The codling 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". 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. This moth has a widespread distribution, being found on six continents. Adaptive behavior such as diapause and multiple generations per breeding season have allowed this moth to persist even during years of bad climatic conditions.
Codling moths are not large, as the full grown adult codling moth has an average length of 10 mm and wingspan of 20 mm. The wings fold into a tent-like shape when the moths are resting. They are distinguished from other similar moths in the family Tortricidae by the distinctive patterns on their fore-wings. These brown spots enclosed in gold rings are called "little mirrors" because they resemble small mirrors with a golden rim. The slender antennae are mildly curved near the distal end. The dorsal side of the abdomen is smooth and bare, while the ventral side is covered in scales. Though most of the moths are brown or gray in color, it has been observed that the maturity of the fruits the larvae feed on can lead to variation in color in the adult moth.
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.
Viability and fitness of the codling moths are highly dependent on humidity levels and climate. Under observation, the optimal conditions for moth growth and survival were 32 °C and 75% humidity. Even if the temperature is favorable, low and high levels of relative humidity (20% and 100% respectively) led to hindrance in pupation. At a temperature below 0 °C, the larvae become completely inactive and turn seemingly lifeless. However, researchers observed that if the temperature is returned to optimal levels, the larvae regained normal activity. Codling moths have been located at altitudes as high as 1000–1500 m.
Because the codling moth is polyphagous, or able to utilize a variety of food sources, the availability of specific food resources does not determine their optimal habitat. Various stages of the moth's life history, from eggs to pupae, can be found on host plants which the larvae feed on. These plants include apple, pear, walnut, chestnut, and even apricot trees.
The codling moth caterpillars bore into a fruit within 24 hours of hatching from their eggs, usually traveling between 1.5 m to 3 m in search of a fruit. Because they are susceptible to predation, drying up, or being washed away between the period of hatching and boring into a fruit, the caterpillars are prompt in finding a fruit to feed on. Although apples are their dominant food source, they are polyphagous, feeding on a wide variety of fruits from pear, walnut, apricot, peaches, plums, cherries, and chestnuts. They are unable to survive by feeding on leaves of the fruit trees.
It was previously believed that the searching behavior of the caterpillar for fruit to feed on or for a pupation site was random. However, the caterpillar is exposed and susceptible to predation, parasitization, drying up, and even energy depletion during this searching period. Thus, it was hypothesized that the searching behavior uses thigmotactic sense, which means the caterpillars use contact reflex to search. Caterpillars also use phototaxis to locate fruits to feed on. They are photopositive, which means they move towards light. This is adaptive because fruits tend to be located at the ends of the branches where there is most sunlight. Therefore, by following light, the larvae are able to move closer to fruits.
Once the caterpillar has located a fruit to feed on, it starts penetrating the epidermis of the fruit. As the caterpillar makes way into the fruit, scraps of the skin, pulp, and frass build up near the entrance of the hole. These pieces are glued together by silk threads released from the caterpillar to create a cap. This cap protects the caterpillar by blocking the entrance. It takes the caterpillar approximately 45 minutes to bore into the fruit and about 15 minutes to cap. The caterpillar bores through the fruit until it reaches the seminal chamber of the fruit. There, the caterpillar bites into the seeds and halts the growth of the fruit. The fruit ripens prematurely as a result. By doing so, the caterpillar gains beneficial resources, such as albumin and fat. Such feeding behavior lasts for 23 to 27 days and the caterpillar feeds on an average of one to two fruits during this time.
Males fly upward near the top of the trees to search for females because females tend to stay near trees that they eclosed from. 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.
