Greetings Ben,
First, I am certainly not an expert in nut orchard management. This is more of a horticulture-oriented question than a traditional forest management question. Regardless, I will give you a brief summary of what I found:
To maximize nut production, fertilization may be warranted. Because I did my graduate research on forest fertilization, I must admit that I do not agree with recommendations that only suggest nitrogen [N] fertilization. Additionally, I am not an advocate of providing blanket fertilization recommendations without first knowing the site’s fertility baseline through soil and foliar analysis. Your pecan’s nut production issue may in fact be non-fertility related. Thus, an onsite evaluation may be necessary (call your local UI Extension office if needed).
According to Oklahoma Cooperative Extension Service, the following are rule-of-thumb guidelines for fertilizing pecan trees in situation where leaf analysis results are not available: individual trees may be fertilized at the rate of one pound of mix fertilizer per year of age or inch of trunk diameter. Trees 15-25 inches in diameter may require two pounds of mixed fertilizer per inch diameter. The rate can be increased to three pounds per inch diameter on trees larger than 25 inches in diameter. “Mixed fertilizer” in the Oklahoma recommendation above refers to N-P-K in the ratio of 10-10-10. Zinc supplementation may also be needed. http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-1010/F-6232web.pdf
If you do not like the idea of fertilizing every year and you want to enhance the beneficial insect population within your grove, you may want to think about establishing a nitrogen-fixing legume cover crop. Alfalfa will work on drier sites, where as mixed plantings of white & red clover will work better on wetter sites.
Regarding an insecticide recommendation, I cannot make blanket pesticide recommendations without knowing the severity and specific insect that is causing the damage. Sounds like pecan weevil, but I am not sure because I do not have tangible evidence. According to the sustainable pecan production website http://attra.ncat.org/attra-pub/pecan.html the only viable control for pecan weevil at the present time appears to be one or two accurately timed insecticide applications (e.g., Sevin®, Imidan®, Ammo®, Asana®, Fury®). However, someone with an Illinois Pesticide Applicators License in fruit corps can provide you with a precise recommendation http://www.kellysolutions.com/IL/Applicators/index.asp or provide me with a positive idea and we can have one of our entomologists provide you with a firm recommendation. As a reminder, pesticides should only be used as part of an Integrated Pest Management system.
P.S. This website has a wealth of information on pecan and pecan orchard management http://attra.ncat.org/attra-pub/pecan.html
Good luck!
