Beetles on oilseed rape
Oilseed rape soon becomes boring for the coleopterist because it hosts only a very few beetles, most prolifically the Pollen beetle (Meligethes aeneus Fabricius, 1775) which is in any case abundant and will be found in numbers on any yellow flowers from early spring until well into the summer, but also a few weevils and leaf beetles. Beyond these there will be the usual ladybirds, because rape is commonly infested with various aphids, and a host of opportunistic species that visit flowers generally. And because rape crops generally host incredible numbers of insects, the ground below is often teeming with various carabids and staphs and other predaceous insects. But rape crops demonstrate much about the ecology of beetles, and on a scale that is easily appreciated, and so they should be studied for a season or two as there is much to be learned at all levels, more especially so as it is a very valuable crop, despite what some growers would have us believe, that has been very intensely studied for many decades and this is very likely to continue. Rape is the third largest source of vegetable oil in the world; over the period 2019/2020 global production was about 30 million tonnes, which is less than Soybean oil at about 50 million tonnes and much less than Palm oil, at about
Oilseed rape field - Hertfordshire, 4/2022
75 million tonnes, but this simply reflects the fact that tropical regions are more productive than subtropical and temperate regions. In context the European Union produces about 15 million tonnes of rape products and this is probably close to what is acceptable from the point of view of land usage because official estimates see only a small increase in production through the next decade; a vast increase in cultivation occurred early in the 21st century following the 2003 Biofuel Directive from the European Union promoting the use and production of biofuels but the situation has now more or less stabilized. Rape is also the second largest source of protein meal globally, after Soybean, and is used in a very wide variety of foods but primarily as animal feedstock’s or food additives. This popularity increased over the late 20th century as new cultivars were developed that had lower levels of certain unsavoury chemicals such as glucosinolates (which are pungent and highly oxidised sulphur compounds detrimental to animal health) and erucic acid (a bitter tasting monounsaturated omega-9 fatty acid with a bad reputation), and now modern varieties of rape have very low levels of both. Rape oil is widely used for both edible oil and as industrial fuels and lubricants and special varieties have been produced that are high in erucic acid to meet specific industrial applications. All of which is a bit academic but goes to show firstly how valuable the crops are, and secondly how much more valuable they are likely to become as we shift away from a global fossil fuel economy; the future looks good for electric cars and all the rest of it but the mass movement of goods by land, sea and air will very likely remain dependent upon the internal combustion engine and steam turbines, both of which will run either on hydrogen (if we ever get a catalyst or a very profitable production line) or, which in the short term is much more likely, on biofuels. The rape that turns fields across England bright yellow in the summer is a cultivar of the nominate subspecies of Brassica napus L. and it was one of the earliest plants to be widely cultivated by humans, it is now predominantly grown as winter rape; sown in the autumn it begins to grow and then remains dormant through the winter, begins growing again in the spring, flowers in spring and early summer and then develops pods which contain the prized oil and protein over the next two months. Spring rape is sown in the spring and develops rapidly but is more susceptible to pests, diseases and crop failure. All forms of rape are grown in rotation with other crops, often on a three or four year cycle in order to reduce the transmission of pests and diseases between crops, and this is important to appreciate because a huge amount of research has gone into the breeding of more resistant varieties and the ways various pests and diseases interact with the crop. Obviously we give the crops a hand; the levels of nutrients in the soil soon become depleted by the crop and so we apply high levels of nitrogen fertilizers (among others), which of course produces large amounts of N2O, we also apply chemicals and use traps to destroy various pests but this is an arms race that continues and one that we devote huge resources to winning, but over recent years the losses to pests has increased; this was no doubt helped by the banning in 1999 of the notorious gamma-HCH seed treatment but many more chemical treatments have been intensively studied and tried since that time. Some companies also try to produce rape cultivars that are resistant to any side effects produced by applications of various chemical they also produce. So it is very big business indeed.
For its part the plant also has a go at protecting itself from pests and diseases. Glucosinolates are secondary metabolites of several plant families of the order Brassicales Bromhead (a worldwide group of about 16 families that almost always have C3 carbon fixation) but in particular of the Brassicaceae Burnett, they typically contain complex mixtures of these chemicals and, using various enzymes (glucoside hydrolases often referred to as Myrosinase), are able to hydrolyse them into several groups of chemicals known commonly as thiocyanates, nitriles and isothiocyanates, these are readily react with amino (-NH2) and thiol (-SH) groups in proteins and peptides and so are variously toxic to many organisms that attack the plants. As would be expected various insects have evolved defences against these chemicals and many, including some flea beetles (Psylliodes and Phyllotreta) are strongly attracted to, and have become specialist feeders upon plants with high levels of glucosinolates, often being attracted to volatile isothiocyanates emitted by crops; this might seem counter-productive for the plants but these metabolites also attract high numbers of predatory and parasitic species from which the plants benefit. We have also tried to utilize this attraction by developing synthetic analogues that are powerfully attractive to pest species such as flea beetles and pollen beetles, these are used in various traps but yellow pan-traps laced with isothiocyanates (etc.) can each attract and kill many thousands of beetles in a single day; they are usually deployed when pest populations reach a certain threshold level and so can be economically beneficial, of course they also attract and kill many natural predators and parasites and so the results must be viewed in purely economic terms. But research continues to develop chemicals that will selectively trap and destroy the pests without otherwise harming the ecosystem; this is important as rape and its relatives are almost exclusively insect pollinated and so attract huge numbers of beneficial insects. This very brief insight is given only to show the enormous level of research and economic investment in this crop, much of which is carried out in the UK, and to suggest that the reader might do very well to delve deeper into the wealth of information available on line. Meligethes aeneus is ubiquitous on yellow flowers generally and will be found in huge numbers anywhere rape is grown, but there are a few other species that can be confidently expected to occur and these are typical of the crop, easily identified and so should contribute valuable material to the reference collection.
The flea beetle genus Psylliodes Latreille, 1829 includes about 200 species and is cosmopolitan in distribution, the host plants of about half are known and these belong to 24 plant families; most species (51%) are oligophagous while about 14 % are polyphagous and about 35% are monophagous, and of the total about 50% are specialist feeders of Brassicaceae. In the UK five species occur on various brassicas; the mostly coastal P. marcida (Illiger, 1807) occurs on Sea Rocket (Cakile maritima Scop., P. luridipennis Kutschera, 1864 occurs on Lundy Cabbage, Coincya wrightii Schulz, P. napi (Fabricius, 1792) is polyphagous on Brassicas, P. laticollis Kutschera, 1860 occurs mostly on water cress, Nasturtium officinale Aiton but also other Brassicas and the widespread and abundant P. chrysocephala (Linnaeus, 1758) is associated with a range of Brassicas but occurs commonly on rape and is among the most serious of the UK pest species. Other species, such as P. luteola (Muller, O.F., 1776), which more usually occur on different host plants may occasionally turn to rape and become minor pests. Adult chrysocephala feed on rape foliage and produce shot holes on the leaves, these can be extensive and leaves can become skeletonised in severe infestations but the real damage is done by the larvae which mine the stems; plants infested with larvae become stunted and lose vigour and in severe infestations local areas of plants die off completely. Chrysocephala larvae are pale creamy or white with a black head and numerous small dark spots along the body, fully grown they reach about 6 mm and they pupate within the stems. Because adults breed over a long season the larvae are present through the spring and into the summer; infested stems have darker and often swollen areas and breaking these open will soon reveal larvae which, in extreme cases may be found in numbers crammed into hollowed stems. Adults are strongly attracted to volatile isothiocyanates which, incidentally, have also been shown to stimulate feeding behaviour, and both adults and larvae have adapted to the plants glucosinolate-myrosinase defence system.
The other flea-beetle genus that has specialized in Brassica feeding is Phyllotreta Dejean, 1836; this includes more than 200 species and is also cosmopolitan although the majority occur in northern temperate regions. Fifteen species occur in the UK and all are polyphagous or oligophagous on various Brassicas although some also occur on other hosts, several species might be expected from rape but P. striolata (Fabricius, 1803), like Psylliodes chrysocephala, is interesting as it has been intensively studied and is known to be able to sequester glucosinolates but here they may be used in defence against predators. All our UK species might be expected from rape crops but among those commonly found in numbers are P. cruciferae (Goeze, 1777), the ‘cabbage flea-beetle’, P. nigripes (Fabricius, 1775), P. nemorum (Linnaeus, 1758) and, especially, P. undulata Kutschera, 1860. P. striolata is a notorious pest of rape in many parts of the world but is very local and rare in the UK.
Three further UK beetles are significant pests of oilseed rape; two are widespread and common and the larvae of both may cause significant damage to crops as they mine stems. The rape winter stem weevil, Ceutorhynchus picitarsis Gyllenhal, 1837, can be difficult to control as adults aestivate through the summer and migrate to young crops in the autumn, they feed young foliage before laying eggs on leaf petioles and the resulting larvae tunnel down through the stems and into the crown where they feed through the winter and leave to pupate in the soil during April or May, adults emerge soon after but migrate away from the crop. Often their presence goes unnoticed until the spring when areas of crops remain stunted or even die off in severe infestations. Oviposition continues through the winter, stopping only when the temperature drops to freezing, and so larvae develop and pupate until late in the spring and so plants that appear healthy in early spring may suddenly cease to develop and slowly die off. The Cabbage stem weevil, C. pallidactylus (Marsham, 1802) (often referred to by the older name quadridens) also overwinters in safe areas and migrates into the crops in the spring when the temperature reaches 12°C, they mate after a period of feeding and females lay eggs into leaf tissues. Larvae enter the stems and bore their way down to the ground where they will pupate in late spring and produce adults from June. Adults remain largely dormant through the summer and are unable to reproduce until the following spring, after they have experienced low winter temperatures. Both species may infest the same crop and adults may be found together. The Cabbage seed weevil, C. obstrictus (Marsham, 1802), is widespread and common in England and Wales but more local and scarce in Scotland and Ireland; here adults move into rape crops in the spring and feed on developing flowers and pods but this causes only minor damage, eggs are laid in seed pods and larvae develop within, feeding on the seeds and causing some damage but very high additional losses may result from Brassica pod midges which can lay eggs into feeding and oviposition scars caused by weevil adults and larvae. Other species e.g. the Turnip Gall Weevil, C. assimilis (Paykull, 1792) occasionally occur on rape but they are not generally considered pests, at least in the UK.
These few pest species should soon appear when sampling rape crops through the spring and summer; all might be found more generally as various similar Brassica plants are common in the wild but sampling the crops can be instructive. Many insects are attracted to the yellow flowers and so can be sampled using yellow pan traps, these often produce a wide variety of insects and often the odd unexpected beetle but they can be destructive (although this is the general intention when they are baited and used to control pests), on the other hand it is interesting that some species e.g. Psylliodes chrysocephala, are not attracted to the yellow colour, such species can be investigated by using traps baited with Brassica extracts, all that is needed here is to crush the foliage and add it to the trap or hang a sample above it. The advice that trapping can be destructive should be taken seriously as well-placed traps, and especially baited traps that collect specimens in a water/detergent mixture, can attract and kill many thousands of beetles in a single day; the majority will be pollen beetles but carefully going through such samples will reveal many other insects including predators and parasites. By late spring the crops can host large numbers of various aphids and these attract predators, mostly ladybirds and lacewings etc. among the stems and foliage but also many terrestrial beetles, among the carabids Notiophilus biguttatus (Fabricius, 1779) and various species of Bembidion Latreille, 1802 and Pterostichus Bonelli, 1810 are usually common but many others may be expected e.g. we have found Asaphidion stierlini (Heyden, 1880) regularly in this situation. The other group of predators that are usually numerous are the staphs, examples that should soon be found include species of Xantholinus Dejean, 1821, Philonthus Stephens, 1829 and Tachyporus Gravenhorst, 1802 as well as many species of Oxytelinae Fleming, 1821, Paederinae Fleming, 1821 and Staphylininae Latreille, 1802. Many of the terrestrial predators are active at night, many will climb stems in search of prey and so sweeping at this time can be productive but searching among litter or turning stones will reveal a steady stream of specimens at any time, pitfall trapping can be very productive but, again, can be very destructive and should only be employed if the traps can be visited regularly.