The diamondback moth is a significant pest that affects crops like cabbage and broccoli worldwide. Its larval stage causes extensive crop damage, leading to yield loss and significant economic damage. This guide covers everything you need to know about identifying and managing diamondback moths, from understanding their life cycle and recognizing signs of infestation to exploring effective control methods, including biological and cultural practices.
Overview:
- What is the diamondback moth?
- What is the impact of diamondback moths?
- How do I know if I have a diamondback moth problem?
- How do I get rid of diamondback moths?
- Summary
What is the diamondback moth?
The diamondback moth (Plutella xylostella), sometimes called the cabbage moth, is a pest that damages a variety of host plants during its larval (caterpillar) stage. It has a wide global distribution and is present on all continents, including Australia. Its exact origin is not clear, but it is now considered to be one of, if not the most widely distributed and destructive Lepidopterans, a grouping that includes all moths and butterflies. Its host plants include cabbage, cauliflower, kale, and broccoli, which belong to the Brassicaceae family and are sometimes referred to as cole crops. The direct feeding of diamondback moth caterpillars damages these plants.
Diamondback moth eggs are oval-shaped and appear in yellow or pale-green colours. They measure roughly 0.45 mm in length and 0.25 mm in width and are laid either as individual eggs or in small clusters of up to eight eggs. The pupae (cocoons) of this species are loosely spun compared to other species, appear in many different colours and grow up to 9 mm in length.
Diamondback moth larvae are tapered at both ends and grow up to 2.5 cm long. The caterpillars appear in different shades of green except in the earliest developmental stage when they are colourless.
Adult moths are slender and grey-brown in colour and usually measure 6 mm in length, with a wingspan of 15 mm. They have a wide characteristic light-brown stripe running along their back and have large antennae. When the moth is at rest, this stripe sometimes appears as smaller diamond shapes, which is where this species gets its name. The hind wings are light grey and pointed, and their forewings are light brown with black spots and typically curve upwards at the edges. This species of moth is not capable of extended flights and typically never rises higher than 2m from the ground. However, the wind can carry it long distances.
Life cycle
Adult females lay eggs on the leaves of host plants and favour laying eggs on plants with an established larval infestation. Both male and female adults live for about two weeks, and egg-laying takes place for about ten days during these weeks. In warmer climates, adult moths can overwinter but die off in colder climates. Eggs hatch after 5-6 days, and larvae progress through four developmental stages, also called instars, during which time they feed on the leaves of host plants. The pupal phase occurs on the lower and outer leaves of host plants, and adult diamondback moth adults emerge after roughly 5-15 days, depending on the climate.
What is the impact of diamondback moths?
Diamondback moth larvae cause damage to plants by directly feeding on plant tissue and have different eating habits depending on the stage of development. For instance, the first instar eats in a way known as leaf mining. After they complete this stage, they begin eating the underside of leaves, leading to a characteristic window-panning pattern. Mature larvae eat directly through leaves. The feeding activity of these larvae can cause significant damage to crops, especially when they feed on the growing parts of the plants. Larvae can also burrow into broccoli and cauliflower heads, which makes the crops less marketable. The damage of diamondback moths can lead to significantly stunted plant growth and reduced yields, as well as contamination, which reduces the economic value of crops. This pest has recently become a problem for canola crops in North America.
How do I know if I have a diamondback moth problem?
Early signs of infestation can include leaf mines created by young larvae, though these are hard to notice. As the larvae grow and consume more plant tissue, the damage and extent of the infestation will become more obvious. A large abundance of adult moths in the area may also indicate an infestation and may warrant a closer examination of crops in the following weeks.
How do I get rid of diamondback moths?
Despite their wide geographical distribution and the significant damage they cause to crops, several effective methods exist for dealing with diamondback moths, including biological solutions that avoid the use of harmful chemical pesticides.
Monitoring
Itās important to consistently monitor the number of larvae on plants or to keep track of the extent of the damage, such as the number of holes. Some farmers set thresholds for treatment, such as finding one hole per plant over a predefined number of plants before applying a treatment. Monitoring of adult moths is also possible using traps. Monitoring can help determine if treatment is necessary, and itās important to remember that levels of diamondback moth may be suppressed by control strategies used against other pests.
Cultural control
Crop rotation can be an effective method for reducing diamondback numbers. This involves planting different crops in a given field over different growing seasons to disrupt pest life cycles and reduce the growth of overwintered pests. Overhead irrigation can help to eliminate smaller larvae, and farmers should thoroughly examine transplants before planting.
Biological control
Biological control involves using naturally occurring substances and organisms to tackle pest numbers and is a more environmentally friendly method for controlling diamondback moth populations.
Natural substances
Natural substances are compounds taken from natural sources that reduce pest numbers through different modes of action. Azadirachtin is a chemical that derives from the seeds of the neem plant and works by repelling pests and reducing their feeding.
Microbials
These are microorganisms, including bacteria, fungi, and viruses, that kill pests in various ways. For instance, the bacteria Bacillus thuringiensis is effective against the larval stage of many pests, including diamondback moths. When consumed, this bacteria releases toxins in the larvae’s intestines that kills them. Isaria fumosorosea is a fungal species that kills diamondback moth larvae and lowers the number of eggs laid by adults. It does this by breaking down the cuticle (outer layer) of the larvae and causing an internal infection.
Macrobials
These are larger organisms that can kill pests by feeding on them or through parasitism. Parasitic wasps of the species Diadegma insulareĀ are important natural enemies of the diamondback moth and lay their eggs inside diamondback moth larvae. As the wasp eggs hatch, the developing wasp larvae feed on the internal tissues of the diamondback larvae, ultimately killing them. Species of the genus Trichogramma are also parasitic wasps but lay their eggs inside the eggs of diamondback moths.
Chemical Pesticides
Chemical solutions can be used to control diamondback moth numbers. However, broad insecticides can actually increase diamondback moth numbers by lowering the number of natural predators such as Diadegma insulare.
As a world leader in nature-based pest management knowledge implementation, CABI encourages Integrated Pest Management (IPM) as the preferred, ecologically-based approach to producing healthy crops. It allows for chemical pesticide use only as needed, and when adhering to measures that limit the exposure of people and the environment to them (see FAO, International Code of Conduct on Pesticide Management).
Before considering the use of chemical pesticides, farmers should explore all available non-chemical control solutions. These might include cultural practices, such as the ones mentioned earlier, and consulting the CABI BioProtection Portal for identifying and applying suitable biological control products.
In case chemical pesticides are considered for use, farmers should look to select lower-risk chemical pesticides which, when used as part of an IPM strategy, help manage pest problems while minimizing harmful effects on human health and the environment. Agricultural advisory service providers can provide information on lower-risk chemical pesticides that are locally available and are compatible within an IPM strategy. These experts can also advise on the required personal protective equipment.
Summary
The diamondback moth is a persistent pest with a widespread presence and a damaging impact on crops. Its larvae cause significant damage to plant tissues, reducing crop yields and quality. Early detection and monitoring are crucial, and farmers have various management options, including crop rotation and biological controls. Cooperative extension between federal, state, and local governments are essential for managing the impact of diamondback moths. Effective management of this pest can help protect valuable crops, reduce economic losses, and promote sustainable agricultural practices.
To discover more ways to tackle the diamondback moth, consider visiting the CABI BioProtection Portal. To learn more about bioprotection, check out this beginnerās guide to biological pest control.
