Previously, I discussed how mites and woolly apple aphids could be flared due to the use of harsh pesticides that are sometimes necessary to control the Brown Marmorated Stink Bug (BSMB) in apple. Another pest that has flared due to BMSB sprays has been San Jose Scale (SJS). SJS is a relatively easy pest to control with many different control options added in the last 10 years. Paradoxically, it is a pest that is better prevented than cured. It is also a dangerous pest to leave uncontrolled. After a few years of heavy infestation, limbs and even small trees can be killed and the percentage of infested fruit can reach over 80% in only a season or two. This is due to a high reproductive rate from three generations in a season during which a single female can produce thousands of offspring.
I believe the resurgence of this pest in apple is due to three things:
- disruption of biological control agents by BMSB sprays
- a move away from the use of dormant oils
- a lack of spray coverage from growers using low spray volumes (generally less than 50 gallons per acre) or only a single side application (i.e., sprays involving alternate row middle applications).
Some growers increasingly relied on Lorsban (chlorpyrifos) alone as the price of generic products drove the price way down, and it is possible that SJS has developed some tolerance to it just as the Rosy Apple Aphid (RAA) has.
SJS is normally considered a secondary pest because it has easily been controlled with a dormant oil spray to supplement biological control agents, which consist mostly of various species of tiny chalcid wasps (Aphytis sp. and Encarsia sp.) that attack the scale under their waxy covering and also some predators of the mobile stage called "crawlers." These predators are generalists that eat many small insects and include minute pirate bugs (Anthocoridae), the predatory mite T. pyri, and predaceous mirid plant bugs. Some commercial tolerance of a few red dots on the fruit (especially for red varieties) and of low levels of scale on the twigs and trunks allows for biological control agents to maintain themselves in the orchards rather than the tiny (< 1 mm) wasps having to fly into the orchards each season. This biological control, however, can easily be disrupted by some broad-spectrum insecticides such as pyrethroids, Lannate (methomyl), and some neonicotinoid insecticides that are used for BMSB. The specific names and biology's of these chalcid wasps are not well known, so we do not know the specific times of the season they exert the most pest control, but it is likely that they will attack all three generations of SJS. I would guess from the recent flares of SJS due to late summer/fall BMSB sprays that it is the wasps attacking the overwintering generation of SJS that are the most critical.
SJS females do not lay eggs; rather, the eggs hatch live from within the females. The immature stages of SJS are tiny (<1/2 mm) and, because they are the only mobile stage other than adult males, are called "crawlers." The flattened yellow crawlers are quite active until they find a place to settle down and feed. They immediately begin to produce the waxy coating that shields them somewhat from predation and pesticides and become immobile. Adult female SJS never move from under this waxy cap, but mate, hatch live young internally and die under this covering--leaving it up to the flying adult males to find them via a pheromone. SJS males can be most easily monitored using a specific sex pheromone available commercially, but care needs to be taken when counting tiny little dots on the sticky cards that you are actually counting the right insect (See photo - note dark band across the back, only 2 wings, long antennae). These adult males only fly about 75 yards and live only 2 to 3 days, so pheromone trapping for them from petal fall to about 2nd cover in orchard locations away from borders will give you a good indication of the resident SJS population in the orchards. Be warned, however, that SJS will feed on many different ornamental trees and shrubs as well as stone fruit crops, so traps placed along the borders of orchards may only measure the amount of SJS on your neighbor's crabapple tree. Monitoring the small, yellowish crawlers can also be done using double sided black electrician's tape, but I have found using the commercial SJS pheromone for males to be less time consuming, messy, and more accurate. Unfortunately, trap thresholds do not exist and whether to spray is often based mostly on if damage was seen at harvest the previous fall. A degree day model exists to precisely time sprays for the crawler stage which for 1st generation is approximately at the 1st cover timing (usually around the first week of June).
Dormant oil applied alone or in combination with an insecticide at the dormant to green tip stage of apple was the most effective way of controlling this pest for several decades. Older spray publications listed 5 to 6% oil concentration (5 to 6 gal per 100 gal of water), which worked for standard sized trees. Recent work from Cornell and some of our own trials have shown that the lower 2 to 3% rate will work as long as coverage is thorough. This usually means at least 100 gal per A of water on larger trees and 50 gal per A on smaller/younger trees. Fruit growers never liked the mess and handling of dormant oil nor the incompatibility of oil with Captan when our choices for scab control fungicides narrowed considerably due to resistance issues. When the price of oil increased greatly about 10 years ago, many growers dropped the dormant oil out of the program and relied on Lorsban alone. Dormant oil applications also used to have the additional benefit of controlling overwintering European red mite eggs, but for growers with good populations of T. pyri predatory mites or that use the prophylactic treatment of the now very inexpensive Agri-mek this benefit has largely disappeared. It is unclear if we have Lorsban-resistant SJS in Pennsylvania and none of the trials I or Dr. Larry Hull have run in grower orchards or on the FREC research station have had control failures when applied at the recommended 1.5 to 2 qt per acre rate. Care needs to be taken to not use Lorsban after ½ inch green, however, as it is very bee toxic and will kill wild bees foraging in the ground cover. It is also unique in that through its high vapor activity, it will act somewhat as an orchard fumigant for several days after application. This is one instance where spraying at night will do little to protect wild bees or local honey bees if applied after ½ inch green.
Pre-bloom Control Options
Esteem (pyriproxyfen) applied at 4 to 5 oz between green tip and pink works best with a quart of oil per 100 gallons to serve as a penetrant and will give good control of overwintering SJS. Esteem is a juvenile hormone analog type of insect growth regulator (IGR) insecticide that works to kill the eggs in SJS females. At this timing it will also give some level of control of RAA, but has not held up under high RAA pressure in our trials. Esteem can also be applied for the crawlers around 1st cover, but has proven to be less effective at that timing. Neonicotinoid insecticides applied for RAA at the tight cluster to pre-pink stages offer the benefit of systemic activity, but they only suppress SJS populations in our experience and do not provide complete control under high SJS pressure. The only neonicotinoid insecticides we now recommend pre-bloom because of possible bee toxicity issues are the relatively bee-safe products of Calypso (thiacloprid) and Assail (acetamiprid). Our research has shown that applying either product no closer than one week before king bloom, minimizes pesticide levels in nectar and pollen that bees will be exposed to, and will still give excellent RAA control.
Centaur (buprofezin) is another IGR insecticide of a subclass that primarily interferes with insect molting and has been used with success by Dr. Hull and a number of growers over the last couple of seasons to clean up SJS problem orchards. This product is locally systemic (translaminar) and the potential for movement into nectar and pollen from pre-bloom applications exists, but we have not measured this as of yet. Applications can be made from green tip to pink, but most of the data I have seen have been for post-bloom applications targeting the crawlers around first cover. A single rate of Centaur at 34.5 oz per acre is an option and the addition of 1 qt of oil or a penetrant surfactant (0.25%) has been shown to increase control.
Post-Bloom Control Options
All of our post-bloom control options target the crawlers of the overwintering generation of SJS with the aim of stopping this generation in its tracks. Little work has been done to look at later summer sprays for the two succeeding SJS generations. As mentioned, Centaur can also be used post-bloom at the same rate and has been shown to provide excellent control of the crawlers at around 1st to second cover. This is the "normally" recommended timing of this product, but as trials from Larry Hull and results in some grower orchards have shown, there appears to be some flexibility. While Centaur is a good control option for pear psylla in pear, on apple it appears to be only effective on SJS and other summer pests such as aphids. The combination product Tourismo (flubendiamide and buprofezin) can be used to target crawlers and control internal worms (codling moth and Oriental fruit moth) at the same time. Care needs to be taken because the rate of the Centaur active ingredient in Tourismo in labeled rates is lower than that needed for anything more than suppression of SJS.
The product I have the most experience with at FREC and with some grower trials is Movento (spirotetramat). Movento is a highly systemic product that is unique in that it will move both up and down within the vascular system of the tree (both phloem and xylem) and kills sucking insects that ingest the tree sap. Although some claims have been made that this movement into the roots and not just from the roots to the foliage will kill WAA root colonies, the data I have seen only shows such an effect at rates much higher than currently legal. Movento works to stop the production of fats in insects and works well on female insects to stop egg production and eventually starve immature stages of both scale and some aphids. Movento must be mixed with a penetrating adjuvant or oil at a rate of 1 qt per 100 gallons in order to move it into the plant to work.
On standard sized trees, however, I have seen control failures due to the difficulty in penetrating the bark of large old trees. Although it is generally the most expensive control option for SJS, Movento has the added benefit of also controlling even the worst outbreaks of woolly apple aphid (WAA). While sprays mid-season timed for WAA only have helped control the later generations of SJS, by that time some fruit injury from SJS has already occurred. A better use for Movento in my experience has been to time a single application for SJS crawlers at 1st to 2nd cover and let the residual of this product take out WAA populations later in the season. Movento also control mealybugs, which is generally not a problem in the mid-Atlantic region, and pear psylla. Although observed in other regions to be somewhat toxic to predatory mites, in several years of my trials it does not affect our main predatory mite, T. pyri. It also appears to be safe for most beneficial insects in apple orchards since it is systemic only within the plant, but there are concerns over exposure to bees from spray drift to non-target plants near orchards where exposure through contaminated nectar and pollen is possible.
We will continue to research SJS control at the Penn State FREC and with grower trials, especially with Centaur, which I have little experience with. Concerns about impacts of any new pesticide or use pattern on pollinators mean that we also have to conduct very expensive residue analysis of apple nectar and pollen to determine if bees are being exposed through contaminated nectar and pollen. It is our hope that BMSB populations will decline in the near future as some native predator or pathogen decides it likes to eat Asian food as well or that we will be able to import and release Asian Chinese parasitoids that already have a taste for BMSB eggs. Then, hopefully our apple IPM programs will return to normal and we will see fewer secondary pest outbreaks as biological control is re-established.