What to watch for: Aphids in strawberries

There has been concern among some strawberry growers in the southeast regarding aphid vectored virus infections in plants from some sources. Dr. Barclay Poling, NC State University Horticulture Department, has reported on these observations as they developed in a series of alerts, found at the NCSU Strawberry Growers Portal.

Aphid vector viruses are rarely significant issues in annual strawberry because even if infections occur during the year, they happen later in the season and do not cause economic losses.  In addition, infection from multiple viruses (virus complexes) is often necessary for visual symptoms or loss to occur.  Where aphid vectored viruses can be significant is in perennial production and in nursery production.

It appears that plants suspected to be infected with viruses are restricted to a few plant sources, so for growers who do not have plants on their farm from suspected sources, no additional management is recommended.  However, for growers with suspect plants, management is recommended to prevent early season in field spread.  Systemically applied insecticides are likely the most effective method of managing aphids, and information on using these materials is in the Southern Regional Small Fruit Consortium Strawberry IPM Guide.  Importantly, these materials have application timing restrictions with respect to bloom to protect pollinators.  This restrictions should be carefully followed and are another good reason NOT to preventatively treat for aphids if you do not have suspect plants!  Most North Carolina growers with a need to treat should be able to meet these restrictions. If systemic insecticide applications cannot be made, the alternative foliar materials are more difficult to use because of pollinator concerns and should be applied only if aphids are present--which requires good scouting!

The recommended aphid scouting program is 40 randomly selected leaves per acre, which should be observed for live, non parasitized aphids. If only parasitized aphids are present, treatment may not be necessary.  This is particularly true if you have already treated, because parasitized aphid mummies will often hang on to plants when dead non parasitized aphids will wash away or decay.

Non parasitized and parasitized (brown, rounded) aphids on cabbage. Photo: IPM Images, Alton N. Sparks, Univ of Georgia.

The aphid species we are most likely to have present on strawberries in North Carolina are strawberry aphids, green peach aphids, and occasionally, potato aphids.  It can be important to distinguish between these species, particularly if faced with the need to treat populations with foliar insecticide applications, because aphid species may respond differently to insecticides.

All three aphids can be green but may also be orange to red. Potato aphids, with their larger size and long legs, are perhaps the most distinctive species on strawberries.

Potato aphids (Macrosiphum euphorbiae) nymphs, adults, and winged adults. Photo: IPM Images, Whitney Cranshaw, Colorado State University.

Strawberry aphids can be distinguished from green peach aphids by the presence of knobbed hairs covering their bodies, which can be observed under 20x or greater magnification. University of Florida Extension has a nice image illustrating these hairs.

Strawberry aphids (Chaetosiphon fragaefolii) posses knobbed hairs that can be used to distinguish them from green peach aphids. Photo: IPM Images, Jeffery Lotz, Florida Department of Agriculture and Consumer Services.

Green peach aphids are among the most common aphid pests in North Carolina crops and have a very wide host range.

Green peach aphid adult and nymph. Photo: Jim Baker, NC State University.

If aphids are observed in fields with suspected virus infected plants that do not appear similar to our common species, contact your extension agent and submit a sample to the NC State University Plant Disease and Insect Clinic (or your state resources) for identification.

Mid season tobacco pests & the burning question of tank mixes

Tobacco budworm moth.  Moths lay their eggs in or near tobacco buds and flowers and are more attracted to flowers. Photo: HJB

Tobacco budworm populations have been booming in our research plots this summer, and as they often are, tobacco budworm larvae are present in fields at or near topping.  The logical questions that follow these tobacco budworm populations is whether or not they need to be managed.  In general, if plants are within 2 weeks of toppping or large enough that contact sucker controls are being applied, tobacco budworm treatments are unnecessary.  This is because budworm larvae prefer to feed on flowers and seed capsules when given a choice between these and leaves.  These are the same plant parts (along with the youngest, smallest leaves) that are removed during topping. Tobacco budworm feeding during the interval between buttoning (when the unopened flowers are visible outside of the bud) and flower does not damage plant tissue that will be harvested and has no economic impact.  Therefore, any insecticide applied at this stage for tobacco budworm is a waste.  Tobacco budworm will not reinfest tobacco after topping because the attractive buds and flowers are no longer there.

Tank mixing insecticides and sucker controls
Among some North Carolina tobacco growers, it has been standard practice to include an insecticide (most commonly acephate) in the first few applications contact sucker control.  Contact sucker control materials are pesticides that, as their name implies, need to come into contact with a leaf axil in order to inhibit growth.  Contacts are typically fatty alcohols, or soaps, which are commonly made from petroleum or, in the case of organic contacts, palm oil.  Fatty alcohols are also used in a number of industrial and cosmetic applications.

Foliar (sprayed on) insecticides in tobacco should only be used when insects are present at damaging levels (the economic threshold).  Acephate is a broad spectrum insecticide, so it could potentially be used against a wide range of insect pests.  The insect pests most commonly present at or near topping are tobacco budworm, the "tobacco form" of the green peach aphid, and, less commonly, tobacco & tomato hornworms.  We already know that pretopping treatments for tobacco budworms are not needed, but what about aphids and hornworms?

Aphids
Aphid management in tobacco has shifted dramatically in the last 15 years.  Aphids used to be frequent pests which often reached treatment threshold (10% or more of plants with 50 or more aphids in the upper third) several times in a season.  The widespread (over 90%) use of systemic neonicotinoid insecticides (imidacloprid & thiamethoxam) has rendered aphids infrequent early season pests who typically only appear near topping.  If aphids are present in a field when contacts are being applied, it may be tempting to include an insecticide in with these materials, but this is totally unnecessary.  Contacts (soaps) can be excellent aphid control materials, because they dry out their soft bodies, and the application method for contacts actually results in decent coverage for aphid control.  In addition, aphids are less attracted to tobacco leaves as they "harden off" following topping and will rarely reinfest.  In an organic aphid management trial we conducted in 2009, none of the organic insecticides were effective against the large aphid populations present, but as soon as the grower started spraying organic contact and topping, the aphids disappeared.  Timely topping and good sucker management can eliminate an aphid problem, so insecticides in the contact do not provide any additional benefit.

Tobacco & tomato hornworms

Tobacco or tomato hornworm eggs.  Hornworm eggs are laid on the upper surface of tobacco leaves. Photo: HJB

Tobacco hornworm (Manduca sexta) larva, note the red posterior "horn".  Tomato hornworm larvae will have a dark blue horn. Photo: HJB

Tobacco and tomato hornworms typically occur in their highest numbers just after topping and, from a timing stand point, are perhaps the most logical insects to be targeted by an insecticide tank mixed with contacts. However, I do not recommend tank mixing insecticides with sucker control materials even for hormworms for two important reasons:

1. Phytotoxicity may occur when insecticides are tank mixed with oils and soaps, and more importantly, 

2. Contact sucker controls and insecticides should be applied to different parts of the plant and will be less effective with different coverage patterns.

The only time I have seen phytotoxicity associated with some of our newer, caterpillar active insecticides in tobacco is when they have been used in combination with contacts.  Although I have never seen truly damaging phytotoxicity associated with either Belt or Coragen, it doesn't make sense to combine these with sucker controls.

Because contacts need to coat leaf axils to be effective, they are applied in a coarse spray, run down spray, sometimes using modified hoods, as described in the 17 June issue of the Tobacco Connection Newsletter.  This application method drives contacts down the stalk, but hormworms are leaf feeding insects and could be missed with a stalk spray or hood application.  If an tank mixed insecticide application fails, it means yet another treatment across the field.  I would rather see one insecticide trip made, done correctly.

Stink bugs

A midseason tobacco ecosystem, tobacco budworm larva and adult brown stink bug.  Hoke County, NC. Photo: HJB

The last tobacco insects that may pop up in the mid season are stink bugs, both brown and green.  Stink bugs feed on the stalk and mid veins of tobacco leaves, which can cause them to wilt.  This damage is rarely, if ever, economically significant, and insecticide treatments for stink bugs are not recommended. In fact, so few insecticides are effective for stink bugs, that is unlikely that any of the materials with workable preharvest intervals would provide control regardless of whether treatment was needed.

Tobacco plant with leaf injured by stink bug feeding, Raeford, NC. Photo: HJB

More information
Tobacco Connection Newsletter, 2(6). 17 June 2011.

Tobacco greenhouse pests - Aphids

Red & green color forms of the green peach aphid (Myzus persicae). Photo: Sterling Southern, NCSU.

I have gotten two calls and emails about aphids in tobacco greenhouses, specifically the red color form of the green peach aphid (Myzus persicae). This topic was among those I addressed in the very first Tobacco Connection Newsletter last March. Although they appear different, the red, green, and orange color forms of the green peach aphid are the same species and are managed in the same fashion. The main difference, other than color, of these different color forms is their environmental preferences and interactions with pesticides. In general, the green color form prefers cool temperatures and is more susceptible to pesticides. The red color form predominates in hot weather and has been documented to develop resistance to pesticides used in tobacco more readily.

We typically seen green color forms early in the season while red color forms dominate during the hotter summer months. Both color forms can be found year round in North Carolina, however. Virtually all conventional tobacco grown in North Carolina is treated with systemic neonicotiniods (imidacloprid and thiamethoxam) to control aphids and flea beetles in the field. For greenhouses with aphid infestations where transplant is within 5 days, these materials can be applied and will kill aphids prior to transplant. For aphid infested greenhouses where transplant is more than 5 days off, other treatments are available (see the NC Ag Chem Manual for specific recommendations or contact me and see A note on pesticide recommendations).

More information
NC State Tobacco Connection 1(1). March 22, 2010

As the covers come off...

Those strawberry growers who have had row covers on their plants all winter are now removing them and performing sanitation on the plants (removing dead leaves and stray weeds). As these covers come off, I am getting reports of lots of arthropods appearing.

Spider mite management begins in earnest now, and growers should begin their weekly sampling of 10 leaflets/acre. The undersides of these leaflets should be examined for spider mites with a 10x hand lens (or microscope if you're well equipped). If the average number of mites/leaflet is 5 or great, miticide treatments should be applied. Scouting should continue post treatment to ensure they are effective and to determine if and when re treatment is needed.

Twospotted spider mite female and eggs on the underside of a strawberry leaf. Photo: HJB

Threshold based treatments are helpful because not only do they often reduce the number of applications, but they also allow us to select the best material for the job by providing a weekly pattern of information. This means we have a better idea if populations are small and newly established (best treated with an ovicide/larvicide) or are already large (best treated with an adulticide plus ovicide/larvicide).

Cast aphid skins (white) and dead aphids (brown) on tunnel strawberries at the Piedmont Research Station, Salisbury, NC. Photo: HJB
Click to enlarge this image for a much better view.

Aphids are also showing up as covers are removed, but I have yet to hear about damaging numbers in NC. It takes a lot of aphids to reduce yield in strawberries, and we have excellent native biological control agents. In fact, also found under the covers have been lady beetle larvae, feeding on the captive aphid buffet. Most of the aphids that were present under the covers morphed to winged (alate) forms, and have dispersed off the plants leaving only their cast skins. These cast skins appear white against the plastic or leaves.

Lady beetle larvae found on newly uncovered strawberries in the Sandhills. Photo: David Dycus.

As the weather warms and spring is finally here, the real fun begins!

A note about pesticide recommendations.

Spring Strawberry Arthropod Management Reminders

No matter what the forecast says, spring is coming. Along with it, come greater management needs. Strawberries have been in the ground since last fall and are the first fruit crop harvested in spring. The cold winter has meant most of our plants have been under row cover for at least part of the winter and our arthropod (insect and mite) issues have been out of sight and out of mind.

Row covered strawberries at Vollmer Farms, Bunn, NC. Spring, 2009.

When these cover come off, the real arthropod management begins. Here are a few key steps to start the 2010 season out right (A note about pesticide recommendations):

Green peach aphid (Myzus persicae) on strawberry. Image from UC IPM.

1. Scout for aphids, but only treat if populations are high. Aphids can rapidly proliferate under row covers, but unless early spring populations are causing sooty mold build up, treatment is likely not needed. Later spring and summer populations are often kept under good biological control in NC. If aphid populations under the covers are producing sooty mold and exceed 10 aphids/newly expanded leaf on average, treatment may be justified. If systemic insecticides are your aphid management tool of choice, the time to use these is now (before covers come off). Once blooms are exposed to bees, the labeled systemic materials for aphids in strawberries can no longer be used.

Female twospotted spider mite, our most common spider mite pest in strawberries. Image from UC IPM.

2. Begin spider mite sampling. Spider mites are the key arthropod pest in southeastern strawberries. Mite activity is typically lower in winter months, but as temperatures warm, spider mites will become more active and reproduce faster. We are currently studying the impact of long term row cover on overwintering mite populations, but our working assumption is that row covers potentially increase mite populations over the winter. This winter, of course, all bets are off. It is important to track mite populations closely and be prepared to treat when they reach our recommended treatment threshold (5 mites/leaflet). This threshold is based on a random sample of 10 leaflets/acre. It is in grower's best interest to treat at threshold levels and not on a schedule basis, because virtually all of the labeled miticides have restrictions on the number of applications and/or amount of active ingredient that can be applied to strawberries per season. We want to keep as many good options in our tool box for as long as possible.

Phytoseiulus persimilis, one of the commercially available biological control agents for spider mites. Image from UC IPM.

Biological control is also an option for spider mites, but we are in the process of optimizing methods for NC and do not have concrete recommendations at this time.

I have heard a lot of rumblings about different materials being applied systemically for mites, and I want to be perfectly clear: There are no systemic miticides. None of the materials currently labeled for soil or drip applications in strawberries are either registered for or effective against spider mites. Our best chemical control for mites is over the top treatments of miticides. I also do not recommend the use of pyrethriod insecticides against mites. The pyrethriods registered in strawberries have either shown no effect in our trials or have actually increased mite populations because they are equally toxic to beneficial insects.

Strawberry clipper damage in Lee County, NC, 2009. Photo from Stephanie Romelczyk, Lee County Horticulture Agent.

3. Do not assume you will have clippers in 2010 just because you had them in 2009. I had several reports of clipper injury in 2009. We do not have a sense of what 2010 will look like with respect to clippers, but I am not recommended a preventative treatment just because damage was observed in 2009. Most strawberry varieties compensate to a certain extent for clipper damage, and rescue materials are available for clipper management. Pre bloom preventative applications may be too early to really protect plants. When scouting for clipper damage, remember, clippers only affect flower buds (as in the above photo), not leaves.

You can find more specific information about pesticides in the NC Ag Chem Manual and the Southeastern Strawberry IPM Guide.

I will post a mid season arthropod reminder for strawberries in May which will cover late season mite management, sap beetles, thrips, and the unusual occasionals we see as the weather gets warmer.

Aphids in tunnel berries

This is just a short note to mention that aphids have popped up in tunnel strawberries at one of the research stations we work on. They haven't been problematic in our plots, and it takes a lot of aphids per plant (10 per newly expanded leaf on average as a rule). Aphids are, however, one of the pests that will likely be more problematic in tunnels than in the field. There are several conventional and organic options for aphid management when they do exceed threshold levels, but the strategy used for each will be different--early and often is the mantra for organic aphid materials, while conventional materials can and should be used much less frequently.

A note about pesticide recommendations.

Check out the newest Small Fruit News

The latest edition of the Small Fruit News has just been posted at the Southern Region Small Fruit Consortium's webpage. I contributed an article on the spotted wing drosophila to this quarter's edition which expands on my previous post here. The most interesting article to me was contributed by Phil Brannen, University of Georgia plant pathologist, which briefly discusses 2 viruses new to eastern blueberry production.

Blueberry scorch and blueberry shock have been detected in locations in Michigan, the nation's largest blueberry producer. Among these locations is the Trevor Nichols Research Complex, where much of the blueberry research in the state is conducted. Blueberry scorch is vectored by aphids (likely non persistently), although the potential vectors are unclear. Blueberry shock is a pollen borne virus whose spread is mediated by bees. These viruses have yet to be detected in the southeast, but their movement across the US from the Pacific northwest to Michigan raises concern.

More information on Blueberry scorch and shock is Michigan can be found here.