Arthropod Pest and Natural Enemy Interactions: Can Pests Become Resistant to Natural Enemies?
Biological control is a well-known pest management strategy available to greenhouse managers to “manage” or “regulate” arthropod (insect and mite) pests. As opposed to discussing the basic concepts of biological control and why it is important to implement some type of biological control, this article will focus on the arthropod pest-natural enemy interactions and characteristics of arthropod pests.
It is often stated in both scientific and trade journal publications that arthropod pests [1] cannot develop resistance to natural enemies. Well, this is not entirely true, as arthropod pests don’t just “sit” and wait to be attacked by a natural enemy such as a parasitoid or predator [2]. Arthropod pests have evolved mechanisms to resist or prohibit natural enemies from attacking them. In fact, arthropod pests have the ability to utilize a variety of defensive mechanisms to protect themselves from attack by either parasitoids or predators. This is why certain natural enemies only attack a specific life stage of a given arthropod pest, will not attack a particular arthropod pest species, and why some arthropod pests are more difficult to control with natural enemies. Despite this, some natural enemies themselves have evolved ways to avoid or circumvent these defenses, which is why biological control is still an effective pest management option.
This article will address the defense mechanisms of arthropod pests, which will provide insight to those interested in the complexities of biological control and explain why a particular natural enemy is sometimes not effective although all procedures were conducted properly to ensure success. This may also explain why some natural enemies are more effective than others. Furthermore, this is why it is important to release natural enemies early or preventatively so they can easily locate arthropod pests before arthropod pests develop their own defense mechanisms. The various defensive mechanisms to be discussed include behavioral and morphological defenses, physiological defenses, and physical defenses.
Behavioral and Morphological Defenses
These types of defense mechanisms include “clumping,” hard integument or covering, webbing, and seclusion or hiding. Mealybugs and scales tend to congregate (clump) in tightly clustered groups on plant parts, which may result in individuals stacking on top of one another. Eggs or individuals on the exterior or near the exterior suffer higher attack rates from parasitoids or predators than those in the interior. This allows eggs and individuals in the interior to escape attack and continue development. In addition, predators may only eat mealybugs or scales located on the exterior, and forage for or consume individuals underneath because by the time they reach the interior they may already be satiated.
Some insects, including hard scales, possess a tough waxy surface that makes it difficult for parasitoids to insert eggs because the ovipositor (egg-laying device) is unable to penetrate the scale outer covering or integument. A thick scale covering increases the handling time or time it takes for the parasitoid or predator to kill a particular scale pest. This is especially the case with older life stages. Mature mealybugs also produce a waxy covering that may make it difficult for parasitoids to lay eggs.
Webbing produced by the two-spotted spider mite, Tetranychus urticae serves to protect populations from predators, especially predatory mites. Predators get “tangled-up” in the webbing and expend energy trying to escape. After which, they spend time cleaning or grooming themselves instead of attacking spider mites. Both parasitoids and predators are very clean insects and can spend exorbitant amounts of time, sometimes hours, cleaning themselves — time that is better spent looking for arthropod pests.
Hiding from natural enemies is a very effective way to escape attack. Western flower thrips (Frankliniella occidentalis) adults, and in some cases immatures, prefer tight enclosed areas such as buds and flowers. Hiding in these plant parts provides protection from natural enemies. Although predators such as minute pirate [insidious flower] bugs, Orius spp., do feed on thrips in flowers, they are generally only effective when flowers have pollen. Searching in the flower buds may be difficult for some predators depending on their size. In addition, this may require energy that must be replenished through pollen feeding. As a result, predators spend more time feeding on pollen instead of searching for thrips. Fungus gnat (Bradysia spp.) larvae may escape attack from predatory mites (e.g. Hypoaspis miles) or entomopathogenic (=beneficial) nematodes (e.g. Steinernema feltiae) by tunneling into rooted or unrooted cuttings. This is a protective environment, which reduces the probability of being attacked by any natural enemies.
Physiological Defenses
Physiological defensive mechanisms include encapsulation and the production of fluids or secretions. Some scale insects protect themselves by encapsulating or entombing parasitoid eggs. This involves the scale immune system forming a capsule around a parasitoid egg or larvae, which is inside their bodies, using blood cells (hemocytes [3]) and the pigment melanin [coloring]. The parasitoid egg or larvae dies from suffocation or fails to develop any further. For example, the brown soft scale, Coccus hesperidum encapsulates the eggs of the parasitoid, Metaphycus helvolus, which reduces its effectiveness in controlling this scale species.
Several insects produce defensive secretions or fluids that may irritate, repel, or are toxic to natural enemies. For example, thrips and mealybugs produce fluids that protect them from being attacked by parasitoids or predators. Western flower thrips adults produce an unpleasant, moist, anal exudate that covers the predatory mites Amblyseius cucumeris [Neoseiulus cucumeris] or A. degenerans [Iphiseius degenerans]. The exudate causes the predatory mites to spend time cleaning themselves, rather than locating and feeding on thrips. Many mealybug species possess two pairs of ostioles [3] or small openings, which produce fluids that “soil” predator mouthparts such as green lacewing larvae, and interfere with their normal behavior. These secretions may also cause parasitoids to halt foraging and instead spend time cleaning themselves. This results in an increase in the time required for the natural enemy to kill a particular arthropod pest. In fact, the greater the quantities of fluids they are covered with the longer natural enemies spend cleaning themselves.
Continutation of Can Pests Become Resistant to Natural Enemies…
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Mike Cherim responds:
Posted: April 2nd, 2009 at 7:00 am →
We’ve always held onto the belief that the only way pests could really develop a resistance to natural enemies was to evolve over time. What we didn’t take into consideration is that pests have been evolving over time all along and have always been at war with their enemies. Everything has enemies, and anything that does has defenses to its enemies. This includes plants. One of the best things a grower can do is to bolster their plants’ defenses against their natural enemies — namely plant pests — by keeping them healthy and growing vigorously. Thank you very much, Ray. This article is very insightful.