15. Semiochemical uses
• Detect presence of a species
• Measure seasonal activity
• Provide decision support
• Mate disruption
• Assess levels of insecticide resistance
17. Aren’t pheromone catches biased?
• Females are the ones that oviposit
• Females are directly linked with larval
abundance
• Females do not move as much as males
in many species
• Some baits that are food based attract
both sexes but are less specific.
18. Mating Disruption
Work best if:
• Pests do not have wide range
• Pests are poor dispersers
• Pest densities are not too high
• Male sensitivity to pheromones is high
Integrated Pest Management: Potential, Constraints and Challengesedited by Opender Koul, G S Dhaliwal, G W Cuperus CABI Publishing
We talked about, some lectures ago, on how parasitoids can perceive plant odors or the combination of plant and herbivore odors. Several parasitoid species won’t go towards a plant smell only. They will only fly towards a signal that indicates that the parasitoid’s hosts (e.g., a caterpillar ) will be there as well. So parasitoids respond to the odor combination produced by what it is called a host-plant complex. This is a way for the parasitoids to look for their hosts but it can be seen as a way of plants to communicate parasitoids that they are under attack. So, in a sense parasitoids and plants are talking.
Not only plants call natural enemies for help but plants can tell other plants that they are under attack so the other plants start getting ready to fight the herbivores that might potentially bother them.
Chemicals that modify insect behavior in any way are referred to as semiochemicals. Not only chemicals stimulate but they might also inhibit certain insect behaviors.
Semiochemicals can be divided in two main classes:
Pheromones: Chemicals that operate intra-specifically. That is, among members of the same species.
Allelochemicals: Chemicals that operate inter-specifically. That is among members from different species.
Pheromones are classified based on their function. Sex pheromones are involved in reproduction by attracting the opposite sex. In some species female pheromones are used to attract the male. In other species male pheromones are used to attract the female.
Alarm pheromones are used to warn members of the same species about imminent danger. Like in the example we used to explain synergism last class. Remember?. In that example the aphids were using an alarm pheromone to inform other members of their species that they were attacked by a lady bug! As soon as aphids smell this alarm pheromone they drop from the plant. In this way, even aphids that are not being attacked by the lady beetle fall to the ground.
Aggregation pheromones are use to congregate members of the same species in relatively big numbers. They are used by organisms that need help to invade a new area or habitat or plant. These conifer beetles for example do a better job, or have a higher fitness when feeding in group than when feeding individually.
Trail pheromones are used by ants to mark their trails so they can follow each other. Have you ever scratch the soil under an ant trail? Do it and you will see how the ants get confuse and lost for a while.
Host marking pheromones are used by parasitoids to avoid ovipositing on hosts they or somebody else have oviposit already. Hyper parasitoids may use these marking pheromones to find their hosts.
Allelochemicals, the second class of semiochemicals, are classified according to the advantage provided to the receiver or sender. Examples of:
Alomones: An oviposition or feeding deterrent produced by a host-plant
Kairomone: An odor produced by a host that helps a parasitoid to find it
Synomone: A volatile produced by a host-plant to attract parasitoids to them when attacked by herbivores.
The use of pheromones and kairomones in IPM allow us to monitor pest levels. Depending on how many pests we catch in our pheromone traps we can decide what control method, if any, needs to be done. This kind of pheromone trap information, for example, may help us decide when to treat a field with pesticides. These semiochemicals are also used to assess the presence of pests in undesired places within quarantine facilities. Or to monitor the colonization of pests in new areas.
Several control methods use semiochemicals that influence insect behavior. For example, mass trapping lures insects using pheromones or an allelochemical into a trap to get rid of pests. It is used in parks and storage situations. In parks it is effective because people is able to tolerate some damage (mass trapping is not very good at keeping damage to zero). In storage situations mass rearing works well due to the small enclosed area involved.
The attract and kill technique use a semiochemical to attract pests to traps loaded with a toxic chemical to get rid of pests.
The Lure and infect technique (also known as autodissemination) relies on attracting a pest using a semiochemical and instead of killing the pest right there, this method infects the pest with an pathogenic agent that is spread by the attracted pest to the rest of the pest population.
These method involves the release of large amount of sex pheromones in the crop land so pests do not mate due to their inability to find each other
So as we have seen, semiochemicals can be used to gather all sorts of information about pests.
Pheromones are powerful little lotions and insects are awfully good at smelling them!!!
Pheromone catches are biased. In Lepidoptera for example, females are not catched by pheromones. Pheromones are released by females in nature thus, pheromone traps only catch males. Aggregation pheromones in beetles attract both males and females though. The slide here show some things to consider when thinking about the biases of pheromone trapping. In many pest species the females are the important sex to track and if they are the ones producing the pheromones we might loose important information by looking at males only. In mosquitoes for instance, it is the females the only ones that do the blood feeding. In many agricultural pests females are very important because they are the ones deciding where to oviposit. Thus, they are directly linked with larval abundance at a particular place. However, it needs to also be considered that females may not move or disperse as much as males do in many species. If interested in females, baits involving food rather than pheromones could be important. Since food attractants will attract females and males. The problem with this last approach is that these food based baits are less specific.
Mating disruption is the most effective semiocemical based control method existing so far.
The antennae of many insects are able to perceive extremely low pheromone concentrations. The insect antennae are amazing sensory organs.
Pheromones are composed by a blend of different chemicals. Male insects are able to recognize specific blends. That is, they respond to a particular ratio of the chemicals present on a pheromone emission. Ratios that are different from the natural blend might not attract the insect. Different blends are perceived by different species. Insects are able to differentiate between different pheromone blends even if the same components are present between one blend and another and the only difference is in the abundance ratio of the different chemicals involved.
One of the problems of using pheromones or semiochemicals to attack pests is that some environmental factors can alter their performance. Wind speed and precipitation can considerably affect pheromone performance for example.
Next class we will study how to modify insect behavior to our advantage.
