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).
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.
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.
*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).
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).
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