Friend or Foe?

Megalopyge opercularis, commonly known in Texas as an “asp” is one of the most toxic caterpillars in North America. Also known as puss moth caterpillars, the larva are teardrop shaped from 1 to 1.5 inches long. Although called a stinging caterpillar, the venom is actually in spines connected to a poison sac and concealed by the outer hairy surface. When touched they break off and remain in the skin releasing the venom. Intense throbbing pain develops within five minutes of contact with pain extending up the affected arm. Other symptoms may include headaches, nausea, vomiting, intense abdominal distress, lymphadenopathy, lymphadenitis, and sometimes shock or respiratory stress. Erythematous spots or hemorrhagic papules may appear at the site of the sting and last for 1-5 days. Some of the more unusual presentations have been in children where they have gotten into the mouth and deposited spines and venom in the mucus tissues. These spines can persist and one researcher describes the experience of removing shoes after being in an infected area and transferring a spine from the shoe to the eye causing severe pain. The number of exposures peaks in June-July with a second peak in October. One researcher reported that there may be two generations within a year. The poison control centers report that about one fourth to one third of the patients experience intensive pain that radiates up the arm or leg. The pain is severe enough for some patients to believe they are having a heart attack. In a reported case of abdomen “sting” severe abdominal pain ensued with an emergency department visit and work up for acute abdomen.


The use of adhesive tape to remove the spines that remain on the skin is recommended. Wash the area with soap and water to remove any remaining venom. Prompt application of an ice pack and a baking soda poultice should help reduce pain and swelling. Over the counter analgesics appear to be ineffective for reducing pain and headache. Oral administration of antihistamines may help relieve itching and burning following up with topical corticosteroids to reduce intensity of inflammation.

Life cycle of the caterpillar

The caterpillars spend the winter in cocoons attached to twigs with the puss moth (aka flannel moth) emerging in the late spring. The adult moth with a short life of 5-7 days deposit eggs on shrubs and trees with no known predilection for a specific species and within days the larva emerges. The caterpillars feed on deciduous trees and shrubs. It is unclear how many instars the moth goes through but it appears to be about 5-7 molts. Their color varies from white gray to reddish brown or a mixture of colors.

Venom Identification

There has not been much work done on the identification or characteristics of the venom found in the spines of the puss caterpillar (megalopyge opercularis). David M. Eagleman, PhD of the Department of Neuroscience at the Baylor College of Medicine in Houston, TX is currently working on the identification of the toxin with the hope of developing antivenom. There is a closely related caterpillar also known in Oklahoma as the puss caterpillar, Lagoa crispata that has had some work on the venom identification by scientists at the Oklahoma State University in Stillwater. They have characterized the venom as a protein with high kallikrein activity. The two species are very similar (see pictures) both belonging to the Megalopyge family. The Lagoa crispata can be found along the east coast of the US and far inland to Oklahoma while the megalopyge opercularus is located along the gulf coast and into Latin America; although the species has been identified as far north as Kentucky and Virginia. Although usually not a public health problem the puss moth caterpillars have historically had large outbreaks including a recorded outbreak in San Antonio, Texas in 1924 closing several public schools. Recent data indicate an increase in reporting of exposure incidents. Of particular importance are children and multiple exposures such as larva falling from trees into clothes.


  1. Diaz, J.H., “The evolving global epidemiology, syndromic classification, management, and prevention of caterpillar envenoming” Am. J. Trop. Med. Hyg. 72(3):347-357, 2005.
  2. Eagleman, D.M., “Envenomation by the asp caterpillar (Megalopyge opercularius)” Clin. Tox. 46(3):201-205, 2008.
  3. Lamdin, J.M. et. al., “The venomous hair structure, venom and life cycle of Logoa crispata, a puss caterpillar of  Oklahoma” Toxicon 38:1163-1189, 2000.
  4. Lee, D., RD Pitetti and ML Casselbrant, “Oropharyngeal manifestations of lepidopterism” Arch. Otolaryngol. Head Neck Surg. 125:50-52, 1999.
  5. Neustater, B.R., NH Stollman and HD Manten, “Sting of the Puss Caterpillar: An Unusual Cause of Acute Abdominal Pain” South. Med. J. 89(8):826-827 1997.
  6. University of Kentucky Entomology

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