Insect Biochemistry and Molecular Biology 30 2000 915–925 www.elsevier.comlocateibmb
Mini Review
Sunlight-activated insecticides: historical background and mechanisms of phototoxic activity
Thameur Ben Amor, Giulio Jori
Department of Biology, University of Padova, Via U. Bassi 58B, Padova 35121, Italy Received 21 January 2000; received in revised form 12 March 2000; accepted 14 March 2000
Abstract
Several photosensitizing agents, which are activated by illumination with sunlight or artificial light sources, have been shown to be accumulated in significant amounts by a variety of insects when they are administered in association with suitable baits. The
subsequent exposure of such insects to UVvisible light leads to a significant drop in survival. Of the photosensitizers tested so far, xanthenes e.g. phloxin B and porphyrins e.g. haematoporphyrin appear to be endowed with the highest photoinsecticidal
activity. In particular, porphyrins absorb essentially all the UVvisible light wavelengths in the emission spectrum of the sun; hence they are active at very low doses. Thus, 1 h irradiation of Ceratitis capitata, Bactrocera oleae also known as Dacus oleae or
Stomoxys calcitrans
which ingested a few nanomoles of porphyrin per fly with light intensities of the order of 1000 µ
E s
21
m
22
causes about 100 death in laboratory tests. Present evidence suggests that such photosensitizers act on the membranes of the midgut with consequent feeding inhibition, as well as on the neuromuscular sheath. No apparent onset of photoresistance has been
observed. The rapid photobleaching of xanthenes and porphyrins when illuminated by visible light, as well as the lack of significant toxicity of such compounds in the dark, minimizes the risk of an important environmental impact of such photoinsecticidal agents.
2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The use of photochemical processes as a tool to con- trol the population of several types of insects has been
repeatedly examined in both laboratory experiments Heitz, 1987; Rebeiz et al., 1991 and field studies
Pimprikar et al., 1980a; Lenke et al., 1987. Most inves- tigations have been performed by using photoactivatable
polycyclic aromatic dyes that absorb near-UV light wavelengths, including thiophenes, furocoumarines and
quinones Robinson, 1983; Cunat et al., 1999. The use of xanthene derivatives such as eosin and its analogues
absorbing selected light intervals in the visible spectral range has also been proposed Yoho et al., 1971;
Fondren and Heitz, 1978a; Heitz, 1997a. All of these dyes require the presence of molecular oxygen to express
their phototoxic action; hence the overall photoinsectici- dal process appears to be of the photodynamic type Jori,
Corresponding author. Tel.: +
39-049-8276333; fax: +
39-049- 8276344.
E-mail address: joriciv.bio.unipd.it G. Jori.
0965-174800 - see front matter
2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 5 - 1 7 4 8 0 0 0 0 0 7 2 - 2
1996. In addition, furocoumarines, upon photoexcit- ation, can generate various types of addition products
with DNA bases, which often results in genotoxic effects Jori, 1985.
More recently, Rebeiz and co-workers proposed the use of porphyrins as photoinsecticides Rebeiz et al.,
1987. In particular, these authors tested protoporphyrin IX and its ZnII derivative, which appear to be
especially promising photoinsecticidal agents since these compounds absorb essentially all the UV–visible wave-
lengths, that is to say these molecules can be efficiently excited by natural sunlight. Along the same line, we
recently demonstrated that haematoporphyrin is an efficient phototoxin to several insects Ben Amor et al.,
1998a. Subsequently, we extended our investigations to some meso-substituted porphyrins Ben Amor et al.,
1998b in order to identify possible relationships between the chemical structure and the photoinsecticidal
activity of this class of compounds. Ceratitis capitata, a Mediterranean fruit fly, was selected as an experi-
mental model.
The present contribution is aimed at describing the historical background and the present state of the art for
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
