Mid-Season Woolly Apple Aphid Management: July 6th, 2018

Brief: Increasing populations of Woolly Apple Aphid (WAA) have been observed this morning, most problematic in Fuji apple in Orange County. The aphid feeds on the sap from plants, excreting or ‘shunting’ excess and concentrated sap will act as a substrate for sooty mold while high infestations can colonize the calyx end of fruit becoming problematic at harvest.

The WAA tends to be a sporadic pest in orchards in the northeastern United States, occurring in high numbers only every few years. However, through the use of pyrethroids for the control of the brown marmorated stink bug we are seeing frequent and high infestations of this insect pest on apple over the past few years. Since the WAA is frequently parasitized by Aphelinus mali, a tiny wasp that is also native to North America, the use of pyrethroids late in the season have likely reduced the biological control provided by this important parasitoid.

If you are seeing WAA in commercial blocks, management may be required to reduce the insect colony, most importantly to reduce honeydew created by the pest along with the white cottony fibers to assure the fruit is commercially acceptable. As we move through to harvest, the use of short PHI materials (7 day or lower), effective at reducing the population may be required over the next few weeks.

Early intervention, prior to heavy infestation levels at harvest, is critical in fresh fruit production.

Wooly Apple Aphid Arial Colony on Commercial Fuji

Biology: Reportedly native to North America, occurs in most apple-growing areas of the world. The WAA feeds mainly on apple, but can also be found on pear, quince, mountain ash, hawthorn, and Cottoneaster. Its reproduction on these hosts is asexual (parthenogenetic). Sexual reproduction has been thought to occur only when elm grows in close proximity to the other host plants. The portion of the life cycle occurring on elm has become more doubtful and less important than in the past, since most elms have disappeared from Eastern forests because of Dutch Elm Disease. The WAA tends to be a sporadic pest in orchards in the northeastern United States, occurring in noticeably high numbers only every few years.

Management: of the WAA should include higher volume applications (>100 GPA) then what would be used for apple maggot and codling moth. The waxy covering protecting the insect from desiccation necessitates the use of a penetrant, providing more effective access to directed contact of the active ingredient to the developing WAA colony. Few insecticides are labeled and effective for use in NYS.

PHI will dictate the use of material choice nearing harvest. Diazinon (High degree of control) and Beleaf 50SG (Moderate Control) having a 21 day PHI. Movento 240 SC, also very effective earlier in the season, takes time for systemic activity to occur with late season applications in September. This insecticide may have greater challenges for uptake and distribution later in the season as sunlight and temperature decrease. Admire Pro 4.6SC and Assail 30SG are two options that should also be considered based on the 7 day PHI. Other products such as horticultural oils (Pure Spray, Bio-Cover) and soap (M-Pede; Gowan) may be options for conventional and organic growers requiring zero days to harvest (0 PHI), yet may prove challenging if high temperature or slow drying conditions prevail so as to cause fruit russett or phytotoxicity on susceptible varieties.

Note: Mixing formulations of diazinon or Danitol with Captan or Captec have caused crop injury in the past. Therefore, diazinon and Captan formulations should not be tank-mixed. This type of phytotoxicity results from either a direct interaction of the active ingredients or an interaction of the “inert” ingredients in one formulation that enhances the toxicity of the other one. (reference from Russ Holze and Dave Rosenberger)

WAA Infestation of Pruning Cuts.

Life Cycle: The majority of nymphs are borne alive on apple trees by the un-mated female. The WAA nymph passes through four instars, changing in size from 0.6 mm (.02 in.) long in the first to 1.3 mm (.05 in.) long in the fourth instar. The nymphs are dark reddish-brown with a bluish-white waxy covering that becomes more extensive in the later instars. The first instar nymphs (crawlers), which are considerably more active than later instars, are a dispersal stage. They initiate aerial colonies in the spring from overwintering root infestations. The crawlers are carried by wind from tree to tree within an orchard or nursery, or move downward from the branches to initiate colonies on roots.

Damage: Cottony-white aerial colonies are found most frequently on succulent tissue, such as current season’s growth, water sprouts, unhealed pruning wounds, or cankers. Heavy infestations can cause honey dew and sooty mold on the fruit, and galls on the plant parts. Underground colonies may be found throughout the year on the root systems of orchard trees or nursery stock. Severe root infestations can stunt or kill young trees, but usually cause little damage to mature trees. WAA can also transmit perennial apple canker, Pezicula malicorticis Jacks.

Biological Control of WAA by Aphelinus mali

Biological Control: The WAA is frequently parasitized by Aphelinus mali, a tiny wasp that is also native to North America. Parasitized aphids appear as black mummies in the colony. A. mali has been successfully introduced to many apple-growing areas of the world, and is providing adequate control of the WAA in several areas. It does not provide sufficient control in commercial orchards in the northeastern United States because of its sensitivity to many commonly used insecticides; however, the wasp is thought to reduce WAA populations in abandoned orchards. Because the woolly apple aphids are somewhat protected by their waxy covering, regular spray programs may not provide adequate control. High volume applications of recommended insecticides may be necessary to penetrate the wax. Failure to control aerial infestations can result in underground infestations on susceptible rootstocks. Chemical control of root infestations is not possible; resistant rootstocks provide the only defense against underground infestations.

Rootstock Susceptability: Failure to control aerial infestations can result in underground infestations on susceptible rootstocks. Chemical control of root infestations is not possible; resistant rootstocks provide the only defense against underground infestations. The Mailing-Merton (MM) rootstock series was developed to provide resistance to WAA infestation. The M-9 series, M-26, M-27 are all susceptible to WAA. Its reproduction on these hosts is primarily parthenogenetic, that is reproduction without mating.

More on Management:
Diazinon is an excellent material against the WAA. . It is principally used prebloom for control of San Jose scale or postbloom for broad-spectrum control of major pests. It is generally less persistent than other standard phosphates. This insecticide has caused russeting or related finish problems on Golden Delicious, R.I. Greening, and Baldwin. No injury has been reported on McIntosh or closely related varieties yet only limited observations have been made on other varieties. Note that the material should not be used in tank mix with Captan under slow drying conditions, strong acids & alkalis and copper-containing compounds.

Spirotetramat (Movento) is a tetramic acid registered for the control of a number of indirect pests in pome fruits and stone fruits, primarily aphids (including woolly apple aphid), mealybugs, pear psylla, and San Jose scale. It has systemic activity, exhibiting 2-way movement in the plant, both upwards in the xylem to new shoots and leaves, and downwards in the pholem to the root tissues. Its mode of action is as a Lipid Biosynthesis Inhibitor (LBI), and it is active by ingestion against immature insects feeding on treated plants. Additionally, adult females have exhibited reduced fecundity and offspring survival.

Acetamiprid (Assail) belongs to the neonicotinoid group of insecticides (along with *AdmirePro and *†Actara). It was registered by the US EPA under the reduced risk pesticide policy and is considered a replacement for older OP insecticides. Assail has a spectrum of effectiveness across several insect groups, and is active against pests such as plum curculio, apple maggot, internal leps, aphids, leafhoppers, leafminers, San Jose scale, European apple sawfly and mullein plant bug, plus pear pests such as pear psylla and Comstock mealybug.

Imidacloprid (*Admire Pro, *Leverage) is a broad spectrum contact and locally systemic chloronicotinyl insecticide with low mammalian toxicity. It is primarily effective against aphids, whiteflies, thrips, scales (crawlers), pyslla, leafhoppers, mealybugs, some beetle and weevil species, and leafminers. The original *Provado formulation has been replaced by *AdmirePro, which is labeled on pome and stone fruits for aphids (except woolly apple aphid), leafminers, leafhoppers, San Jose scale, pear psylla, mealybug, Japanese beetle, cherry fruit flies and San Jose scale. It has also shown activity against pear midge when applied at petal fall. It is additionally labeled for use as a soil-applied product against woolly apple aphid. This material has no effect on any mites, beneficial or phytophagous, but is hard on Stethorus. Mite flare-up is often associated with use of Imidacloprid during the season.

Flonicamid (Beleaf) is labeled against aphids and plant bugs in NY for pome fruit and stone fruit; the label classifies it as a member of the pyridinecarboxamide family, an IRAC Section 9C material, which is defined as “Unknown or non-specific mode of action – selective feeding blockers”. It has not been tested in NY field trials, but other researchers have reported good efficacy against green peach aphid and tarnished plant bug in peaches. The label also lists apple aphid, black cherry aphid, rosy apple aphid, spirea aphid and woolly apple aphid. It has a low bee poisoning hazard.