Formica planipilis

Formica planipilis - "Flat-haired Mound Ant"

Formica planipilis is a large North American ant belonging to the rufa group, known for building impressive thatched mounds. Workers are robust and measure 4-9mm, with majors having a head longer than broad and a dark reddish-brown coloration heavily infuscated on the head and thorax ^[1]. These ants are native to western North America, ranging from British Columbia and Alberta south through Washington, Nevada, New Mexico, Colorado, and Kansas ^[2]. They inhabit grasslands, open forests, and ponderosa pine forests up to 3,000 meters elevation, with most records coming from 6,000-9,000 feet in Nevada ^[2]. This species is very similar to Formica obscuripes and may actually be synonymous with it ^[2].

What makes F. planipilis stand out is their polydomous nesting behavior, they create multiple connected nest chambers rather than a single nest ^[3]. They build characteristic thatched mounds using pine needles, grass culms, and juniper sprays, with typical domes reaching 53cm in diameter and 28cm in height ^[2]. Workers forage up into tree canopies, following established trunk trails to collect honeydew from aphids and scale insects ^[3].

Quick Summary

• Difficulty: Medium
• Origin & Habitat: Western North America, Canada (Alberta, British Columbia) and United States (Washington to North Dakota, south through Nevada, New Mexico, Colorado, Kansas). Found in grasslands, open forests, ponderosa pine forests, riparian areas, and poplar-spruce forests up to 3,000 meters elevation ^[2].
• Colony Type: Polydomous, colonies have multiple queens and maintain multiple connected nest sites. This is different from monogyne (single queen) or polygyne (multiple queens in one nest), polydomous colonies spread across several locations that workers move freely between ^[3].
  • Colony: Polygyne
  • Founding: Claustral
• Size & Growth:
  • Queen: Estimated 9-11mm based on rufa group patterns
  • Worker: 4-9mm (majors larger) ^[1]
  • Colony: Likely several thousand workers based on rufa group patterns
  • Growth: Moderate
  • Development: Estimated 6-8 weeks at optimal temperature based on genus-level data (Development time is inferred from related Formica species, actual timing may vary. First workers (nanitics) are typically smaller than normal workers.)
• Antkeeping:
  • Temperature: Keep nest area at 20-24°C. They tolerate cooler temperatures given their high-elevation habitat (found up to 10,200 ft in Nevada). Room temperature is likely suitable. A heating cable on one side of the nest can create a gentle gradient if needed.
  • Humidity: Moderate humidity. These ants naturally nest in areas with some moisture but good drainage. Keep nest substrate moist but not waterlogged. Provide a water tube as their primary water source.
  • Diapause: Yes, they require a winter rest period. In their native range, broods and reproductives are found in nests from late June to mid August ^[2], suggesting they enter dormancy by fall. Provide 3-4 months of cold storage at 5-10°C.
  • Nesting: Y-tong (AAC) nests work well for founding colonies. Established colonies can be moved to naturalistic setups with soil and thatching material. They prefer nests with chambers they can connect, polydomous arrangements mimic their natural behavior. Avoid overly humid plaster nests as they prefer better drainage.
• Behavior: Workers are moderately aggressive and will defend the nest vigorously, this is typical of rufa group Formicas. They are active foragers that establish trunk trails leading to food sources in trees ^[3]. Escape risk is moderate, they are large enough to be contained with standard barriers, but use fluon on test tube rims as a precaution. Workers can spray formic acid as a defense, which is irritating but not dangerous to humans. They are diurnal and heavily active during daylight hours.
• Common Issues: polydomous colonies can be difficult to house, they may try to expand to multiple locations, they require protein-rich food and honeydew/sugar sources, diet is more specialized than some ants, escape prevention is important during feeding times when the outworld is open, winter diapause is mandatory, colonies that don't cool down may fail to produce brood the following year, founding colonies are slow to establish, patience is required during the claustral period

Nest Preferences and Housing

In the wild, Formica planipilis builds thatched mounds in open areas. Nest material includes grass culms (about 5cm long), juniper sprays, and pine needles ^[2]. Typical domes measure 53cm in diameter and 28cm in height, with some soil bases reaching 76cm ^[2]. They often start nests at the base of small plants, frequently sagebrush, and prefer open areas without ground cover ^[2].

For captive care, start founding colonies in test tubes with a water reservoir. Once established (6+ months with 20+ workers), you can move them to a Y-tong (AAC) formicarium with soil chambers. These ants do well in naturalistic setups where you can provide thatching material like shredded paper or dried grass. Because they are polydomous, consider providing multiple connected chambers or a setup that allows them to expand. Avoid overly humid environments, they prefer better drainage than some ants. A water tube should always be available.

Feeding and Diet

Formica planipilis workers forage up into tree canopies to collect honeydew from aphids and scale insects ^[3]. This means their diet in captivity should include both sugar sources and protein. Provide sugar water, honey, or diluted maple syrup as a constant energy source. For protein, offer insects like mealworms, crickets, and other small arthropods.

These ants are not strictly predatory, they prefer collecting honeydew and will tend aphids if given the opportunity. You can offer them small pieces of fruit or provide a sugar water feeder. Feed protein 2-3 times per week, and keep sugar water available at all times. Remove uneaten prey after 24-48 hours to prevent mold. Given their arboreal foraging behavior, they may be more interested in elevated food sources, try placing sugar water on a cotton ball or in a small dish elevated above the substrate.

Temperature and Seasonal Care

These ants are found at high elevations (most records from 6,000-9,000 feet in Nevada) and tolerate cool conditions ^[2]. Keep the nest area at 20-24°C, room temperature is likely suitable for most keepers. They can tolerate temperatures toward the lower end of this range better than excessive heat.

Winter diapause is essential. In their native range, broods and reproductives are found in nests from late June to mid August ^[2], indicating they enter dormancy by fall. Provide 3-4 months of cold storage at 5-10°C during winter. This can be in a refrigerator (not freezer), garage, or basement that stays cool but above freezing. Reduce feeding to once monthly or less during diapause. Do not skip diapause, colonies that remain warm all year often fail to produce healthy brood the following season.

Colony Structure and Growth

Formica planipilis is polydomous, meaning colonies maintain multiple connected nest sites rather than a single location ^[3]. This is different from simple polygyny, the colony spreads across several chambers or locations that workers move freely between. This behavior is reflected in their natural nest construction, where they build extensive mound systems.

Colony growth is moderate. Founding queens are claustral, they seal themselves in a chamber and survive entirely on stored body fat until their first workers emerge. Expect 6-8 weeks for first workers to develop, though this is estimated from related species. Initial colonies grow slowly as the queen transitions from producing nanitics (smaller first workers) to normal-sized workers. Once established, colonies can grow to several thousand workers over several years. The polydomous nature means you may need to provide expansion space as the colony grows.

Behavior and Defense

Workers are moderately aggressive and will vigorously defend their nest, this is typical of the rufa group. They establish well-defined trunk trails leading to food sources ^[3], and foragers will patrol established paths. When threatened, they can spray formic acid as a defense, which causes irritation but is not dangerous to humans.

These ants are diurnal and most active during daylight hours. They are effective foragers and will readily explore their outworld for food. Escape risk is moderate, they are large enough to be contained with standard barriers, but always use fluon on test tube rims and formicarium openings as a precaution. Their large size makes them easier to contain than tiny ants, but they are strong and determined.

Their polydomous behavior means established colonies may try to expand to multiple locations. If housing them in a multi-chamber setup, ensure all chambers are connected and accessible.

Frequently Asked Questions

References