Pests of Field Crops in Southern Africa

THRIPS

(Thysanoptera: mainly Thripidae)

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Text Box: Thrips are slender, torpedo-shaped insects of little more than 1or 2 mm in length. They have peculiar fringed wings, but while they are very mobile, they are not strong fliers. They are often found in abundance in flowers, or in the tender growing points, where they take shelter between petals or bracts. There are many different species of thrips, and a brief description of some of the more commonly found species of agricultural importance is given below. TOP Text Box: One of the most troublesome genera is Frankliniella of which two species are of importance. These are F. schultzei (the common blossom thrips) and F. occidentalis (the “western flower thrips”). The latter is a fairly recent introduction to southern Africa. They tend to be very small (about 1.5 mm long), and are yellow with greyish brown markings on their abdomens. Distinguishing between the two species is difficult without a microscope and even then depends on some very minute structures. Both species are widespread and common, and have been recorded on legume crops (such as groundnuts and peas), cotton, capsicums, cucurbits, deciduous fruits, granadillas and many flower species of various families, such as asters, chrysanthemums and roses. They also infest many weed species and flowering trees growing in the bush. Another commonly found thrips is Thrips tabaci, known as the onion thrips. It is a small and slender species, usually pale yellow in the sub-tropics, and tends to be found at the bases of onion leaves, or as a leaf feeder (rather than in the flowers) in other host species such as cotton (particularly seedlings) and vegetables. Fortunately, the serious introduced pest, Thrips palmi, has not yet been reported in Zimbabwe. The bean flower thrips, Megalurothrips sjostedti, is a rather larger species (about 2 mm long), coloured very dark brown or black, with the third antennal segment pale. It infests the flowers and flower buds of legumes, including Phaseolus spp., sunnhemp, cowpeas and groundnuts. The citrus thrips, Scirtothrips aurantii, is another small species (less than 1 mm long) and yellow to orange in colour. It is chiefly a pest of citrus species, but also of banana and cotton, and has been recorded on Acacia species. The tea thrips and also a common glasshouse species, Heliothrips haemorrhoidalis, is dark brown in the adult stage, with pale legs and strong “sculpturing” on the body, while the nymphal stages are white to pale green. Many other thrips species are common among crops, but may not be of economic importance. One of these is a species of Sericothrips, which can become very abundant on soyabean crops in hot, dry weather. The thorax is dark and the abdomen striped yellow and black. A very similar species can be found causing serious damage to leaves of marigold, a flower crop which has become grown commercially for the colouring agent extracted from its petals. A species of Retithrips, which is dark and also heavily sculptured, may be found on castor bean and the hedging plant, Jatropha, and also sometimes cotton. Other large dark species belonging to the suborder Tubulifera are commonly seen. These are very slender, have the last abdominal segment extended and tubular, and are often predatory species or fungus feeders. TOP Text Box: Both the nymphs and adults feed by puncturing and damaging the top layer of the plant tissue rather than piercing and sucking, or chewing. This produces scarring or flecking on young leaves, petals or developing fruits. If the damage is done early before the leaves or fruits have fully expanded, they may subsequently grow distorted and puckered. Often, the damage can be related back to a hot, dry spell, and there may be few or no thrips currently to be found. If the damage is done when the leaves or fruits are fully grown, the scarring takes the form of silvering or bronzing of the surface, as the cell contents are consumed and replaced by air. The damaged area is also usually speckled with little dark, oily-looking spots of excreta. (This sort of damage is seen particularly in marigolds where the affected leaves can lose almost all their colour, and also in onions and various ornamental plants.) In addition, thrip nymphal stages can pick up plant viruses, which are usually transmitted by the feeding of infected adults. One virus which has been a cause for concern particularly in flower and paprika production is the “tomato spotted wilt virus”, which is recorded to be transmitted by Frankliniella spp. and Thrips spp. A third form of damage is seen in garden peas, particularly the “mange tout” variety, damaged by Frankliniella spp. Here, the plant tissue reacts to the insertion of eggs by forming little raised bumps on the fruit. A pale blemish usually surrounds these, and blemished vegetables are unacceptable to sophisticated export markets. The tiny egg can usually be detected by microscope in the centre of the bump. In commercial flower and vegetable production, the mere presence of thrips in a consignment intended for export, even in very low and possibly “non-economic” numbers is a serious problem, as some thrips species have quarantine status. TOP Text Box: Minute, bean-shaped eggs are inserted into the plant tissue. Tiny pale yellow or pinkish first instar nymphs hatch from them and soon change into the next stage, which is similar in size but still wingless. Next are the prepupal and pupal stages, which show signs of wing development. They are mobile but do not feed and in many species, pupation may take place in the soil below the plants. Adults emerge from the pupae in a few days. They are winged and very mobile, but can only disperse long distances by wind. If the larval stage has picked up a virus, the adults are able to transmit it by feeding on plant tissues. Duration of the life cycle can be as little as two weeks in the hot, dry summer weather, but much longer in the cool weather. Studies of Frankliniella occidentalis in Zimbabwe indicated duration from egg to adult of 18 days at 25o C, and 13 days at 30o C. The pupal stages in the soil were 4.9 and 3.2 days respectively. Many species survive in the sub-tropics throughout the year, having 12 or more overlapping generations. In the above-mentioned studies, the winter months seemed to be passed as adult females, and numbers began to increase very rapidly with rising temperatures in the hot, dry months of August and September. Conveniently, these are also the months when “spring flowering” occurs so that there is an abundance of pollen available, which ingredient included in the diet favours prolific breeding. These are also the windiest months, so that the spread of the population into new vegetable and flower crops can be incredibly rapid. The thrips as a group exhibit some very peculiar reproductive behaviour. Many species exhibit arrhenotoky, where only males result from unfertilised eggs. Overwintering females, then, are able to give rise to a generation of males with which they can mate and produce normal fertilised eggs. The sometimes remarkable differences in appearance between the males and females of the same species make identification based on size and other features difficult. This is the case with the bean thrips, Megalurothrips sjostedti. TOP Text Box: A flare-up of thrips may be associated with very dry weather, as is the case occasionally in cotton seedlings, which tend to be planted early before the rains appear. A similar situation can result from unwise application of insecticides, which kills the target insect pest but allows thrips to build up and flourish in the absence of natural enemies. However, in recent years some species of thrips have become a serious recurring problem in agriculture. This is particularly the case with the “western flower thrips”, and the reasons for this are numerous. This species has a particularlyost plantsh rapid generation time (short life cycle); it has a huge host range, offering year-round availability of feeding and breeding sites; it is highly resistant to many chemical groups; there are pesticide application problems (the pest being hidden inside tight buds); soil-inhabiting stages and eggs are inaccessible; and greenhouses offer a very favourable environment (better survival through the winter months). In addition, many of the insecticides that were used in the past are now in the “severely restricted” category which gives exporters very little choice in their arsenal of insecticides, and as mentioned already, there are very low levels of tolerance for the presence of this pest in products destined for export. If the thrips species is not one that has exhibited resistance to insecticides, then control may not be a serious problem, if it is warranted at all. A number of commonly available insecticides may be used either as sprays, or as systemic seed dressings or soil-applied granules. With some crops, such as onions, thought must be given to the application method, because the leaves are fairly hydrophobic and the chemical may simply run off the plant. Overhead irrigation, if it is feasible, may go a long way towards solving a thrips problem, since thrips outbreaks are often associated with hot, dry weather. Likewise, plenty of moisture in the soil will be disadvantageous to the soil-inhabiting stages. When considering control of “western flower thrips”, one has to accept that insecticides alone will not control this species. A more holistic control strategy must be followed, depending on whether the situation is a flower crop within a greenhouse, or a vegetable crop in a field. Amongst the considerations are: Bear in mind other crop hosts – cotton, groundnuts, paprika, peas, beans, etc. If possible, plant the new crop or site the greenhouse at a distance upwind of these to limit immigration of the pest into it. It is advisable to detect thrips infestations as early as possible, so that control measures can be started before the problem gets out of hand. The use of blue or yellow sticky traps or strips near greenhouse openings and at flower level is advised. (Since many other insects are also attracted to yellow, sometimes blue is preferred, particularly if this method is to be used mainly for monitoring). Train crop scouts to look for thrips, and concentrate scouting prior to the danger period. Flowers, buds and growing points should be gently beaten over a white card or plastic sheet. The tiny thrips falling onto it are instantly visible and countable. Chemical control options include the use before flowering of soil-applied systemic insecticides, and the use of foliage-applied contact insecticides with added sugar to act as “bait” (based on scouting results). Once flowering has begun, the use of insecticides with translaminar or systemic action should be considered to assist with coverage problems. Rotation of chemical groups is essential to avoid or limit development of resistance. Amongst the groups are organophosphates (e.g. malathion, DDVP, dimethoate), carbamates (e.g. methiocarb, dicarzol, methomyl), inorganics such as sulphur, organochlorines (endosulfan), and some insecticides of newer chemistry (e.g. imidacloprid, spinosad). Application of contact insecticides to the soil and the mulching at the plant bases (rather than to the plants themselves) is possible to try to control prepupae, pupae and newly emerged adults. Control of flowering weed populations in the vicinity to limit alternate hosts. In commercial flower production, crop “hygiene” is important – e.g. unwanted flowers should be picked and removed from the greenhouse as open flowers are very attractive to thrips. Be aware that most thrips problems may be associated with an immigrant population from a nearby alternate host, such as flowering cotton or citrus. With short-lived insecticides, reinfestation from outside the greenhouse may be a recurring problem. A system that would limit the invasion of thrips into greenhouses would be ideal, such as “thrips netting”. A system referred to in much of the literature with regard to thrips control in extreme infestations is attempting to break the cycle in warm weather by the use of three sprays, three days apart. This can be three of the same insecticide, but thereafter, a new chemical group should be used. However, it should not be a continual situation. It is assumed that these three sprays knock the population down for some time, and only when it again reaches economic levels, should another group of three sprays be used. Thrips do have natural enemies, and in an unsprayed crop, these may play an important part in keeping the population at an acceptable level. It is many years since tiny blackish bugs of the genus Orius were noticed by the author in abundance in a study on soyabean insect populations, where there were also large numbers of Sericothrips sp. (It has been mentioned that the latter, although they can be numerous in hot, dry seasons, never seem to be of any economic importance in soyabeans.) Orius spp. (Hemiptera: Anthocoridae) are now known to be fairly effective predators of thrips. Like most of the natural enemy species, however, they are highly susceptible to insecticides, if only because their “hunting behaviour” exposes them to far more of the chemical than the hidden thrips. Other predators are some other species of thrips and some mite species, which may also become very abundant in a natural situation. In northern hemisphere closed greenhouses, considerable control is achieved by the artificial introduction of these predators. TOP

Identification

Identification

Host Plants

Damage

Life Cycle

Control

Host Plants

Damage

Life Cycle

Control

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Thrips life cycle.

Typical thrips "drawstring" damage, done while leaf was still small, in shoot.  In this case on a citrus leaf.

Thrips damaged areas of Marigold leaves with typical dark spots from excreta.

Western flower thrips. (Photo: Jack T Reed, Miss State Univ., Bugwood.org)

Scarring and flecking on groundnut caused by thrips.