• Jul 7, 2020

Clover Root Curculio - A Small Pest That’s Tough to Control

Forage Genetics International
Clover root curculio (CRC) is an economically important alfalfa pest, increasing in numbers since the phase-out of several popular synthetic soil fumigants in the early 2000’s.  CRC causes both direct and indirect damage to alfalfa plants, decreasing persistence as well as stand density and production. Monitoring for these pests is difficult because most of the damage occurs to the roots below the soil surface. Control is currently focused on cultural methods including crop rotation and planting date.

Identification:

  • Adults are grey-brown with a checkerboard pattern on the elytra and approximately 1/8 – 3/16 inches long
    • Include semi-erect hairs on wing covers, which helps when distinguishing them from other pest lookalikes including the alfalfa weevil
  • Eggs are <0.5 mm long and are yellowish-white when laid, maturing to shiny black within several days after fertilization
  • Larvae are off-white and semi-translucent, appearing grub-like with a brown head capsule and develop through five instars
  • Pupae is cream colored, and usually found near the soil surface

Life cycle:

  • CRC only produce one generation per year
    • In eastern states, eggs and adults can overwinter, with adults quickly resuming ovipositioning in the spring
    • In western states, CRC overwinter primarily as eggs with few adults surviving through the harsher winters
  • Once spring temps are ≥ 50°F, surviving adults are again stimulated to feed and oviposit
    • Eggs are typically found within the top 1 inch of the soil
  • First-instar larvae feed on root nodules, damaging the plant’s N-fixing ability
    • As larvae develop, they begin to feed on other parts of the plant, including the lateral roots, as well as the main taproot
  • Pupae are usually found near the soil surface, where they can remain for 8-22 days before emerging as a new generation, typically in June through July
  • After emerging in midsummer to feed and oviposit, adults are most active at the soil surface where it is cooler and more humid
    • They are largely inactive at higher temperatures, and remain in the litter, burrowed in plant crowns, or in crevices in the soil
  • In early fall, adults begin more vigorous feeding and begin to lay eggs, as well as migrate to other areas

How can we look for them?

  • Collect the top inch of soil around plants showing damage, and then wash the sample through a 60-mesh sieve
    • This material can be sent to Extension and University personnel for evaluation and egg counts
  • In early spring, digging up symptomatic plants and evaluating the root system for larvae or pupae can help in determining if CRC is the source of the damage
  • Sweep net samples collected when adults are active can help to establish presence 

What does their damage look like?

  • Adults are more likely to damage newly established stands, while the larvae can damage new or existing stands
  • Leaf maturity has been suggested to influence feeding behavior, with greater damage often seen on the newer leaves
  • Larvae can consume entire nodules, as well as sever lateral roots and even create girdled taproots 

How do they migrate?

  • In warm summer months, most migration is likely due to crawling to adjoining fields and pastures
  • In the fall, adult flight is important for longer-distance dispersal of populations

Impact on plant production

  • Nitrogen-fixing ability can be reduced due to damaged nodules, creating N stress in the plants
  • Feeding on lateral roots can interrupt nutrient and water movement throughout the plant
  • Adult feeding on new seedings can decrease stand establishment
  • The physical damage to the roots can predispose the plant to secondary diseases
  • CRC larvae and other pathogens may interact synergistically, decreasing crop yields and density, affecting overall stand life
    • Can also delay green-up, decrease nonstructural carbohydrate root reserves, and increase winterkill and injury 

What can we do to control?

  • Currently no economic thresholds are available and damage often unnoticed until year 2
  • Only available insecticides are licensed for adults, not for larvae
    • Not recommended due to limited effectiveness and impacts on beneficial populations
    • Timing of application is critical, always read the label for recommended practices
    • Research has found inconsistent results when evaluating effectiveness of chemistries labeled for other insect pests
    • Ongoing research to evaluate potential of nematodes, fungi, bacteria, and parasitoids for control
  • Best management practices currently advise rotation to a non-host crop (ex. corn or potatoes) for at least one growing season to disrupt populations and limit damage
  • Early spring seeding allows for more vigorous plants to withstand larval damage
  • Healthy plants are better able to handle some feeding compared to unhealthy plants, so manage irrigation and fertilization appropriately
  • Contact your local Pest Control Agent, Certified Crops Advisor, or Forge Genetics International team member to discuss all options available for control
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