- Aphids spp.
Aphids, commonly called “plant lice”, are insects, whose adults have varying dimensions of about 1,5-4,0 mm in length, possess an almost transparent thin external integument, in colours that range from clear yellow to black.
Aphids with and without wings (Aphis Gossypii)
They have a strong polymorphism between the winged forms, whose task is to reach new plant hosts up to a distance of hundreds of kilometers away, and those apteral (wingless) that, with the loss of wings and wing muscles, assemble all of their resources in the reproductive function, producing in a short time numerous colonies.
Aphids possess piercing-sucking mouth-parts (rostrum) through which they pierce the most tender tissues (vegetable apexes and young leaves) from which plant sap is withdrawn. This is rich in sugar but poor in amino-acids, for which reason they seek to retain the necessary nutritional elements and to expell the surplus water and sugars in the form of sticky honeydew, through a posterior appendix of the abdomen (siphuncle).
Aphid with egg
In nature, these Homopterons have a very complex biological cycle; in greenhouses, on the other hand, they have a simpler cycle because the insect has at it’s continual disposal a vegetable host (Gerbera) and a mild climate. All of this allows it to have only parthenogenetic generations, with a phenomenal ability to multiply. On average a female during her life cycle produces from 50 to 100 progeny, most of which other females that, within 8-10 days of their birth, are sufficiently mature to begin reproduction.
The damage caused by these can be direct and indirect.
- direct damage consist in the suction of plant sap, lesions and necrosis of the tissues subject to puncture by the insect. They also produce gall and hyperplasia, because of the effect of different substances which are injected into the plant, including Indoleacetic acid (vegetable growth hormone).
- indirect damages however, consists of the excretion of honeydew that accumulates on the foliage of the plant. This honeydew supports the growth of black sooty-mould fungi, wich often renders the effected plant unsightly and their flowers unsalable (the photosynthetic activity of the plant is hindered).
Further indirect damage is causes by their capacity to transmit viruses(carriers) from healthy to sick plants (the viruses can be transmitted even from infected plants that live outside the greenhouse).
From this point of view one of the most dangerous carriers is the aphid Mizus Persicae.
Control of Aphids
Aphids usually do not cause serious damages to the Gerbera, firstly because they are kept under control by other insecticidal treatments. Furthermore, their activity as carriers is also negligable because the plantlets that originate in the laboratory are unlikely to be infected by a virus (micropropagation).
In cases in which it is necessary to resort to chemicals (hot environments), we can use insecticides such as:
- Pirimicarb – 200 gr/100 l ( 17.5% c.a.).It acts via contact and asphyxia, with translaminar properties (*)
- Imidacloprid (Confidor) – 50-75 ml/100 l ( 17.8% c.a.).See control against Aleyrodidae (*)
- Methomyl – 150-200 ml/100 l ( 19% c.a.). Cytotropic insecticide, it acts via contact and ingestion. It is largely used also against Thrips, Aleurodids and Lepidopters. The latter also possess an ovicidal activity, and it is used for the preparation of poisoned bait (*)
(*) Product of prevalent adulticide activity
One or two treatments are generally enough to control the insect.
Source: p. 138 & 139 of “Gerbera Cultivation in Greenhouse” by Gerardo Mercurio
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