Maintaining a good fertility program and cutting schedule may reduce plant stress. Also avoid planting new seedings immediately adjacent to older stands since adults can readily move to new seeding and cause damage or even plant death in the seeding year. Avoiding compaction to the field may also help. Under heavy infestation and drought conditions, however, crop injury can still occur.
Pesticides are poisonous. Read and follow directions and safety precautions on labels. Handle carefully and store in original labeled containers out of the reach of children, pets, and livestock. Dispose of empty containers right away, in a safe manner and place. Do not contaminate forage, streams, or ponds.
Authored by: Dennis D. Calvin, Professor Arthur A. Hower, Jr., Emeritus Professor Sources: Information from scientific journal articles by various researchers including A.A. Hower, and Destructive and Useful Insects by C.L. Metcalf, W.P. Flint, and R.L. Metcalf
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