The codling moth or (especially Australia) codlin moth (Cydia pomonella) is a member of the Lepidopteran family Tortricidae. They are major pests to agricultural crops, mainly fruits such as apples and pears, and a codling moth larva is often called an "apple worm". Along with the apple maggot, it is the worm that people encounter when biting into an infested apple.

Because the larvae are not able to feed on leaves, they are highly dependent on fruits as a food source and thus have a significant impact on crops. The caterpillars bore into fruit and stop it from growing, which leads to premature ripening. Various means of control, including chemical, biological, and preventive, have been implemented.

thumb|Cydia pomonella illustration by [[Des Helmore]]

Geographic distribution

Although the geographic origin of codling moths is unclear, there are theories of these moths originating from either Europe or the Mediterranean. Scholars believe that the codling moths were introduced to the Americas in the mid-1700s. There is still debate on whether or not these moths have been distributed by humans. Today, the codling moths are spread all over the world, ranging from Europe, Asia, Africa, North and South America, Australia, and islands in the Pacific. Although apples are their dominant food source, they are polyphagous, feeding on a wide variety of fruits from pear, walnut, quince, apricot, peaches, plums, cherries, and chestnuts. They are unable to survive by feeding on leaves of the fruit trees.

Adult

Adult moths feed little if at all. Though feeding may prolong life a little longer, abstaining from feeding does not significantly reduce their reproductive success. Their ability to copulate and oviposit are unaffected, and embryonic development in eggs is not affected by lack of feeding behavior of the parent moths. If they choose to feed, the moths feed on sweet fluids, such as juice from the fruit, diluted honey, and diluted molasses. Codlemones, or (E,E)-8,10-dodecadien-1-ol, is a major male-attracting sex pheromone secreted by females. Plant volatiles create a synergistic effect with the codlemone, which increases the degree of male attraction. These volatiles include racemic linalool, (E)-β-farnesene, or (Z)-3-hexen-1-ol. The optimal ratio at which attraction is maximized is at a 1:100 ratio of codlemone to synergistic plant volatiles.

Pear-derived kairomone has been found to be a species-specific attractant as well. Ethyl (2E, 4Z)-2,4-decadienoate, which is found in ripe pears and is a minor volatile secreted from ripe apples, attracts both mated and virgin males and females.

Codling moths can copulate as early as the day of their eclosion, as long as the climate is appropriate. Males have developed hooks near the end of the abdomen, which are used to grasp onto the female during copulation. Because these hooks hold onto the female tightly, copulation that lasts up to several hours has been observed. Copulation occurs within 24 hours of eclosion. Males have two brown spots near the end of the dorsal side, which become the gonads. However, the majority of the moths travel between 60 and 800m. Such wide range of flight behavior is an adaptive characteristic; their habitat is usually determined by the availability of the fruit, so there is normally no need to travel far, but if the fruits are spread out in a patchy manner, the moths are capable of traveling longer distances to find food and re-colonize.

High genetic correlation (0.84 to 1.00) was found between total distance traveled and flight traits, such as flight duration and velocity. Flight characteristics are shown to be heritable for both sexes. Selection for and against genes that increase flight traits showed rapid changes in flight traits in both directions.

Ants are among the most significant of the insect predators because they are numerous and are active in masses. They attack all the stages of the codling moth life cycle, including the caterpillar, pupae, cocoons, and eclosing moths. Some of the commonly known ant predators include Solenopsis molesta, Lasius niger, Formica fusca, Formica pallidefulva schauffussi inserta, Aphaenogaster fulva aquia, Tetramorium caespitum, and Monomorium minimum. Solenopsis molesta can kill 90% of caterpillars they attack, which are usually those moving between fruits or fifth instar caterpillars looking for a pupation site.

  • Lissonota culiciformis – an ichneumonid wasp
  • Mastrus ridens – another ichneumonid
  • Pimpla examinator (caterpillar and pupa) – another ichneumonid
  • Pristomerus vulnerator (caterpillar) – another ichneumonid that attacks both generations
  • Trichomma enecator (caterpillar) – another ichneumonid
  • Ascogaster nov. sp. (egg or pupa) – another ichneumonid. A parasitised pupa may die or eclose as a stunted adult. Perilampus tristis, a chalcidoid wasp, is a hyperparasite of this Ascogaster species.

Pathogens

Fungi

Beauveria bassiana is a parasite to the caterpillar and pupae of the codling moth. The spores of this snowy white fungus are dispersed in locations where the caterpillars pass by or where they pupate. The mycelium of B. bassiana grows radially out from the body of the caterpillar, turning the caterpillar soft and mushy. It has a killing rate of 13.1% in caterpillars. B. bassiana grows favorably in humid and warm environments.

Hirsutella subulata is another entomophagous, or insect-eating, fungal parasite to the codling moth larvae. Unlike B. bassiana, this fungus type can grow even if the humidity is low.

Three main Cydia pomonella granulovirus isolates have been identified: Cydia pomonella granulovirus-M, E, R. These can be categorized into four genome types: genomes A, B, C, and D. It is believed that genome C is ancestral to the other genomes. Genome C is also less pathogenic to codling moth neonates compared to other genome types. Isolates from Iran have also been identified and were found to have same genome types as the other isolates.

As bioinsecticide

Cydia pomonella granulovirus-M, which is a Mexican isolate strain of Cydia pomonella granulovirus, has been used as a bioinsecticide. Codling moth larvae can be infected with Cydia pomonella granulovirus just by crawling or browsing on infected leaves and fruits. A linear log time/mortality relationship has been found, which indicates that the longer the larva remains in contact with the bioinsecticide sprayed surface, the higher the degree of infection. Aqueous suspension of the granules are sprayed on leaves and fruits, which are then taken up by the larvae. Cydia pomonella granulovirus works most effectively when taken up by neonate larvae, so the bioinsecticide should be applied in concordance to the hatching time schedule of the eggs. Once the granules reach the midgut, which is basic (pH>7), the granules are dissolved and invade the tracheal matrix, epidermis, fat body, and other tissue bodies. During the later stages of infection, the virus form clusters, which causes apoptosis, or programmed cell-death, of host cells and eventually host death. Death of the host occurs within 5–10 days.

Some colonies of codling moths have developed resistance against the commonly used Cydia pomonella granulovirus-M isolate. However, some Cydia pomonella granulovirus-M isolates have overcome that host resistance. Identified isolates, such as 112 and NPP-R1, have shown increased pathogenicity against RGV isolates, which are Cydia pomonella granulovirus resistant colonies of codling moths used in laboratories.

Olfaction sexual dimorphism

α-farnesene is a major volatile released by ripe apples that attracts codling moths. Sexual dimorphism in olfaction has been discovered with regards to this volatile. Both mated and virgin females were attracted to α-farnesene at a low dosage but repelled by it at a high dosage. The degree of response was stronger for mated females. Compared to that, mated males were attracted to a higher dosage of α-farnesene, while mid to low dosage elicited neutral responses. The threshold for mated male attraction was higher, which means the males are not as sensitive to the volatile as the females are. Butyl hexanoate is another sex-specific volatile released by ripe apples. Mated females were attracted by this volatile while males were not affected.

Pest control

thumb|Codling moth caterpillar on an apple

Because the caterpillar of the codling moth bore into fruits and stop their growth, codling moths are major agricultural pests. They are widespread and attack a wide range of fruits. In order to control these pests, insecticide has been used extensively. Another case study at Codling Moth Areawide Management (CAMP) project sites in Washington, California, and Oregon showed that the number of hectares of farms treated with mating disruption insecticides to control codling moths has grown exponentially from 1990 to 2000.

Preventive measures

Regular pruning of fruit trees allows insecticide to reach the inner part of the tree's crown more effectively, and exposes it to sunlight, sunlight being toxic to eggs and larvae. Scraping bark from the tree trunk decreases the number of sites available for pupation. Since caterpillars bore into the fruit, causing it to ripen and fall prematurely, fallen fruit should be removed to eliminate those that remain inside.

Particle films

Particle films are hydrophobic solutions used to spray crops and plants to prevent damage from pathogens and arthropod pests. A common type of particle film is composed primarily of kaolin clay and adjuvants. Particle films slow down the activity of larvae and moths. On trees coated with particle films, the larvae display decreased walking speed, fruit scavenging activity, and fruit penetration, as well as a lower rate of oviposition by moths.

Molasses traps

Molasses traps (inverted bottles containing dilute molasses) are a simple, low-cost, and effective mechanical control of the adult moths. The molasses scent attracts the moths inside the bottles where surface adhesion incapacitates them. The traps are placed at the beginning of the season when the fruit is smaller and the larvae are emerging from hibernation.

Chemical control

thumb|left |Pheromone traps for codling moth |250x250px

Synthetic attractants

Successful development of synthetic fruit volatiles has led to increased control of codling moths. Codling moths can be managed and controlled with the use of synthetic apple volatiles, such as (Z)-3-hexen-1-ol, (Z)3-hexenyl benzoate, (Z)3-hexenyl hexanoate, (±)-linalool and (E,E)-α-Farnesene, and other synthetic attractants, such as pear ester ethyl (E,Z)-2,4-decadienoate and its corresponding aldehyde, E,E-2, 4-decadienal.

Growth inhibitors and regulators

Insect growth inhibitors (IGIs) and insect growth regulators (IGRs) are used in insects, especially Lepidoptera, to prevent the synthesis of chitin during development. Chitin is one of the major component which constitutes the exoskeleton of arthropods and cell walls of fungi. Without chitin, insects cannot develop properly. Ovicidal IGIs, such as diflubenzuron, hexaflumuron and teflubenzuron, have been shown to be effective against egg development. Fenoxycarb is an ovicidal IGR, while Tebufenozide is a larvicidal IGR. Flufenoxuron and Methoxyfenozide are an IGI and IGR respectively and are equally effective in preventing growth in eggs as in larvae. In addition to IGRs, reduced-risk pesticides are also used to control codling moth population in apple orchards.thumb|Trap hanging in tree |250x250px

Insecticide resistance

In recent years, codling moths with insecticide-resistant strains arose. They have become resistant to avermectins, benzoylureas, benzoylhydrazines, neonicotinoids, organophosphates, macrocyclic lactones, pyrethroids. It is believed that this resistance is due to increase in enzymatic activity of the larvae. Low genetic structuration and high rate of gene flow have led to a rapid spread of resistance in European populations.

However, there is a fitness cost associated with increased insecticide resistance. Pesticide-resistant codling moths are less fecund, less fertile, slower in development, lighter in weight, and have a shorter life span compared to non-resistant moths. This is believed to be caused by increase in the metabolically costly activities of oxidase and glutathione-S-transferase. Oxidase is an enzyme that catalyzes oxidation-reduction reactions. Glutathione-S-transferase is an enzyme that catalyzes a detoxification process involving a conjugation of glutathione into xenobiotic substrates.

Biological control

The codling moth is not a great candidate for biological pest control, as the larvae are well protected within the fruit for the majority of development. However, their eggs are susceptible to biological control by Trichogramma wasps. The wasps deposit their eggs into codling moth eggs, and the developing wasp larvae consume the moth embryo inside.

Another candidate for a biological control agent is the parasitoid wasp Mastrus ridens, also known as the Mastrus ridibundus. This wasp has exhibited a positive response to higher densities of codling moth larvae, a short generation time compared to other parasites of the codling moth, and a high number of female offspring per host larva. These 3 characteristics improve M. ridibundus<nowiki/>'s ability to control codling moth populations. Parasitism of overwintering codling moth cocoons has reached up to 70%, but most field tests have not demonstrated a dramatic result. M. ridibundus as a biological control is recommended as part of a broader management strategy.

References

  • Codling moth on UKmoths
  • Codling Moth Information Support System (CMISS)
  • UC IPM Pest Management Guidelines – Apple Codling Moth
  • Lepiforum.de