916 T. Ben Amor, G. Jori Insect Biochemistry and Molecular Biology 30 2000 915–925
the use of photoactivatable insecticides, as well as dis- cussing the limitations, scope and potential of this novel
technique. The interest in this topic is enhanced by safety and environmental considerations. The use of sunlight
as the promoter of the phototoxic activity is in line with the growing trend to utilize and validate natural
resources. Moreover, a careful selection of the chemical structure of the photosensitizing dye can modulate the
nature of the subcellular photodamaged sites, which is quite important for optimizing the efficiency of the cyto-
cidal effects and minimizing the risk of selecting photor- esistant insect species.
The photoinsecticides represent a possible alternative to traditional chemical insecticides. The latter com-
pounds are known to cause several important problems, including widespread toxicity to plants and animals and
in particular to humans, as well as the prolonged persist- ence in the environment which may cause severe pol-
lution. As we will discuss in the present review, at least some of these issues can be adequately addressed by the
use of photodynamic insecticides.
2. Historical background
The first scientific documentation that sunlight can be toxic to biological systems was provided by Marcacci in
the late nineteenth century Marcacci, 1888. This author reported that the fermentation of plant alkaloids and
amphibian eggs
becomes more
important under
UVvisible light than in the dark. Shortly thereafter, Raab 1900 observed that the presence of some exogen-
ously added visible light-absorbing compounds, such as acridine orange, was necessary for sunlight to promote
the death of paramecia. Acridine orange and other dyes with similar properties were defined as photosensitizers.
Raab’s observations were repeated with a variety of multi- and unicellular organisms Spikes, 1985. Finally,
Jodlbauer and Von Tappainer 1904 demonstrated that the presence of oxygen is an essential requisite for pho-
tosensitization to occur. The combined effect of the three elements, namely light, photosensitizer and oxygen, has
been termed photodynamic action Blum, 1941.
While most organisms have developed specific protec- tion or repair mechanisms to counteract the damaging
action of light, several attempts have been made to con- trol the photoprocesses involving specific biological sys-
tems with the aim to obtain beneficial effects. Thus, pho- totherapeutic techniques have been developed by taking
advantage of the property of some photosensitizing agents to be accumulated in significant amounts by dis-
eased tissues, such as tumours, atheromas or psoriatic plaques Brown, 1997. At the same time, the photosen-
sitized inactivation of yeasts and bacteria is used for the decontamination of microbially polluted waters by sun-
light Merchat et al., 1996. The possibility of using photoactivatable compounds
for controlling the pest population was first explored by Barbieri 1928 who used a combination of xanthene
derivatives, including fluorescein, erythrosine and rose bengal, against Anopheles and Aedes larvae. Barbieri
showed that light or photosensitizer alone had no detect- able toxic effects on insects.
This research line underwent no further developments until the early 1950s when Barbieri’s experiments on the
photosensitized killing of insects were repeated by Schildmacher 1950, who concluded that the chemical
structure of the xanthene dyes exerts a profound influ- ence on the degree of phototoxicity. Thus, the highest
photoinsecticidal activity is displayed by rose bengal fol- lowed by eosin, erythrosine and fluorescein. The poten-
tial of xanthenes as promoters of lethal effects on insects exposed to sunlight was explored in detail by Heitz and
co-workers Heitz, 1987; Heitz, 1997a, who investi- gated the role of various experimental parameters on the
photoinsecticidal efficacy of this class of compounds. Of particular interest were the field studies based on several
combinations of bait and xanthene dyes Carpenter et al., 1981; Sakurai and Heitz, 1982. Xanthenes undergo
rapid photodegradation in aqueous media by analogy with the well-known tendency to photobleach which is
typical of many polycyclic aromatics when exposed to non-ionizing radiation Spikes, 1992. This topic was the
subject of intense discussions at a recent congress of the American Society of Photobiology Heitz, 1997b.
At the same time, independent investigations perfor- med by various laboratories demonstrated that near-UV-
or visible light-absorbing dyes belonging to different cat- egories of organic compounds can develop a phototoxic
effect towards a variety of insects Robinson, 1983. It seems reasonable to propose that any photodynamically
active drug, which can be combined with a suitable bait to facilitate its uptake by the target insect and avoid its
uptake by useful non-target insects, has the potential for acting as an insecticide. In fact, UVvisible light can
penetrate to a depth of about 1 cm in most biological tissues; as the extent of penetration is wavelength depen-
dent Svaasand et al., 1990, it is possible to modulate the volume of the photoinduced tissue damage by sel-
ecting a photosensitizing agent with specific absorption properties. In this connection, porphyrin-based photoin-
secticides which have been recently developed Rebeiz, 1992; Ben Amor et al., 1998a are particularly interest-
ing since they exhibit a multiplicity of absorption bands throughout the UVvisible spectrum; hence they can be
efficiently activated by either white light or selected wavelength ranges depending on the desired extent of
photodamage.
917 T. Ben Amor, G. Jori Insect Biochemistry and Molecular Biology 30 2000 915–925
3. Mechanisms of photodynamic sensitization
