Itís common to hear that healthy plants have fewer insects. Is that really true? The answer is a definite yes and no. It is generally true that healthy plants are in better shape to handle insect attack, but healthy plants can actually be more attractive to some insects.
Borers are attracted to unhealthy trees and shrubs. Trees in decline are known to produce certain volatile chemicals (odors) that attract bark beetles and other borers to them. Borers are generally unable to survive in healthy trees. Beetle borers commonly chew niches through the bark to lay their eggs. Moth borers, unable to chew a hole through the bark with their siphoning mouthparts, lay their eggs in wounds. Healthy trees have a high sap flow that frequently washes the borer eggs out of natural wounds and those chewed by borers. Predators eat these eggs, or the hatching larva are not be able to eat through the bark. Larvae that enter healthy trees are likely to drown in the heavy sap flow or be crushed by the high internal pressures caused by healthy sap flow. Recent research has shown that the Asian longhorned beetle, which attacks seemingly healthy maples and other trees, actually attacks trees that are under stress, as measured by instrumentation. In this case, trees that appear healthy are really not as healthy as they seem and are thus vulnerable to borer attack.
Properly fertilized plants tend to be healthier plants, and those that are fertilized tend to have higher nitrogen content. Many insects seek high-nitrogen sources. Insects tend to contain 8 to 14% nitrogen, but plants typically contain only 2 to 4% nitrogen. Compared to plant tissue, the liquids in plants are even lower in nitrogen. The phloem sap contains only one-half of 1% nitrogen or less, but xylem fluid contains only one-tenth of 1% nitrogen or less. Aphids and other sap-feeding insects are known to build up to higher populations more quickly on nitrogen-fertilized plants. However, nitrogen is an important component of tannins and alkaloids, chemicals that reduce insect digestion. Thus, many sap-feeding insects are more likely to attack healthy plants, but other insects may be more likely to attack unhealthy plants that are too weak to produce chemicals to defend themselves.
Healthy plants tend to have higher water content. Caterpillars need high amounts of water and develop 40% slower on plants with a low water content, even levels that are not low enough to cause wilting. On the other hand, desert grasshoppers are attracted to drought-stressed plants. Sugars and nitrogen are more concentrated in water-stressed plants, allowing insects to obtain more useable food in a shorter time. Water-stressed plants produce fewer chemicals that deter insect feeding.
Younger leaves tend to contain more water and nitrogen than older leaves, making them more vulnerable to some insects. Tree leaves tend to be much lower in water and nitrogen than low-growing perennial plants. The water and nitrogen content of leaves decreases later in the growing season.
In summary, borers are attracted to unhealthy plants, sap-feeding insects tend to be attracted to healthy plants, caterpillars prefer well-watered plants, and the rest of the insects are variable in preference. Whether a plant is more or less attractive to insects when healthy, a healthy plant is more likely to survive an insect attack than an unhealthy plant.