Drepanothrips reuteri

Drepanothrips reuteri

The vine thrip (Drepanothrips reuteri Uzel, 1895) is an insect belonging to the Thripidae family.

Systematic –
From a systematic point of view it belongs to:
Eukaryota domain,
Kingdom Animalia,
Subkingdom Eumetazoa,
Bilateria branch,
Phylum Arthropoda,
Subphylum Hexapoda,
Class Insecta,
Subclass Pterygota,
Exopterygota cohort,
Subcohort Neoptera,
Superorder Paraneoptera,
Section Thysanopteroidea,
Order Thysanoptera,
Suborder Terebrantia,
Thripidae family,
Subfamily Thripinae,
Genus Drepanothrips,
Species D. reuteri.
The terms are synonymous:
– Drepanothrips viticola Mokrzecki, 1901;
– Thrips betulicola Reuter, 1901.

Geographic Distribution and Habitat –
Drepanothrips reuteri is a thrips widespread in North America, Europe and western Asia. In Italy it is present from North to South and in Sicily.
It is a polyphagous insect that can live on various tree and shrub species, but is particularly frequent on Vitis spp., where it attacks the vegetation in the early stages of development, causing deformations, growth delays, flower drop and berry suberosity. Among the various species of thrips hosted by cultivated vines (about ten), it is the only one capable of causing damage of a certain importance.
Other plants on which it lives are some broad-leaved forest trees and of landscape interest (hazel, willow, maple, oak).

Morphology –
Drepanothrips reuteri is a small insect, less than a millimeter in size, yellowish or greenish-yellow in colour, depending on the stages of development.
The eggs are about 0.2 mm long and hyaline white in colour
The first age nymph is 0.3 mm long and has a white color and red eyes.
The second age nymph is 0.8 mm long and is light yellow in colour.
The prenymph and nymph stages, before adulthood, have wing buds, non-functioning mouthparts and are almost inactive. They are also called subpupae, they measure approximately 0.8 mm and are light orange in color.
The insect in the adult stage, as mentioned, is less than mm in size; females measure 0.7 –0.85 mm in length and males 0.5-0.6 mm. The insect is yellow-brown in colour. The antennas are made up of 6 items and have black ends.
The head is wider than long; three pairs of ocellar setae are present; the male has two sickle-shaped appendages (drepanae) on the sides of the IX abdominal tergite which extend beyond the abdominal apex.

Aptitude and biological cycle –
Drepanothrips reuteri survives the winter thanks to the fertilized females, which overwinter inside the crevices of the plants, especially in wood that is at least 2 years old. In the period of April these migrate to the young shoots where they lay at the rate of one egg per day per individual, up to 60 eggs in total.
The eggs hatch within 6-10 days giving rise to nymphs which reach nymphal maturity in 5-9 days.
The nymphal stage occurs in the crevices of the wood or in the superficial layers of the soil; this stage is completed within 3-4 days.
The first adults then emerge in the period between May and June.
The overall duration of the cycle from egg to adult is 20 – 25 days, which varies depending on the climatic trend of the area. The adults are very mobile and move towards the young shoots, so they are rarely observed on the leaves when the symptoms are already evident.
The insect, also depending on the climatic conditions, can carry out 5 to 6 generations per year.
This thrips attacks the vegetation when the buds open, the period in which it causes the greatest damage; this is represented by the trophic punctures which cause a slowdown in the development of the shoots, at the base of which superficial and suberified necrotic notches often form. These notches are caused not only by feeding bites but also by injuries due to the ovipositor.
Furthermore, insect bites cause:
On young leaves: deformations that tend to curl;
On old leaves, affected during the summer: deformations and wrinkling of the blade; upon closer inspection, on the lower page, necrotic punctuations, corresponding to the punctures on the upper page; these are surrounded by a chlorotic halo.
On the grapes, punctures can cause notches or superficial streaks with reactions on the part of the epidermis which tends to suberify; the damage, however, is minor.

Ecological Role –
Drepanothrips reuteri is a thrip, whose trophic activity occurs mainly at the expense of leaves and shoots. These parts, when stung by the insect (both for feeding and for oviposition), slow down their development, subsequently presenting necrosis, drying and blistering of the leaf blade. The attack can also occur on the inflorescences: punctures to the flower peduncles can cause drying and premature fall.
Following the attacks, the plants present various symptoms; these are caused by the feeding bites of young and adults which cause the death of the affected cells and those nearby which stop dividing and growing. Brownish necrotic spots form on the leaves, visible on both pages, especially along the veins; subsequently, with the growth of the leaf, lacerations, wrinkling and sometimes slight marginal curling of the leaf edge (spoon leaf) are observed. These symptoms are more evident in case of drought or windy periods.
On shoots, petioles and rachises, the stings cause the formation of light areas with brownish streaks; Furthermore, there are delays in budding and deformations of the young shoots which tend to take on a zig-zag conformation, which is particularly evident if the climatic trend is unfavorable for optimal growth of the vine.
If the attacks are very intense, the bunches can also be damaged with consequent abortion of the flowers or fall of the young grapes. Nutrition punctures made on already formed berries cause the appearance of suberose spots.
We remember, however, that the symptoms can be confused with those of acariosis, in particular from Calepitrimerus vitis; however, thrips cause stickiness of the leaf blade, necrotic areas and zigzag growth of the shoots. Furthermore, they are found on all the buds of the shoot, while Calepitrimerus vitis prefers the basal buds. However, observation with the naked eye or with a lens allows you to easily distinguish thrips from mites.
The techniques to fight against this thrips must take into account various factors.
Until now, the fight was often carried out with synthetic products but in recent years it has been increasingly understood that at the basis of the balance of these insects, as well as others, it is necessary to think first of all about the design of the viticultural systems, their biodiversification, the techniques of grassing, mulching, presence of boundary hedges, etc.
These are agroecology techniques that allow infestations to be significantly limited to the point of lowering them even below the intervention threshold.
Furthermore, generally the development of thrips is limited by the predators naturally present in the vineyard. However, it can become problematic in the case of widespread use of insecticides that interfere with the growth of beneficial insect populations; for this reason it is not usually dangerous in organic farming.
Furthermore, it is useful to adopt monitoring techniques to identify the intervention threshold, where necessary.
The adults are easily observed, perhaps with the help of a thread counting lens, on the leaves and shoots, while the juvenile forms are more difficult to notice.
It is advisable to proceed with visual checks from when the first leaves are visible (phenological phase D-E according to Baggiolini), examining the shoots in search of females that have survived the winter. Later, as the foliage grows, controls become more difficult. An empirical method to evaluate the extent of the infestation is to use a black or silver sheet on which to shake the vegetation to identify the adults and juveniles.
Chemical control was carried out in the past especially in case of infestations exceeding 2-4 individuals during the development of the first leaves (phenological phase D-E according to Baggiolini); in this case an insecticide intervention was planned with spinosad or phosphoric esters. This often allowed the elimination of females before they laid their eggs, thus reducing the size of summer populations.
In pre-flowering an intervention could be useful if the populations were high and damage to the bunches was feared; the threshold is about ten individuals on the apical leaves. In this period, green pruning operations help reduce summer populations.
In recent years we have been increasingly moving towards biological control and agroecological cultivation techniques.
For biological control, take into account that Drepanothrips reuteri is preyed upon in its juvenile stages by phytoseiids such as Typholodromus pyri or by thysanoptera such as Aelothryps intermedius. In organic farming you can intervene with summer oils and products based on pyrethrum or azadirachtin. The most problematic period is the bud breaking phase, during which natural enemies are not yet present.

Guido Bissanti

Sources
– Wikipedia, the free encyclopedia.
– GBIF, the Global Biodiversity Information Facility.
– Russo G., 1976. Agricultural Entomology. Special Part. Liguori Editore, Naples.
– Pollini A., 2002. Manual of applied entomology. Edagricole, Bologna.
– Tremblay E., 1997. Applied entomology. Liguori Editore, Naples.

Photo source:
– https://www.uvadatavola.com/come-contenere-il-tripide-della-vite-drepanothrips-reuteri/