Entomology Program Update: Weevil Biology and Management in Berries
Volume 5 Issue 10
Beverly Gerdeman, Entomologist at WSU-NWREC and Hollis Spitler Email: firstname.lastname@example.org
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).
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.
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).
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