These important spray decisions are covered in Fruit Times articles by Dr. Krawczyk. My research concentrates more on the potential of biological control for BMSB and how pesticides affect wild bees and secondary orchard pests such as mites, San Jose scale and woolly apple aphids. I summarized the effects of BMSB sprays on mites in a previous article. In this issue, I summarize what we know about pesticide impacts on the biological control of woolly apple aphid.
The "American Aphid"
You may be surprised to know that while we can blame many Asian countries as the origin for many of our recent invasive pests such as BMSB, the spotted wing drosophila, the Asian longhorn beetle and emerald ash borer, woolly apple aphid (WAA) is known by some fruit growers in Europe as the "American Aphid" because it is one of our unique contributions to the worldwide apple pest community. Originally evolving on native hawthorn trees and using elm trees as an overwintering host, WAA has adapted to using apple as its sole host in many areas. Sexual reproduction occurs only on elm, and for apple, we are dealing mainly with clones of the parent produced asexually. This lack of genetic variability may mean that when resistance to pesticides occurs it is rapidly spread in these clones of the resistant mother and there are no susceptible genes in the population to pass on during sexual reproduction. WAA is unusual for an apple aphid in that it attacks both the roots and the woody aerial parts of the tree, such as the shoots and pruning wounds.
In addition to leaving a mess of wool and sticky defecated plant sap called "honeydew," late season WAA can infest the fruit directly and its presence in the stem or calyx end can constitute a quarantine issue for export or a rejection for "bug parts" in processing. What is less known is that infestations can kill nearby fruit and flower buds being formed for the next year and result in long areas of "blind wood" without leaves or fruit. Feeding on pruning wounds causes galls that can prevent healing and encourage the recurrence of perennial cankers. Feeding on the roots may be the main cause of injury to the tree, although this is generally invisible to the grower. Root galls interfere with root function and reduce water and nutrient uptake which is presumably much greater with smaller, dwarf trees at high density than larger well established semi-dwarf or seedling trees. Currently there are no effective treatments for preventing root infestations other than dipping plants just before planting. Claims of systemic movement of products like Movento from foliar sprays to the root populations of WAA have not proven to be effective at labeled use rates, and drenches of neonicotinoid products such as Provado have also been ineffective in the eastern US. WAA has been one of the few pests of tree fruit where host plant resistance has been quite successful using rootstocks developed from the resistant "Northern Spy" variety, notably the Malling-Merton series of rootstocks including MM 111 and MM 106. While some of the newer dwarfing rootstocks coming out of Geneva, NY are WAA resistant, unfortunately M9 and other popular dwarf rootstocks remain very susceptible to this pest. Anecdotally, it has been said that for every above ground colony seen in most orchards, 10 colonies exist on that same tree underground. I don't know if this has ever been proven, but it is generally accepted that if effective pesticide controls existed to control underground WAA colonies, significant increases in apple yield and fruit quality could be realized. Additionally, in the mid-Atlantic region certain apple varieties appear to be more susceptible to WAA, including Fuji, Rome, Greening, York and Ginger Gold.
For U.S. apple growers, however, the "made in the US" label of this pest gives us something of an advantage when it comes to biological control. We don't have to send off to China or some other country of origin to find a predator or parasitoid, as we already have them here and export them to other countries. The example of the tiny parasitoid wasp, Aphelinus mali, was one of the earliest and most successful examples of exporting a biological control agent for a major pest to other countries. A. mali has also been shown to be resistant to most organophosphate insecticides including diazinon and seems to tolerate single applications of some neonicotinoids midseason without flare-ups. Almost as important in controlling WAA in the mid-Atlantic are several species of syrphid fly larvae. Other generalist predators such as green and brown lacewings, minute pirate bugs and some lady beetles may prey on WAA colonies when the nymphs and colonies are still small and unprotected by the waxy white coating and white filaments that protect larger nymphs and adults (giving them their "woolly" appearance). Unfortunately, these biological control agents are considered to be largely ineffective on the underground colonies deeper than a few inches into the ground.
For the majority of apple growers who had to give up on their IPM programs to control BMSB, the following are some guidelines and control strategies for minimizing the impact on woolly apple aphid biological control while still giving acceptable management of the pest.
- Use resistant rootstocks if possible. Keep a closer eye on what appear to be favored varieties such as Fuji, Rome, Greening, York and Ginger Gold.
- Conserve biological control of WAA if possible. See table 4-4 in the Pennsylvania Tree Fruit Production Guide for pesticide impact ratings on A. mali and syrphids.
- Avoid pyrethroids, including combination products like Endigo, Lannate and the IGR insecticide Rimon which consistently flare WAA.
- Avoid the use of Delegate for 2nd generation codling moth. Use it for 1st generation codling moth, but use Altacor or Belt, which won't affect WAA biological control, for 2nd generation codling moth. Refer to the previous article on mites, as this also applies to biological control of mites.
- Avoid multiple applications of neonicotinoid insecticides after June and try to restrict use to products that are "softer" on beneficials, which include Calypso, Assail and Provado/Imidacloprid products, at lower than maximum rates. Multiple applications of all products other than Calypso can also affect mite biological control. For late season control of BMSB, neonicotinoids are the least disruptive control options, but we have seen evidence that at least Belay may flare mites the following season. Depending on the timing these sprays may also affect WAA biocontrol, but in general, sprays after mid-September probably don't have much effect, as WAA populations have crashed at this point and the biocontrol agents are moving to overwintering sites.
- Control of WAA - Diazinon is the most effective and cheapest product for control of WAA at the 2 lb/A rate. This is the only labeled use for this product and only a single application is allowed or necessary. Unlike Movento, it only controls WAA. If you also have San Jose scale (SJS) in your orchards, Movento will control both pests even if you timed the spray early in the season (around 1st cover) for SJS crawlers. Movento has not always proven to be reliable on large standard or semi-dwarf trees due to trouble penetrating through the bark. When used with a penetrant adjuvant or oil in a 1% solution, the 9 fl oz of Movento has proven very effective in controlling both pests, albeit at a much higher cost than Diazinon. None of the neonicotinoids have proven effective on WAA.
- Another relatively cheap option, if you have had both SJS and WAA problems in the previous season, is to use a 3 to 4 pint rate of Lorsban Advanced at tight cluster in a higher rate of water to get complete coverage. This has proven to give control of SJS as expected, and the long residual control of the product also has given excellent control of mid-season WAA in several grower trials. Some growers, however, seem to be having problems getting adequate control with Lorsban in the last few years, which I hope has more to do with spray coverage than with the onset of resistance. The label for all Lorsban/chlorpyrifos products allows only for applications pre-bloom (only as late as ½ inch green). Later applications will kill bees. Most growers have Lorsban-resistant rosy apple aphid, but this spray may in some cases help with control of this pest and also tarnished plant bug.