Injection or fertilizer treatments for pecan tree fall webworm?
I see a lot of treatments that amount to spraying a pecan tree for spring/fall webworm. I had an arborist treat my pecan tree with little "injectors" drilled into the base of the pecan tree. He warned it would take time to work but the effect was dramatic and immediate and lasted for years. He has since passed, and I dont know what treatment he used. I tried a homeowner applied injector treatment which was not effective. Another arborist suggested a "fertilizer" that would be applied in the fall (after leaves fall?) to the ground that would be taken up into the tree through the root system. I expect fertilizer is the wrong word, but the product was supposed to remain in the tree and leaves for the coming season, so would protect it from webworms (caterpillars). Again dont know the brand/insecticide. It is supposed to allow the pecans to remain safe to eat. Also see a bacteria treatment, which I assume is also a surface spray. My tree is 80 feet tall at least. I cannot possibly spray the top half. I need a treatment that goes through the trees water supply system.
Certified GKH Gardening Expert
With the size of your tree you are right, spraying the entire foliar canopy would only be possible with a large high-pressure hydraulic sprayer. Some tree services are capable of this, most are not.
Contact insecticide sprays and B.t., the bacterial biological control that you read about, would require complete coverage of the foliar canopy. Even then, fall webworms are harbored inside the web and difficult to cover and control in this manner. And at a late stage of infestation, spraying is minimally effective. It's best to start early with preventive treatment.
Systemic insecticides are applied by stem injection into the tree vascular/conductive tissue, or by soil drench for root uptake. There is still some controversy and conflicting research reports on whether the systemic chemistry can end up in the fruit or nut. In reality if it does it's probably minimal and insignifcant as a harmful toxin, but for some consumers (and pesticide regulators) any amount of a chemical pesticide in the edible nut is unacceptable. Rightfully so.
Because of these issues for controlling fall webworm, I usually recommended mechanical control, climbing or bucket-truck access for pruning out the webs with a pole pruner. For a tree the size of your pecan with a lot of webs, this can be a half day job for a climber.
The stem injection with a systemic insecticide treatment that your first arborist did, is best done professionally; there are learned techniques and specific equipment and chemical concentrations that are essential to know for safe and effective applications.
What he used for effective stem injection was probably the same chemistry as the soil drench for root uptake suggested by the other, a product with the neonicotinoid chemical active ingredient "imidacloprid", which is slow (one month or more) to take up and translocate throughout the foliar crown, but has a long (one year or more) residual effectiveness . There are other neonicotinoid systemic formulations that are faster for uptake, but have a shorter residual period. Safari (dinotefuran) is the fastest.
I know this is not a definitive or satisfactory answer, but this is a difficult situation and can be a budget buster if you have to hire professionals. And there is no good do it yourself method for a tree like that. And the chemical considerations and toxicity issues make it more complicated.
The good thing is that webworms are not an outright tree killer, but primarily unsightly. Although heavy repeated infestations can affect tree health.
As an example, not a recommendation, this is the type of product used for soil drench of imidacloprid systemic insecticide.
It is used like this:
Soil Drench: Uniformly apply the dosage in no less than 10 gallons of
water per 1000 square feet as a drench around the base of the tree,
directed to the root zone. Remove plastic or any other barrier that will
stop solution from reaching the root zone.
It would be mixed with water at: 0.2 fl oz of the concentrate (6 mL) per inch of trunk diameter (D.B.H.)