Plant Interactions with Pests – The insects

 

1. Insect Evolutionary History.

 

Existed on Earth for >300 Million years

 

>1million different species

 

2. What are the reasons behind their success?

 

Flight

          External skeleton

          Small size

          Rapid Reproduction and extreme fecundity.

 

3. Economic Importance

 

          Pollination

          Production of important products - honey and silk

Crop Loss. FAO of the UN estimates that 1/3rd of all food is lost to insects.

          Disease vectors for livestock and humans.

 

4. Overall Insect Structure

 

          Rigid external skeleton; highly impermeable to water/

segmented. May be fused, as in head in head and thorax, or articulated – as in the abdomen.

Wings; Generally two pairs positioned in 2nd / 3rd thorax segments.

In Diptera (Flies) reduced to a single pair. The reduced remnants of the second pair of wings are known as halteres, and function as stabilizers or as airspeed detectors.

In Coleoptera (Beetles), the forewings have hardened into horny cases

 

5. Mouth Parts

 

Biting and Chewing

 

Labrum covers mandibles…the inner edge of which are often serrated.

Slightly different arrangement in larvae. Note – new structure, the spinnerettes.

 

          Piercing and sucking

 

          Seen in Hemiptera – leaf hoppers, aphids and mealy bugs.

          Probocis is formed from labrum.

          Salivary glands…

Well developed in most insects. Discharge saliva through the mouth which is re-absorbed. .

 

6. Growth and  Development

 

Hard exoskeleton necessitates growth by moulting or ecdysis

Growth stages between moults in known as the “instar”.

The Number of instars varies between species – House fly has three instars,

Mayflies have between 30-40 instar stages.

Moulting is controlled by moulting hormones (MH) – ECODYSOME a steroid.

 

7. Metamorphosis.

 

          Usually associated with complete body pattern changes

Also used to describe incomplete metamorphosis, where juvenile stage resemble  

Adult but not identical.

          In juvenile stage – “nymphs” sexual organs are not mature.

          Metamorphosis is controlled by juvenile hormone (JH)

Negative regulator, its disappearance induces insect metamorphosis.

          JH is produced in corpara allata in the brain.

          JH is a terpenoid.

If added at an inappropriate time – normal developmental progress is disrupted and synthetic versions have been used as an insecticide.

 

8. Reproduction

 

Attract mate by producing chemical attractants – sex pheromone, which may work over several Km.

Very Specific.

Most are mixtures of compounds, rather than single substances.


 

Host Selection

 

Host selection – occurs for feeding and also oviposition.

 

Insects respond to plant colour, shape, touch and chemical cues .

 

Sensory systems –

 

1.    COMPOUND EYES – ocelli,

2.    MECHANORECEPTIVE SENSILLA.

 

Hair sensilla are the most abundant mechanosensor.

 

Used mainly for controlling flight and also sense deformations and stresses in the body.

 

But also in host selection by detecting plant surface properties.

 

 

3.    CHEMIORECEPTIVE SENSILLA.  These are very important and sensitive but not abundant.

 

Rabbits have 108 olfactroy receptors compared to only 48 in caterpillars.

 

Divided into two types –

          Oliofactory  -     high sensitivity.

          Contact     low sensitivity.

 

 

The Host Selection Process

 

1. Odour, colour and shape.

 

Apple maggot flies – fly to host fruit following a concentration gradient. Lands and moves to apple based on shape and colour.

 

In onion fly (Delia antiqua) – oviposition was encouraged by alkyl sulphides and a smooth yellow cylinder positioned vertically near the plants.  

 

2. Surface testing.

 

Once in physical contact- chemoreceptors on the antennae, mouth, tarsi, ovipositor perceive stimuli from the plant surface.

Examples of surface testing –

 

A)     “Fluttering” of butterflies – drums the leaf surface before oviposition.

Surface palpation by larval and adult beetle and moths with labial and maxillary papili.

          Both of the above stimulates the release of cuticular lipids.

 

B)     Test biting. e.g. locust pierces tissue but is not necessarily removed.

Typically discriminates nutrient status and specific plant secondary metabolites.

         

 Pieris brassicae larvae – application of mustard oil glucoside stimulates biting and feeding.    

 

3. HOST ACCEPTANCE

Chemical composition of food is continuously monitored. Continuation of feeding depends on

-olifactory and gustatory feeding stimulants

-intensity of repellents/deterrents

 

The continuation of Colorado Potato beetle feeding depends on

a)    the presence of stimulating nutrients e.g. sugars, amino acids, phospholipids

b)    absence of several alkaloids. 

 

Grasshopper rejection after tasking is attributable to alkaloids and monoterpenoids.


OVIPOSITION

 

Normal egg production >100 per female.

 

Influenced by cues which correlate with the prospect of larval survival.

 

 

Initially vision is the dominant sense –

(i)              orientation

(ii)           landing

(iii)        assessment –

 

ASSESSMENT

 

(I) Olfaction,  - oviposition stimulants

 

Black swallowtail butterfly (Papilio polyxenes) on carrot.

          LUTEOLIN and trans chlorogenic acid

 

Pieris brassicae – glucosonolate hydrolysation products – allyisolthiocyanate –

 

2. CONTACT ASSESSMENT

 

          Drumming e.g. Piersis brassicae

a)    Stimulates the secretion of non-volatile chemicals

b)    Resonance – shows the physical properties of the plant/leaf

 

3. SPACING ASSESSMENT

 

Female olive fruit fly (Dacus oleae) spreads olive juice over the surface to deter further oviposition.