Attractants to insects

Chromoplasts are pigmented plastids that contain carotenoids but lack chlorophyll. These plastids are attractants to insects and animals. 

There is also the possibility that pollutants alter susceptibility of the plant to pathogens (36) or insect attack. Of the latter there is the decreased resistance of ponderosa pine to bark beetle attack caused by ambient oxidant exposure (37). The investigations of others with respect to the effects of fluoride on ponderosa pine indicated that although foliar injury was associated with increased resin exudation pressure, which could be interpreted as an increased capacity of the tree to overcome bark beetle attack, degree of insect infestation was not associated with amount of foliar injury (38). As more is known about pheromones, the botanical investigation of the secondary products of metabolism, such as terpenes and phenolics, may become more important in investigating the mode of action of pollutants in the entire plant. The switch to alternate pathways, while resulting in the same products, may reduce the intermediates needed in biosynthesis and thereby affect the plants resistance to disease or attractiveness to insects. 

Insect-pollinated plants are called Entomopkilous. These, being dependent upon the visits of insects for fertilization, possess brilliantly colored corollas, have fragrant odors, and secrete nectar, a sweet liquid very attractive to insects, which are adapted to this work through the possession of a pollen-carrying apparatus. Example Orchids. 

Alterations in monoterpene content can determine the degree of resistance to bark beetles and, thus, whether or not a tree is colonized. Resin monoterpenes (Fig. 1) may be either repellents or attractants to Insects or fungistatic or chemotroplc stimulants of fungal growth, depending on the precise monoterpene composition and the particular herbivore or pathogen (9-13). 

Baits include mixtures of toxicant, usually at l ndash 5%, with a carrier especially attractive to the insect pest. Carriers include sugar for the houseflies, protein hydrolysates for fmit flies, bran for grasshoppers, and honey, chocolate, or peanut butter for ants. 

The function of the essential oil in the plant is not fully understood. Microscopic examination of plant parts that contain the oil sacs readily shows their presence. The odors of flowers are said to act as attractants for insects involved in pollination and thus may aid in preservation and natural selection. Essential oils are almost always bacteriostats and often bacteriocides. Many components of essential oils are chemically active and thus could participate readily in metaboHc reactions. They are sources of plant metaboHc energy, although some chemists have referred to them as waste products of plant metaboHsm. Exudates, which contain essential oils, eg, balsams and resins, act as protective seals against disease or parasites, prevent loss of sap, and are formed readily when the tree tmnks are damaged. 

In Alloxysta victrix, 6-methylhept-5-en-2-one 16, which is produced by both males and females, was identified as potentially attractive to the males and slightly repellent to the females in Y-tube olfactometer assays [60]. In this study, the activity was also dependent on prior exposure of the insects to the compound. Naive insects responded more strongly than previously exposed ones. This underscores a second difficulty in the bioassay-guided identification of parasitoid hymenopteran pheromones the responses are very dependent on the context and on prior exposure. Learning has been demonstrated in several species of parasitic hymenoptera [61-65]. 

An example of a larval parasitoid that responds to the host sex pheromone is seen with Cotesiaplutellae (Braconidae), also a parasitoid of the diamondback moth. These insects were attracted equally to the pheromone blend (31,32,33, see above), the acetate 32, or aldehyde 31 components [80]. This larval parasitoid, however, was also strongly attracted to host frass volatiles, in particular, dipropyl disulfide 34, dimethyl disulfide 35, allyl isothiocyanate 36, and dimethyl trisulfide 37. In contrast, the egg parasitoid Trichogramma chilonis was only weakly attracted to 36. In both, T. chilonis and C. plutellae, plant volatiles, in particular (3Z)-hex-3-en-l-yl acetate 38, significantly enhanced attraction by the pheromone [80]. 

The sex pheromone communication system basically involves the release of specific chemicals from a pheromone producer (emitter), the transmission of these chemicals in the environment to a receiver, and the processing of these signals to mediate appropriate behavioral responses in the receiver. The chemicals transmitted downwind have been the most obvious targets for characterization. The code was first broken with the publication in 1959 (3) of the sex pheromone for the domesticated silkworm Bombyx mori after extraction of a half million female silkworm pheromone glands and 30 years of classical chemical analyses. The pheromone was found to be (E10, Z12)-hexadecadien-l-ol, which was called bombykol. This work showed that there was nothing magical about the communication system, and chemists around the world were "attracted" to this area of research on insect pheromones. 

Many hunters react to their prey s overall scent or some of its components, perhaps the smell of fur or some less complex odor. One of the world s most injurious insects, the African malaria mosquito (Anopheles gambiae), prefers humans to other sources of a blood meal. Oddly, whenever possible the mosquitoes bite people on their feet. This predilection reflects their strong attraction to the mixture of fatty acids that we associate with smelly feet. Humans may find the odor offensive, but these mosquitoes know it as a fragrant guide to blood. The same fatty acids also draw them to another odor that offends some people, the smell of Limburger cheese. 

Both moth and plant are well adapted to their lifestyles. Because most yuccas produce little nectar, they offer no reward to attract other insects but entrust their pollination solely to yucca moths. The lack of nectar in yucca flowers means nothing to the moths. They have an incomplete gut and cannot feed in any event. Without nourishment, they live only long enough to fulfill their reproductive tasks. Two to four days after emerging from their cocoons, they die. 

Among the many factors that affect cotton production are insects and diseases. Probably no other cultivated crop is so attractive to such a wide variety of insects, and in tropical conditions they multiply rapidly and spread. Therefore, the economical production of cotton in these areas, as elsewhere, often depends on whether or not the insects are controlled. 

Insects are attracted to flowers by their color (including light of wavelengths—or colors—that we cannot see) and sometimes by their scent Typical examples of flowers that will attract beneficial insects are ... 

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