Background

Blood worms are larvae of midges belonging to the family of Chironomidae. It has been reported that these worms are found in swimming pools and this paper serves to provide some introductory remarks on this insect as well as its prevention and control.

Insect Biology

2. The family Chironomidae is grouped under the Order of Diptera which includes housefly and mosquito. The insect undergoes complete metamorphosis with life cycle divided into four stages, i.e. egg, larva, pupa and adult. Although the adults are somewhat mosquito like in appearance, they do not have piercing and sucking mouthparts and therefore non-biting. They often occur in huge swarms, usually in the evening. They are attracted by light so light traps could be used for the control of adults. However, this behaviour also implies that lights concentrate the insects near people at night and pose a nuisance. Adult chironomids are short-lived, reported to be about a few days. Their flight ability is generally weak and they disperse mainly by passive means like wind, vehicles, etc.

3. After mating, female lays eggs in water in masses. Under favourable condition, the eggs hatch in about two to three days. The larvae live about four weeks and then pupation follows. Pupae usually last about two days before adults emerge.

4. Most of the chironomid larvae are aquatic and red in colour because of the presence of haemoglobin in their blood. However, a few species are green or yellow. Polluted water apparently favours their multiplication and emergence. Most of them are detritus feeders. They eat tiny particles of organic matter together with some algae. The larvae can be found in large number when the water is polluted by organic matters.

Local Situation

5. Members of this family are quite common in Hong Kong. However, there is little study and publication about the distribution and biology of the local fauna. Larvae have been collected at many of our streams which are polluted by organic matters.

Public Health Importance

6. The adult midges lack piercing and sucking mouthparts. They are unable to bite and are not regarded as an important disease vector. However, the midges can induce allergic diseases.

Control Methods

7. Adult midges resting in vegetation, shaded sand traps, indoor environment, etc. can be knocked down by insecticidal aerosol. Unlike mosquito larvae, they do not come to the surface for air but obtain dissolved oxygen from the water. So, larvae cannot be suffocated by means of malarial oil. Insecticides and larvicides such as B.t.i. can provide better control against the larvae.

Advice to the Public

8. As the larval and pupal stages of the midges are aquatic, source reduction and water management are regarded as the fundamental control measures. Drains should be checked regularly to prevent accumulation of water. Larvicidal fish is a possible biological agent when the water bodies cannot be removed.

9. As the adults are attracted by light sources and they are most active during sunset, the nuisance can be abated, to a certain extent, by avoiding the use of unnecessary lights in this period of time.

10. Adult midges proofing measures such as installation of protective nets and screens of small mesh can ward off its intrusion into indoor environment.

Pest Control Advisory Section