Behavioural Bioassays and Identification of Cashew Leaf and Stem Volatiles Mediating Attraction to the Stem Girdler, Analeptes trifasciata (Coleoptera: Cerambycidae)

The cashew stem girdler, Analeptes trifasciata Fabricius (Coleoptera: Cerambycidae), damages cashew by its girdling activities in the stem thereby causing huge economic losses. The stem girdler is managed through cultural practice of burning girdled stems and beetles, though this has drawbacks. The objective of this study was to explore the cues mediating attraction to the cashew host plant;hence the role of olfaction in host plant location by A. trifasciata underlying the semio-chemical option for controlling this insect pest. A diffusional Y-tube olfactometer was used to study the behavioural response of A. trifasciata, to freshly cut cashew stem and leaves odour sources. Methanol-extract of these plant tissues was subjected to the coupled gas chromatography-mass spectrometry (GC-MS) analysis. Y-tube olfactometric assays demonstrated that both sexes oriented towards and spent significantly more time in stem odour arm compared to the leaf odour arm in both male (male: t = 2.228, d.f = 11, P = 0.040) and female (t = 2.341, d.f = 11, P = 0.040). A combination of fatty acids, amino acids and carbohydrates were detected in cashew stems. Some of these fatty acids are attractants to other insect pests. It is suspected that these fatty acid blends may possibly be responsible for facilitating host plant location by both sexes. In conclusion, both sexes were independently and strongly attracted to the stem volatiles;this study opens the possibility of utilizing cashew stem volatiles as surveillance and control tools.

The “sequential cues hypothesis”: a conceptual model to explain host location and ranking by polyphagous herbivores

Successfully locating a host plant is crucial for an insect herbivore to feed and/or oviposit.However,locating a host within a complex environment that may contain an array of different plant species is a difficult task.This is particularly the case for polyphagous herbivores,which must locate a host within environments that may simultaneously contain multiple suitable and unsuitable hosts.Here we review the mechanisms of host selection used by polyphagous herbivores,as well as exploring how prior experience may modify a generalist's response to host cues.We show that recent research demonstrates that polyphagous herbivores have the capacity to detect both common cues from multiple host species,as well as specific cues from individual host species.This creates a paradox in that generalists invariably rank hosts when given a choice,a finding at odds with the“neural limitations”hypothesis that says generalist insect herbivores should not have the neural capacity to identify cues specific to every possible host.To explain this paradox we propose a model,akin to parasitoid host location,that postulates that generalist herbivores use different cues sequentially in host location.We propose that initially common host cues,associated with all potential hosts,are used to place the herbivore within the host habitat and that,in the absence of any other host cues,these cues are sufficient in themselves to lead to host location.As such they are true “generalist”cues.However,once within the host habitat,we propose that the presence of a smaller group of cues may lead to further host searching and the location of preferred hosts:these are“specialist”cues.This model explains the current conflict in the literature where generalists can respond to both common and specific host-plant cues,while also exhibiting specialist and generalist host use behavior under different conditions.