This required a large selection of trees to verify or disprove my theory, and possibly lots of space to accomodate all of these. I basically have one of each of those: Calamandrin, Limequat, lemon and 'Chinese orange'. The Chinese orange and lemon are in my garden. The Limequat is in a pot with garden soil, and the Calamandrin is potted with my potting mix, containing equal parts of washed sand and peat + fertilizer.
All but the Calamandrin (Not sure of the spelling) have succumbed to the citrus leaf-miner. In spring, the infestation caused fertilized flowers to drop. The Calamandrin had yellow leaves and did not flower, but was not infected. The Chinese orange had a milder infestation, and is setting fruit in autumn along with the Calamandrin. The others did not even flower.
My observation was that the leaf-miners avoided the calamandrin. Strange enough, but another article which I have found on http://www.sare.org/sanet-mg/archives/html-home/32-html/0482.html refers to soil nutrients affecting resistance to leaf-miners. That made me think: If the soil chemistry did affect availability of nutrients, which in turn affected resistance, was my potting mix (50% sand + 50% peat) a good solution ? Is the mixing ratio an ideal one ? I had lots of success in growing cacti and succulents, and even trees (mulberry, brachychiton, tamarind) and especially cacti seem to prefer it over pure peat.
