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Entomology Program Update: Weevil Biology and Management in Berries

Volume 5 Issue 10

Beverly Gerdeman, Entomologist at WSU-NWREC and Hollis Spitler
Email: bgerdeman@wsu.edu

Identifying Weevil Damage

  • Sometimes but not always, there is leaf notching.
  • Leaf notching – half circles cut out of leaf margins (Fig. 1).
  • Evidence of chewing or girdling roots (Figs. 2 & 3).
  • Plants look weak, diseased or water stressed.
  • Sampling for root weevil larvae by digging is necessary to verify infestations (Fig. 4).

Fig. 1. Not all root weevil adults feed on blueberry foliage. Those that do, typically notch leaves near the ground.
Fig. 2. Blueberry root nearly girdled into two by root weevil larvae.

Sampling

  • Begin sampling in April for weevil larvae and pupae.
  • Weevils vacate weak bushes to attack healthy adjacent plants, so sample a plant next to a weak one.
  • Dig 2-3 inches in the soil near the crown to locate the white larvae and pupae easily seen in the dark soil Fig. 16).
  • Continue sampling by digging midway down the shoulder of the row hill.
  • Sample more than 1 plant to increase chances of finding weevil larvae and pupae.

Fig. 3. Early instar root weevil larvae collected in September 2016 next to a penny as a size reference.
Fig. 4. Severe root weevil damaged blueberry from feeding on the left. The blueberry plant on the right exhibits less damage and could recover. Young plantings are highly suseptable to root weevil damage.
Fig. 5. Digging for root weevil larvae and pupae in blueberry.

Timing for Sampling and Spraying to Control Root Weevils in Blueberry (Table 1)

  • Eggs hatch in the summer so by fall they are young larvae, with few adults and no pupae (Fig. 6).
  • In the fall young larvae are near the crown where they hatched.  Concentrate spraying near the crown.
  • In the fall there are no concerns about the 75-day PHI with Platinum.
  • In the fall there are no worries about postbloom restrictions for Advise or Admire Pro.
  • In the fall before the rains, the ground is not muddy and you can easily spray.
  • Make applications before a rain to move the insecticides into the soil.  If the ground is not wet, run irrigation before and during the application.
  • Weevils are typically a chronic problem.  Expect control to require multiple treatments across several seasons, followed by continual vigilance!
  • A new label registration for Capture LFR (bifenthrin) may provide additional opportunities for controlling root weevil larvae.  See Table 2.

Fig. 6. Root weevil pupa and its protective soil hibernaculum where pupation occurs. This is not a life staeto target for control since it is non-feeding and in a protected location.
*A new label for the bifenthrin soil formulation, Capture LFR, is due out in 2017 and is expected to include blueberry and be labeled for drip.
** Label allows use as high volume drenching application.
*** Advise 2F has been discontinued and replaced with Advise 4, which has a different rate.

Field Trial

  • A fall field trial in 2015 was conducted in Skagit County on 21 October.
  • Pre-treatment sampling verified the presence of weevil larvae but exact population size and distribution were unknown.
  • The field trial consisted of 2-bush treatment plots, replicated 6 times in an RCB design to minimize variability.
  • Four products were tested using a CO2 backpack sprayer (Table 2).
  • Water equivalent to 250 gal/acre was applied to the treated areas to help drive the materials down into the soil at a target depth of 2-3 inches.
  • Treatments were evaluated by digging for larvae and also by using leaf notching as a secondary method of evaluation.

Evaluation – larvae/pupae

Larvae and pupae were recovered (26 total) from one side of each bush the following April.  Recovery was low but all 4 treatments were better than the untreated but none were statistically different from each another (Fig. 7).

Fig. 7. Fall treatments were evaluated in April, 6 months after treatment. NOne of hte treatements out-performed the others but all were significant;y different from the untreated. P<0.05 Fisher test.

Evaluation – Leaf Notching

Leaves with notching were counted from every bush (12/treatment) in the spring to provide additional evidence of product performance.

Results indicate activity but none of the treatments were any worse than the others (Fig. 8).

Fig. 8. Twelve bushes/treatment were examined for leaf notching in the spring following fall treatments. Results suggest some efficacy compared with the untreated but non of the treatements were better than the others. P<0.05 Fisher t test

Summary

  • Pyrethroids such as bifethrin are non-systemic, nearly immobile in the soil, so placement is important. Typically effectiveness would result from adults crawling through the toxic barrier.
  • Neonicotinoids are systemic, uptaken by the roots and provide a wider zone of protection.
  • Neonicotinoids are uptaken into the roots and work best as ingestion poisons but have some contact efficacy