Helicoverpa zea
Helicoverpa zea, commonly known as the corn earworm, is a species in the family Noctuidae. The larva of the moth Helicoverpa zea is a major agricultural pest. Since it is polyphagous during the larval stage, the species has been given many different common names, including the cotton bollworm and the tomato fruitworm. It also consumes a wide variety of other crops.
The species is widely distributed across the Americas with the exception of northern Canada and Alaska. It has become resistant to many pesticides, but can be controlled with integrated pest management techniques including deep ploughing, trap crops, chemical control using mineral oil, and biological controls.
The species migrates seasonally, at night, and can be carried downwind up to 400 km. Pupae can make use of diapause to wait out adverse environmental conditions, especially at high latitudes and in drought.Distribution
The corn earworm is found in temperate and tropical regions of North America, with the exception of northern Canada and Alaska as it cannot overwinter in these areas. Helicoverpa zea found in the eastern United States also does not overwinter successfully. They live in Kansas, Ohio, Virginia, and southern New Jersey, but survival rate is mainly affected by the severity of the winter. Corn earworm moths regularly migrate from southern regions to northern regions depending on winter conditions. They are also found in Hawaii, the Caribbean islands, and most of South America, including Peru, Argentina, and Brazil.
Cotton earworms have also been reported from China in 2002.
The taxonomy of Helicoverpa was poorly understood for a long time. Many older works referring to "Heliothis obsoleta", a synonym of H. armigera, are actually about H. zea.Lifecycle and description
Eggs
s are individually deposited on leaf hairs and corn silks. The eggs are initially pale green in color, but over time they turn yellowish and then grey. Eggs are 0.5 mm in height and average about 0.55 mm in diameter. They hatch after 66 to 72 hours of development. Once larvae have breached the chorion, they spend up to 83% of eclosion making an exit hole larger than their heads. Larvae spend the rest of the time making a silk meshwork around the exit hole; this both helps them escape the shell and helps them find the shell afterwards so they can feed on it. After feeding on their shell, larvae rest about 3 minutes before they begin feeding on the plant material around them.Larvae
Following hatching, larvae feed on the reproductive structures of the plant and usually develop through four to six instars. Initially, the young larva feed together, and this stage is their most destructive stage. Through maturation, older larvae become aggressive and cannibalistic, leaving one or two larvae per feeding site. They usually have orange heads, black thorax plates, and a body color that is primarily black. Their bodies can also be brown, pink, green, and yellow with many thorny microspines. Mature larvae migrate to the soil, where they pupate for 12 to 16 days.Pupae
Larvae pupate 5 to 10 cm below the soil surface. Pupae are brown in color; they measure 5.5 mm wide and 17 to 22 mm long. The biggest environmental factor that affects the pupal developmental rate is temperature, primarily soil temperature. This is because proper insulation facilitates development, and soil temperatures below 0 degrees Celsius correlate to higher pupal mortality. Another factor that influences pupal development is soil moisture. Pupal mortality is high in wet soil, where the moisture level is between 18 and 25 percent. Dehydration can also lead to high death rates among pupae, if soil moisture is as low as 1 to 2 percent.Adults
Adults have forewings that are yellowish brown in color and have a dark spot located in the center of their body. The moths have a wingspan ranging from 32 to 45mm, and live over thirty days in optimal conditions. However, the life span ranges from five to fifteen days on average. They are nocturnal and hide in vegetation during the day. Adult moths collect nectar or other plant exudates from a large number of plants, and live for 12 to 16 days. Females can lay up to 2,500 eggs in their lifetime.Economic impact
Damage
The corn earworm is a major agricultural pest, with a large host range encompassing corn and many other crop plants. H. zea is the second-most important economic pest species in North America, next to the codling moth. The estimated annual cost of the damage is more than US$100 million, though expenditure on insecticide application has reached up to $250 million. The moth's high fecundity, ability to lay between 500 and 3,000 eggs, polyphagous larval feeding habits, high mobility during migration, and a facultative pupal diapause have led to the success of this pest.Control
Two kinds of control measures have been advocated since the 19th century. One aims at total pest population reduction, while the other is aimed at protection of the particular crop., integrated pest management, an array of techniques and approaches to control pests, was recommended. Practices such as deep ploughing, mechanical destruction, and trap crops are also used to kill different instars. Chemical control is widely successful, and includes the use of applying mineral oil inside the tip of each corn ear, which suffocates the young larvae. Pesticides are one method by which corn earworm populations are controlled; however, since they have been widely used, the insects have become resistant to many pesticides. The use of biological controls, such as the bacterium Bacillus thuringiensis and various forms of nematodes, is also common, although not without their own problems. Corn earworm moths are not always vulnerable to the bacterium, and they are only afflicted by nematodes once the larvae have pupated and dropped to the ground. Strains of maize have been genetically modified to produce the same toxin as the bacterium, and are referred to as Bt-corn.Survival
Natural enemies
More than 100 insect species prey on H. zea, usually feeding on eggs and larvae. The insidious flower bug, a pirate bug, feeds on the eggs of H. zea, thus acting as a biological control agent. Some plants emit a blend of chemicals in response to damage from H. zea, which attract parasitic insects. Cardiochiles nigriceps, a solitary endoparasitoid wasp, makes use of these volatile plant compounds to identify the presence of H. zea. When the wasps find damaged host plants, they hover around and then search for the host with their antennae. When the females find their prey, they use their antennae to position themselves and deposit eggs into the host. The braconid wasp Microplitis croceipes, which deposits its eggs inside a living caterpillar, is also an important parasitoid of both H. zea and the related species Heliothis virescens. When larval densities are high, a fungal pathogen, Nomuraea rileyi, can cause an outbreak of disease. However, pupal mortality is high not because of predators, but because of harsh weather conditions, collapsing pupal chambers, and disease.Larval predation
As the larvae mature, they become increasingly aggressive. Although they have host plants surrounding them, H. zea larvae attack and eat other insects. When presented with a second-instar larva of Urbanus proteus, the corn earworm larva grasps the insect, rolls onto its side to form a semicircle, and begins feeding on the insect's posterior end. If the U. proteus begins to bite out of defense, H. zea rotates the larva 180° and uses its mandibles to puncture the head capsule, killing the insect. Then, the H. zea larva rotates the U. proteus back to its original position and continues feeding until the insect is entirely consumed. Even when presented with up to five U. proteus larvae, H. zea engages in the unique behavior, as the larvae have a higher affinity for lepidopterous prey over plant material. H. zea raised in a low-moisture environment has a lower pupal weight and a longer developmental time than those raised in environments of high moisture, so a nutritional benefit exists to such aggressive feeding behavior under such conditions.Movement
Migration
Helicoverpa zea is a seasonal, nocturnal migrant, and adults disperse, weather permitting, when there are poor reproductive conditions. In short-range dispersal, the moths move within the crop and low over the foliage. This type of dispersal is mostly independent of wind currents. Long-range dispersal involves adults flying up to 10 meters above the ground and moving downwind from crop to crop. Migratory flights occur up to 1–2 km above the ground and can last for hours. Migration of 400 km is common for such flights as moths are carried downwind. Helicoverpa zea caterpillars are usually intercepted on produce transported by air-freight transportation. Most activity is restricted to the night-time. Some moths display vertical take-off flight, which carries them above the flight boundary layer and allows them to undertake migratory movement in upper wind systems. During mating, males engage in high-speed directed flight in search of pheromone plumes.Diapause
have the ability to enter facultative diapause, the state of arrested development and growth in response to a change in the environment. By preparing themselves for a major change in environmental conditions, they can increase reproductive success. Diapause increases with increasing latitude. In tropical conditions, populations breed continuously, and only 2-4% of pupae diapause. In subtropical and temperate regions, most individuals diapause. Individuals who don't enter diapause in these areas emerge in late fall and die without reproducing. Drought-responsive diapause has also been observed in the summer.