What to watch for: Strawberry clippers active

I am starting to cross post more information on the NC Extension Portals, and my latest post at the Strawberry Growers Information Portal addresses some of the many questions have gotten this week and last on strawberry clippers.

More information
What to watch for: Strawberry clippers - NC Strawberry Growers Information Portal 

What to watch for: strawberry update

This morning, I visited our strawberry research plots at the Central Crops Research Station, Clayton, NC.  We will be using these plots for sap beetle, caterpillar, and spotted wing drosophila research this spring, and, like most of the southeast, our plants are pushing early.  It looks as though we will have berries in about 3 weeks, which is a month ahead of our normal schedule.

Along with this potentially early strawberry crop comes early questions about pest management.  I have fielded lots of questions about strawberry clippers and two spotted spider mites in the last few weeks.  I posted several times last spring about strawberry clippers, and you can find those posts here.  You can also find my posts on spider mites from the last several years here.  For grower with spider mite populations at threshold (5 mites/leaflet in a sample of 10 leaflets per acre) and plants that have not yet started fruiting, now is a good time to get those populations under control.  You can prevent mite issues later in the season by treating now with a miticide and avoiding the use of broad spectrum insecticides (pyrethroids and carbamates) unless absolutely necessary for other insect pests. If a broad spectrum material is needed, for example against sap beetle or SWD infestations, scout for spider mites before and watch populations carefully follow a treatment.  For growers who do not have mites about threshold (or no mites at all), you can breathe a little easier--barring a major slow down in plant growth, you may avoid the need to treat at all for mites.  Once plants are actively fruiting and rapidly growing, mite populations must be very high to cause damage--and very high mite populations typically only develop if you start off with mites earlier in year (around now). 

More information
Strawberry clipper posts - NC Small Fruit and Specialty Crop IPM
Two spotted spider mite posts - NC Small Fruit and Specialty Crop IPM

Southeastern strawberry preplant meetings underway

The 2011 strawberry preplant meetings are underway, and I will be attending the Sandhills meeting tonight in Carthage, NC. You can find a list of all upcoming strawberry preplant meetings here. Tonight I will be discussing spotted wing drosophila (SWD) biology and significance in strawberries and strawberry clipper considerations.

Strawberry clipper is a relatively infrequent pest in the southeast, but the North Carolina Sandhills is a clipper hot spot. I am planning an on farm trial in this region next spring to address some of the key questions about strawberry clipper management. Namely, how much does clipper damage impact strawberry yield, and how do southeastern strawberries respond to damage?

For those not able to attend preplants, the SWD slides I will be covering are embedded below. Bear in mind that the maps in this presentation are current as of July 2011 and are subject to change.



I will also be attending the South Carolina preplant on August 18, 2011 where SWD will be the main topic.

More information
Preplant meetings and fumigation resources - NC Strawberry Association
Strawberry clipper posts - NC Small Fruit & Specialty Crop IPM

Strawberry clipper update

Last Wednesday, I visited Lee County to check on our strawberry clipper traps and take counts. We continued to see clipped buds (20 of 30 plants checked), but the damage appeared older and no adults were captured on the traps. It will be interesting to see if the damage continues this week. Clipper counts are also picking up at other locations in the Sandhills, I have now heard reports from 3 different locations where clipper has been observed.

Strawberry clipper weevil adults on wild blackberry buds. How many clippers can you find? There are 11 in this container. Photo: HJB

More exciting, however, was what I found this morning in my growth chamber. The clipper eggs that I collected 2 weeks ago have completed their larval stages and are now adults. This answers the first question I had in mind when I brought them back to the lab. At a constant temperature of 28C, the strawberry clippers took about 15 days to complete their larval stages. Now that I know I can successfully rear beetles from infested buds, my next goal is to collect enough adults to conduct a laboratory bioassay of registered and unregistered materials to control clipper weevil adults. This will go a long way to making more meaningful treatment recommendations, particularly for newer, reduced risk materials. However, we still need to understand the real impact of clipper damage in terms of yield for our production system before I am willing to advocate an aggressive pesticide program.

In the meantime, I will be keeping my adult beetles on flower buds, since they have been documented to feed on flowers and pollen. This appears to be correct, because the beetles went straight for the unopened blackberry buds I placed them on and started chewing away.

Strawberry clipper update

I returned to the Lee County farm I visited last Friday to check the sticky traps I placed and count any additional damage.  Also visiting were Seth Holt, Lee County agriculture agent; David Dycus, NCDA & CS regional agronomist; and Brenda Gwynn, new Lee County Horticulture Agent.  The sticky traps I placed at the end of the rows nearest to the woods caught 3 clipper weevil adults, which is a promising result.

Yellow sticky trap placed at the end of a strawberry clipper damaged field in Lee County.  Photo: HJB

Yellow sticky trap with a strawberry clipper weevil (among other insects) in center, circled.  Photo: HJB

Strawberry clipper from sticky trap, up close.  Photo: HJB

 Clippers were also readily distinguished from other weevils on the traps.  If traps can detect strawberry clipper movement from their overwintering sites in the woods into fields, perhaps they can be used to time scouting efforts and insecticides treatments (if necessary).  The four of us also counted damaged plants in the 3 untreated row ends the grower left.  Damage had increased since last week.  Twenty-one of the 31 plants observed had at least one clipped bud, and damage ranged from 1 to 6 clipped buds per plant.  In the treated parts the rows, 11 of 13 plants observed at least one clipped bud.  This suggests that the treatment applied last week is no longer effective and that clippers remain active.  I replaced traps and will be returning next week to check them and assess damage.  My goal is to determine how long the clippers remain active and if the sticky traps continue to catch adults.

Even though flower buds continue to be clipped, I am uncertain that the plants will experience economically significant yield loss from damage at this time of year on young, unopened buds.  Before leaving today, we made plans to conduct an on farm trial at this locations next spring designed to determine if aggressive clipper treatment will significantly improve yield.

What to Watch for - Strawberry clipper weevil

I spent last Friday morning visiting a strawberry grower in Lee County, North Carolina who had noticed the first damage from strawberry clipper weevil (Anthanomus signatus) in his field on Thursday.  Because his farm has a history of strawberry clipper, the grower had been scouting every other day and observed that damage went from zero to several clipped buds per plant in the 2 days since he last scouted.

Strawberry clipper weevil females lay their eggs in developing flower buds of strawberry, blackberry, raspberry, dewberry, and red bud.  Female beetles then chew through the pedicel, which supports the flower bud.  This causes the bud to drop from the plant.  The larvae develop in the dropped flower bud over the course of 3 to 4 weeks.  Adults emerge in mid summer, briefly feed on pollen and then overwinter.  There is only one annual generation of strawberry clipper.  Beetles overwinter in wooded areas, so fields located near the woods or rows closest to the woods often experience the greatest clipper injury. 

Mating strawberry clipper weevils. Photo: Tom Murry, BugGuide.

Rain in Lee County in Thursday had knocked many of the damaged buds off the strawberry plants, but there were still many plants with multiple clipped buds.  I collected several damaged buds to take back to the lab and examine for eggs or larvae.  On Friday, all I found were eggs, but by today (Monday), the buds contained quickly developing larvae.

Clipped strawberry buds (center).  Photo: HJB

Stawberry clipper egg inside strawberry blossom.  Note oviposition hole to the upper left of the egg.  The female weevil will chew a hole through the sepal and  petals and then deposit the egg inside the flower.  Females lay one egg per flower.  Photo: HJB

Strawberry clipper larva after 4 days in a 28C (82F) growth chamber.  I was surprised to see how rapidly they had grown over the weekend!  Photo: HJB

The grower had treated the night before with Sevin XLR, a relatively "safe" pesticide to bees when dry.  No pesticide is completely safe for bees; therefore, I do not recommend pesticide use during bloom if at all avoidable. If pesticides (insectides, fungicides, herbicides, or others) are necessary when flowers are present in fields, they should be applied in the evening, after bees are done foraging, to allow for the longest drying time possible before bees become active again. See "A note on pesticide recommendations". The grower left 3, 15 ft sections near the edge of the field untreated for me to observe over the next few weeks.  I placed yellow sticky traps to determine if we could trap beetles walking into the field and counted the damage on 10 plants per row.  Of the 30 plants I observed, 11 had at least 1 clipped bud, and damage ranged from 1 to 5 buds per plant.

A recently clipped bud. Photo: HJB

The big question in this case is whether treatment was necessary to prevent further strawberry clipper damage.  Insecticides have been demonstrated to reduce adult clippers, and therefore the number of clipped buds.  Research on the impact of clipper in New York (English-Loeb, et al.1999, subscription needed to view full article) found that all strawberry varieties tested compensated well for early season clipper damage, or in the words of the authors, damage to primary and secondary buds.  Only damage in late season (tertiary) buds resulted in a significant yield loss because the the plant was not able to mature additional fruit.  A closely related species has been studied in Europe in perennial strawberry plantings (Aasen and Trandem 2006, subscription may be needed to view full article), where yield does appear to be improved when insecticides targeted to clippers are applied.  I am not aware of any studies on compensation on Chandler, Camarosa, or Sweet Charlie, the strawberry varieties most commonly grown in North Carolina, so we cannot necessarily say if they will behave similarly to those observed in New York (Earliglow, Kent, Jewel, and Seneca).  However, I think it seems likely that our varieties can and will compensate for early season clipper damage.  For this reason, I believe that the thresholds currently recommended by entomologists in Virginia (0.6 clipped buds per ft) or New York (2 clipped buds per meter) are certainly conservative enough to prevent economic loss and are probably more conservative than necessary.  Further work is needed on compensation in our key strawberry varieties as well as strawberry clipper biology in North Carolina.