This nicely colored insect are abundant in the garden. It is the commonly found Green shield bug (Palomena prasina) .
The animal is green in the summer and changes to a brown color in the autumn.
The Green shield bug is one of the most common large true bugs in the Netherlands. In the garden they are represented every year as well in larger numbers.
The bug is active between March and November and has a peak between Juli and September. Nymphs appear in June. The adults overwinter .
True bugs are hemimetabolous, which means that they develop by molting a number of times towards adulthood. With each molt they change a little bit towards the final adult stage, which is contrast with holometabolous insects that will undergo a complete metemorphosis, like a butterfly.
P. prasina has one generation per year, and ovipositioning occurs between sprin and Juli . In 5 molts, instars, adulthood is achieved .
Eggs are laid in clusters on plant stems and hatch after about 5,5 – 12 days, in a period of 0 – 4 days, depending on temperature .
Tempearature infleunces the development speed of instars (in the referenced research it varied between 16 – 28°C) :
1st intar: ±4 – 8,6 days
2nd intar: ±7,5 – 20 days
3rd intar: ±7,5 – 20 days
4th intar: ±6,5 – 17,8 days
5th intar: ±8 – 23 days
As the species can only overwinter as adult, the nymphs undergo growth accellaration as the day length becomes shorter . The new adult generation appears mid to late Juli .
The first instars of shield bugs aggregate in a group after hatching. The reason for this behaviour is that it increases their survivability. First instars do not eat and only consume moisture . Research has shown the following advantages of aggregation :
- Protection agains dehydration
The nymphs in an aggregation are better protected against dehydration as the air pockets that occur between the aggregation members contains moisture which remains than accessible to the nymphs. Individuals that are alone do not have this
- Temperature regulation
Aggregation provides a slightly lower mortality rate at higher temperatures.
- Increased development speed
The development speed of the nymphs in the aggregation at higher humidities, is higher than of those that are alone. This benefit exists up to a temperature of 31°C. The aggregation is capable of temperature regulation and positively influences the growth speed.
- Increased adherence to the substrate
Aggregation increases adherence to the plant survace and prevents nymphs from falling from the plant. The effect becomes lower as the aggregation increases in size because at a threshold of 11-15 nymphs the individuals start to stack in layers and hold each other instead of the plant.
- Increased protection agains predators
The aggregation offers protection agains certain predators:
- Hempitera: less individuals are eaten with the lowest amount in aggregations of 3 nymphs and the most in aggregations of 15
- Ants: least casualties in aggregations of 10 to 15 nymphs, and the most in aggregation of five or less
- Against wasps aggregation does not seem to offer protection
Aggregation is controlled by pheromones as described in paragraph 3.4.
De nymphs are polyfagous on different plants and trees and coprofagous on bird dropping, as shown by soem remarkable observations.
The first instar nymphs don’t eat and only consume moisture . The second instar feeds on herbs . The other stages feed especially on fruit .
This as well as other true bug species feed on the white part of fresh bird droppings , which consists almost entirely of uric acid and is rich in nitrogen. It’s the fifth instars that show this behaviour.
The species is a pest on a number of commercially cultivated plants [6, 7]:
In Turkey the species is a major pest on hazel and the damage caused by the insect is characterized by :
- Premature dropping of nuts
nuts are light colored and shrunken on the bottom
- Spots and damage to the kernel
- Shrunken kernels
(note: the quoted article indicates that this damage cannot be addressed unambiguously to bus damage)
On raspberry the bug damage gives the fruit an unpleasant taste .
On apple and pear the damage causes spots and dents in the fruit surface .
In shield bugs communication between the sexes occurs in two phases [15, 16, 18]:
- Using pheromones the insect is brought to the proper plant
- Using song the insect is guided to the partner
3.1. SEX PHEROMONES (long distance)
The male secretes a species specific volatile pheromone to guide the female towards him. She detects the pheromone using her antennae that she will hold in a ‘V’ shape in the direction of the source, which enables her to find the proper plant where the male resides .
The pheromones are produced in glands in the thorax of the adult animals .
3.2. SONG (short distance)
Like many other insect families true bugs produce songs for:
- attraction / repulsion (repulsion only by the female)
After the female has arrived on the plant where the male resides she will produce, stimulated by pheromones, a calling song that is followed by the male while producing the its own answer song and excreting pheromones [10, 16].
The songs are produced by the tergites in the abdmomen . The instrument consists of tergites I and II and is vibrated using muscles .
The song is soft but is amplified by the plants on which the bug resides. The vibrations from the tergites spread through the body and the legs into the branch and can extend tens of centimeters through it.
The bugs have a number of vibration receptors in their legs to listen to vibrations . Using time difference in the arrival of the vibration waves between the spread out legs, the bug can determine the direction of the source. The bug will stop at branches to listen in order to take the proper exit or direction. In this way the male can accurately track the sending female.
Each species has it own preference in plant. P. prasina has specialized in woody plants and produces frequencies that are best amplified for those types of plants . The species is capable of adapting the frequency based on the substrate it is standing on .
The song uses tones between 70 – 150 Hz with peaks towards 900Hz.
The adult insects emerge everywhere but most stricking is in Lilac (Syringa vulgaris).
When pruning the dried flowers they often contain bugs, sometimes in pairs. The relation to the plant is described in one observation (Assmann, 1854, als Pentatoma dissimile) .
Possibly it is related to larvae as one research article mentions feeding the larvae leaves of this plant  and another reference mentions the it as a food plant for other species of shield bugs .
I suspect it has to do with mating and the fact that Lilac is a woody plant that works well in communication between the sexes. This would explain the relatively high number of pairs I find in the plant.
When the sexes finally meet they will touch each other using their antennae. The bugs will turn around each other and the male encourages the female by pushing her behind up with his head. When the female accepts the male during this process she will point her abdomen in the air and the male will move against her in reverse.
If she does not accept the male during this process she will repell him using pheromones .
3.4. AGGREGATION PHEROMONES
Aggregating first instar nymphs communicate using pheromens to keep the group together or disperse in case of imminent danger .
Nymphs produce pheromones in glands in the abdomen .
3.5. DEFENSE AND OFFENSE
The bugs use pheromones with the purpose to :
- Distract the predator
Pheromones contain components that occur in the alarm pheromones of their attackers, which confuses them.
- Repel / intoxicate their attacker
Some components are very distastefull or even lethal to their ennemies.
The egges of the Green shieldbug are bright grass green in color and become gradually more green-yellow as the nymphs inside them develop. They are clearly barrel shaped with a round lid on top.
The eggshell is thin so themore developed nymphs become visible through it.
When emerging from the eggs the first instars are light yellow-green in color with transparent white legs. They change color as they harden and adopt a dark red-brown scheme color with pink-orange spots or a black scheme with light green spots.
The second instar is black with a light green abdomen with whitish and black spots on the back and lower edge.
The fourth instar has a flatter and rounded shape and is either complete green in colour covered with small black dots or green with a black thorax similar to the second instar but bigger in size.
Adults change color depending on the seasons, a phenomenon called seasonal polyphenism. The new generation is green and colors brown towards the autumn. In spring the brown color is replaced by green again , a more darker green than the summer generation .
The adults are identifiable using the following characteristics:
- Evenly green or brown color
- Dark wing membranes
- Shield without a light base point
- Antennae segments II and III almost equal in length
- Antennae segments IV and V partly red colored (clearly visible in summer dress)
- Front neckshield slightly hollow
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