Introduction
The greenhouse millipede, Oxidus gracilis, is a common nuisance pest found throughout the United States. While millipedes generally play a beneficial role in ecosystems by breaking down decaying organic matter, the greenhouse millipede's propensity to invade structures and its defensive odor make it a concern for homeowners and greenhouse operators. Understanding the biology, ecology, and control methods for this species is crucial for managing infestations.
Description and Identification
The greenhouse millipede is a member of the class Diplopoda. Millipedes are characterized by having body segments that each constitute two combined segments (diplosegments). Each segment bears two pairs of legs that are used for walking and climbing.
Key characteristics of the greenhouse millipede include:
- Size: Adults typically range from three-quarters of an inch to 1 inch (18 to 24 mm) in length.
- Color: They are typically black to dark brown with cream-colored or white legs and paranota (side extensions of the segments).
- Eyes: Greenhouse millipedes have no eyes and rely on their antennae to navigate.
- Body Shape: They have a more flattened appearance, and the body segments have a rougher texture. The legs are longer in proportion to the width of the body compared to larger millipedes.
Other common names for Oxidus gracilis include garden millipede and hothouse millipede. Species within the family Paradoxosomatidae are commonly referred to as dragon millipedes because of their vibrant colors and interesting external structures. By comparison, this nonnative species is drab in appearance.
It's important to distinguish millipedes from centipedes. Centipedes have only one pair of legs per body segment, and their legs are usually longer. When disturbed, millipedes often curl up into a "C" shape and remain motionless, while centipedes are more likely to run quickly.
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Life Cycle and Ecology
The greenhouse millipede is believed to have been introduced to the Americas and Europe from Asia through gardening soil. They thrive in tropical and temperate climates and are now widespread across the United States. They are often found invading garages and other open structures, as well as on paved surfaces. True to their name, they can also infest greenhouses.
Millipedes are detritivores, meaning they feed on decaying organic matter such as leaf litter and other debris. They prefer moist environments like flower beds and leaf piles. They also scrape calcium carbonate from sidewalks and rocks. When moisture, debris, and mineral-rich surfaces are readily available, their populations can grow rapidly.
Like many millipedes, the greenhouse millipede releases foul-smelling toxins when threatened, deterring most predators. While poisonous to predators if consumed, they are harmless to humans.
Millipedes grow continuously, adding a new segment with each molt until they reach adulthood. Adult males possess modified legs called gonopods, which function as reproductive organs. These gonopods replace one pair of legs on a diplosegment, leaving it with a single pair of legs and another pair of gonopods. Adults may mate multiple times, and females lay their eggs in moist soil. The egg and juvenile stages of millipedes are not well-documented. During molting, millipedes protect themselves by forming a ball of mud until they are ready to emerge.
Ecological Significance and Pest Status
While millipedes generally benefit ecosystems by recycling decaying organic matter, the greenhouse millipede, being a non-native species, often thrives in urban environments. Their tendency to invade greenhouses and other structures, coupled with their unpleasant defensive odor, makes them a nuisance pest. Their adaptability to new environments and lack of natural predators contribute to their success in artificial settings.
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Millipedes are beneficial and essential to ecosystems in that they help decompose plant materials and return nutrients to the soil. Millipedes are not dangerous to humans or animals. They do not bite, sting, transmit disease, or cause structural damage. Their propensity to invade structures makes them a nuisance, however, especially when they invade homes and greenhouses in large numbers and then die. When threatened, millipedes release a liquid that has a foul odor and can irritate eyes and skin, and when crushed, the millipede’s liquid can stain fabrics. Millipedes typically do not feed on or damage plants unless the plants are already damaged or decaying.
Factors Contributing to Infestations
Several factors can contribute to greenhouse millipede infestations:
- Moisture: Millipedes require moist environments to survive.
- Organic Matter: Decaying leaves, mulch, and other organic matter provide a food source.
- Harborage: Dark, cool, and moist hiding places, such as under stones, sidewalks, driveways, concrete slabs, and inside logs, provide shelter.
- Weather Conditions: Hot, dry weather or periods of heavy rain can drive millipedes indoors in search of more suitable conditions.
- New Construction: Construction materials like wood scraps and brick piles offer ideal refuges for millipede populations to grow. New sod and mulched beds also provide hidden habitats.
Too much or too little rain can make outdoor millipede habitats unsuitable. Heavy rain can flush thousands of millipedes out of their habitat in search of a moist but less-saturated environment. Their biological behaviors may also cause mass migration.
Control Strategies
An Integrated Pest Management (IPM) approach is the most effective strategy for controlling millipedes. IPM focuses on preventing infestations through habitat modification and exclusion, with chemical control as a last resort.
Non-Chemical Control
- Habitat Management:
- Remove leaves, grass clippings, heavy accumulations of mulch, wooden boards, stones, boxes, and similar items laying on the ground beside the foundation. Keep mulch 6-12 inches away from the wall.
- Reduce moisture around the foundation by ensuring proper drainage. Keep gutters and downspouts free of debris and use splash guards or perforated pipe to reduce puddling. Repair leaking exterior water spigots and prevent water from puddling where there are drip lines from air conditioning units.
- De-thatch the lawn and keep the grass mowed close.
- Exclusion:
- Seal cracks and openings in the outside foundation wall, and around the sills of doors and basement windows.
- Install door sweeps on all exterior entry doors, and apply caulk along the bottom outside edge and sides of door thresholds.
- Seal expansion joints where outdoor patios, sunrooms, and sidewalks abut the foundation.
- Trapping:
- Use sticky traps and glue boards inside and outside to capture millipedes.
- Vacuuming: Inside a structure the fastest and most appropriate control for a few millipedes would be to use a vacuum to remove them.
Chemical Control
- When to Use: Pesticides are rarely required unless millipedes continue to invade in large numbers after all nonchemical methods have been attempted.
- Application: Outdoor applications are generally limited to spot or crack-and-crevice treatments to the base of structure walls around pest entry points.
- Insecticides: For outdoor applications with residual contact insecticides, products include Gamma-Cyhalothrin, dinotefuran, bifenthrin, cyfluthrin, lambda, or carbaryl.
- Diatomaceous Earth (DE): Homeowners may try applying diatomaceous earth (DE) powder directly to potential entry points. DE is not poisonous but causes insects to dry out by absorbing the cuticle’s oils, fats, and water. It remains effective if kept dry and undisturbed but must be reapplied after rain or irrigation.
- Perimeter Sprays: Treat a 2-5 foot wide area of ground along the foundation in mulched, ornamental plant beds and grassy areas, as well as an 18-24 inch wide vertical band of the foundation wall.
Important Considerations for Chemical Control:
- Always read instructions before applying chemicals and be sure to apply products consistent with label directions.
- The key to successful chemical control is spray volume, i.e., that amount of diluted chemical that you apply over an area. The spray must penetrate the soil, not simply lightly coat the surface. The best means of application for homeowners is a garden hose attachment.
- Spraying mulch is ineffective because the chemical can bind to the mulch and not penetrate to the soil.
- Application of insecticides along baseboards and other interior living areas of the home do not really stop millipede invasions. Once indoors, millipedes end up in kitchens, living rooms, etc. and soon die from a lack of moisture. Remove them with a vacuum cleaner or broom.
- Dust formulations may work well in some areas, but they will wash away easily during heavy rains and should not be used in areas where children and pets will come into contact with the chemical.
Preventing Future Infestations
The most effective way to manage greenhouse millipedes is to prevent infestations in the first place. Key strategies include:
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- Moisture Control: Maintaining dry conditions around the foundation of buildings is crucial.
- Habitat Reduction: Removing decaying organic matter and other potential harborage sites will reduce millipede populations.
- Exclusion: Sealing entry points will prevent millipedes from entering structures.
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