Effects of Atmospheric Pollution on a Lichenophagus Lepidopteran
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Publication
Atmospheric pollutants such as
crop sprays and exhaust emissions can be deposited long
distances from their points of release. There are three
routes of entry for such pollutants into animals:
respiratory absorption of inhaled airborne material, skin
absorption from contact with contaminated substrates, and
absorption through the gut from the ingestion of
contaminated material. The experiments described here are
concerned with absorption due to ingestion and contact. The
effects of acidic gases such as sulfur dioxide on epiphytic
lichens and algae have long been known. Recently it has
become apparent that these organisms can accumulate large
amounts of other atmospheric pollutants such as heavy
metals, organic compounds, and radionuclides. Many
arthropod assemblages are associated with epiphytic algae
and lichens, most using these as a source of nutrients. In
this study one such group, the bagmoths (Lepidoptera:
Psychidae), were used for measuring biological effects of
atmospheric pollutants. Bagmoth larvae were used as they
constitute a "front-line" or sentinel group of indicator
organisms because of the ability of their algal diet to
absorb large quantities of atmospheric pollutants.
The global
distribution of the Psychidae ranges from Finland to the
tip of South America. They are commonly referred to as
bagmoths because their larvae construct portable dwellings
from silk coated externally with their various pabulae.
These bags protect the larvae from desiccation and provide
them with camouflage enabling them to avoid the attention
of predators.
MATERIALS AND
METHODS
Test
Organism
The species of bagmoth used for this work was Luffia
ferchaultella on account of its size and features of its
life cycle and biology, notably that it reproduces by
parthenogenesis. Its larvae feed on terrestrial epiphytic
algae of the Pleurococcus group. Larvae were collected from
a woodland (UK OS Map Ref. SU 769 855) and reared in the
laboratory on a diet of epiphytic algae (Pleurococcus sp.)
under controlled environmental conditions [20.8
(20.6-21.0)oC, photoperiod 14 h light, 10 h
dark].
Experimental Design
The practical work was divided into two phases: (1) a
laboratory-based simulation of contamination due to
atmospheric pollution, designed to investigate the
sensitivity of the test organism; (2) collection of samples
along an impacted transect followed by an investigation of
their toxicity using the protocol derived during the first
phase.
RESULTS
Laboratory-Based Spray Drift Simulation
The results for feeding of bagmoth larvae exposed to
pesticide-amended algae showed that exposure of the bagmoth
larvae to lindane resulted in a classic dose-response
relationship. The no-observed-effect concentrations (NOECs)
ranked as follows: 1st. feeding, 2nd immobility and 3rd
mortality (Table 1).
Feeding was the most
sensitive endpoint examined, the 10-day NOEC (feeding)
being 1.7 ug/g algae. With aldrin, the three endpoints
produced common NOECs of 7.5 ug/g algae. Effects were not
seen at lower concentrations as all larvae remained mobile,
with no inhibition of feeding observed below the NOEC.
Permethrin was the least toxic of the three pesticides
examined. No significant mortality or immobilization
occurred at any of the treatments, but feeding was
adversely affected at 24 ug/g algae, the NOEC (feeding)
being 8.1 ug/g algae.
Exhaust Emission Simulation
The results for exposure of bagmoth larvae to algae amended
with vehicle exhaust gases showed that the lead content of
the algae increased with increasing time of exposure to
exhaust gas/air mixture. Larvae exposed to algae containing
lead at 200 ug/g and above did not feed during the 96-h
exposure period, by which time they had become immobilized.
Field Transect
Lead concentrations in the algae were highest at the
downwind site closest to the M40 (site 4), and declined
gradually with increasing distance downwind from the
motorway. Chromium concentrations were elevated at both
sites closest to the motorway (sites 3 and 4), but were
higher downwind than upwind. Algae from the control site
contained less lead and chromium than at any of the
transect sites. Feeding was reduced at site 4, the closest
downwind site (10 m) to the M40, where the concentration of
metals was highest. Feeding increased at sites 5 and 6,
further downwind of the M40 (300 and 700 m, respectively).
Feeding was also depressed at site 1, an area surrounded by
arable farmland located furthest upwind of the M40 (700 m),
but increased upwind of the M40 as the sites approached the
motorway and distance increased from the farmland (sites 2
and 3, located 300 and 10 m from the M40, respectively). It
is significant that larvae supplied with algae from the
"clean" control site, an isolated rural woodland far
removed from arable farming and the influence of road
tra|c, fed much more than those supplied with algae from
the transect sites. This is evidence of the impacted nature
of the environment along the transect. However, traces of
lindane (0.2 ug/g algae) were found in samples of control
algae analyzed prior to spiking, indicating that the
control site was not pristine.
DISCUSSION
The most important feature of this work is that
epiphytic algae have a high a|nity for atmospheric
pollutants and that bagmoth larvae feed on this material.
This makes them front-line or sentinel indicator organisms
as far as the toxicological effects of airborne pollutants
are concerned. The epiphytic algae Diplococcus sp., on
which Luffia ferchaultella feeds, can be attached to
filters and used to absorb pollutants from an air stream.
This material can then be used to establish air / algae
equilibrium kinetics and for laboratory feeding trials.
Laboratory experimentation has demonstrated that bagmoth
larvae are adversely affected by common atmospheric
pollutants absorbed by this material. Feeding was found to
be the more sensitive of three toxicological endpoints
investigated. The metabolism of aldrin to dieldrin,
detected by analysis of larval body burdens, is significant
when one compares the toxicity of aldrin with that of
lindane. As the average third instar larval wet body weight
was 1.5 mg, we calculate that the lethal quantity of aldrin
ingested over a 10-day period was 35 ng/larva. The
corresponding value for lindane was 1 ng. The lethal dose
for permethrin could not be calculated because of its low
toxicity to these organisms. A subsequent field
investigation using a transect across an impacted
environment identifed that reduced feeding by bagmoth
larvae was related to proximity to sources of atmospheric
pollution. The presence of large quantities of heavy metals
in samples collected near a motorway confirms the ability
of epiphytic algae to accumulate heavy metals from the air.
The high concentrations of chromium and lead proximally
downwind of the M40 indicate the strong effect of
prevailing wind direction in dispersal of pollution from
motorway traffic at this locality. Lead is a commonly used
additive in petrol, while chromium is an additive used in
the cement that this section of the M40 was surfaced with
at that time. The detection of measurable amounts of
lindane in algal samples collected from a "clean" site
demonstrates the ability of these unicellular plants to
accumulate this and probably other organic substances from
the atmosphere.
CONCLUSIONS
There are many features of
the biology of Luffia ferchaultella that make it a useful
organism in ecotoxicological studies. Of most importance
for assessing the toxic effects of atmospheric pollutants
is that the diet of its larvae is capable of sequestering
such pollutants, resulting in measurable detrimental
effects on their feeding. Results of laboratory experiments
designed to simulate pollution from agricultural sprays and
vehicle exhaust emissions identified feeding as the most
sensitive endpoint of those examined. A field transect
demonstrated that larvae supplied with algae collected from
an environment impacted by atmospheric pollution exhibited
reduced feeding compared with those fed algae from a
"clean" control site.