Control of Flea Beetles and Other Key Insect Pests of Leafy Salad Brassica Crops

Factsheet 22/10 Horticultural Stoneleigh Park Development Kenilworth Warwickshire CV8 2TL Field Vegetables Company T: 0247 669 2051 E: [email protected] Project No. FV 301 Control of flea beetles and other key insect pests of leafy salad Brassica crops John Colvin, NRI, University of Greenwich Flea beetles, diamondback moths, leafminers and weevils are key pests of speciality leafy salad Brassica crops. This factsheet provides brief descriptions of the pests, outlines their biology in Britain and provides guidance on their control. Action points strong adhesive in order to monitor • Mesh covers provide an extremely populations accurately. Traps must be important element of protection for • Learn to identify flea beetle species checked and replaced regularly and the crops. On their own, however, they (Figure 1) and other key pests. The flea pest numbers recorded. cannot deliver the crop quality de- beetle distinguishing feature is their manded by retailers and so it is neces- enlarged hind-leg femurs (the thickest • Flea beetle larvae cause substantial sary to provide additional protection in section of their legs) which gives them damage to the roots of young Brassica the form of insecticide treatments. great jumping ability. plants, which results in yield loss and plant death. Ensure that mesh covers • Products containing deltamethrin, • The composition of the insect- are placed over beds prior to crop bifenthrin and spinosad are highly ef- pest complex that attacks leafy salad germination and emergence. fective against flea beetles (see Table Brassicas can change markedly, year- 1, page 7). A coded product was also on-year. Learn to distinguish the char- • Crop damage early in the season is effective against weevils and diamond- acteristic types of damage caused by caused by migrant flea beetle adults back moth larvae and may become the different insect-pest species. that overwinter on other Brassica available in the future. crops, as well as on wild and volun- • Flea beetles can escape from some teer Brassicas. Old Brassica crops are • Flea beetle adults were detected sticky trap surfaces and so it is im- a source of flea beetle adults and so migrating into crops as early as the portant to use yellow traps with a must be removed. end of February and pest pressure was 1 Adult flea beetles - The striped flea beetle, Phyllotreta undulata (Kutschera) is on the left. One of the turnip flea beetles, Phyllotreta nigripes (Fabricius) is on the right. Note the characteristic enlarged hind-leg femurs. high by mid-March. Protective mesh • Be aware of potential technological • Seed treatments can be an effec- covers, therefore, need to be deployed advances. Insecticide-impregnated tive control strategy for a range of pest right from the beginning of the growing netting on its own, for example, was species, but trial data has shown that season. highly effective at preventing insect- damage due to some pests, such as pest damage, even with a mesh hole leafminer adults can actually increase • Holes and unsealed edges are the size through which the smaller flea with seed treatment use. main entry points for flea beetles into beetle species could theoretically the crop. It is vital to ensure a good pass. This technology works by a • The use of sacrificial lure-and-kill seal at the edges of the mesh during rapid knock-down effect and research crops may work, but they will require periods when the immigrant pest pres- to prolong the insecticidal efficacy of the availability of insecticidal sprays sure is high. the nets for longer than two seasons effective at killing diamondback moth is ongoing. adults rapidly. sa), Chinese cabbage (Brassica pekin- mesh; use of insecticide sprays and Introduction ensis) and pak choi (Brasica rapa var seed treatments as well as; a potential chinensis). For these businesses, it is lure-and-kill technique using a sacrifi- the leaves that are sold and so even cial crop. The project also demonstrat- Most flea beetle, Phyllotreta species low populations can cause sufficient ed the economic importance of other have a wide host-plant range which damage that reduces crop quality. The damaging pests, such as leafminers, includes many Cruciferous plants main damage caused by adult beetles, weevils and diamondback moths. such as cabbage (Brassica oleracea which appear in large numbers in the L.), cauliflower (Brassica oleracea var. spring, is the characteristic ‘shot- Some of the control techniques tested Botrytis), radish (Raphanus sativus L.), holes’ in the young leaves (Figure 2). worked well, but they currently have horseradish (Armoracia rusticana), tur- limitations that would prevent their nip (Brassica campestris L.), wild mus- This factsheet is based around re- wide scale adoption. In addition, sev- tard (Sinapis arvensis L.) and speciality search carried out in the HDC funded eral effective insecticides and potential leafy Brassica crops. The complex of project FV 301, which generated new new products were identified, which flea beetle species presents a par- information on the biology and move- could be of use to growers as they ticularly serious problem for speciality ment from overwintering sites of flea would reduce the risk of insecticide leafy Brassica growers who produce beetles; data on the use of mesh barri- resistance development. crops such as tatsoi (Brassica narino- ers including insecticide-impregnated Flea beetles Flea beetle species are recognised easily by the enlarged femurs which gives them their characteristic jumping ability. Flea beetle adults move into hedgerows or wooded areas in the autumn to overwinter. At this time of year, adult feeding rates are usually low and the risk of economic damage is consequently reduced. Adults were found to leave overwintering sites as early as March and begin to feed, mate and lay eggs in the soil near host plants. Larvae feed on root hairs and roots and, dependent on soil tempera- tures, develop into the pupal stage within 8-10 weeks. A new generation of adults then emerges from the pupae. 2 A young tatsoi leaf with holes and areas of leaf where the epidermis has been removed. As the leaf grows, the holes expand to create the ‘shot-holed’ appearance. It is this generation that is particularly damaging, because newly emerged flea beetles have a strong propensity beetle species, P. vittula, has also to feed. The peak pest pressure from Host plants been reported in Europe as damag- flea beetles, therefore, usually occurs ing monocotyledonous plants such as from mid-June to late July. maize and other cereals. A wide range of Cruciferous plants Experiments with emergence cages are acceptable hosts for flea beetle Most Brassica farming regions in showed that no emergence took place species and these include kale, cab- the UK have ‘green bridges’ present from ploughed farm fields, which led bage, sprouts, cauliflower, radish, throughout the year in the form of to the conclusion that pest pressure horseradish, turnip, mustard, as well winter crops such as cauliflower and arose mainly from immigrant flea bee- as the speciality leafy Brassicas. They oil seed rape, as well as large areas tle adults. also survive well on wild species such of grassland with wild and volunteer as field mustard (charlock) and black plants. In southern England particu- mustard (Brassica nigra). One flea larly, these Cruciferous species are present throughout the winter provid- gence cages situated in farm fields. hole’ appearance of leaves as they ing a refuge for overwintering adults. This pest pressure clearly arose from grow. Flea beetle larvae feed on plant surrounding hedgerows and other roots, an activity particularly damag- On the first warm spring days in both adult overwintering areas supporting ing to seedlings. When high numbers 2007 and 2008, flea beetle adults were wild and volunteer Brassicas. Adults of larvae are present in the soil, the collected after landing on the surface first attack the leaves of young plants damage they cause can be sufficiently of mesh-protected seedling Brassica and feed on the undersides of leaves serious to kill young seedlings. Some crops. The rate of immigrant arrival by peeling off the epidermis and sub- flea beetle species cause an additional was approximately 24-30 adults per sequently create larger holes. As a problem in that they can act as vectors hour, which would have been sufficient result, the leaves become sieve-like, for several plant pathogenic viruses to cause significant economic dam- which interferes both with photosyn- and P. striolata, for instance, has been age. No flea beetles, however, were thesis and the water balance of the reported to be able to transmit radish either seen or caught inside the emer- plant. The damage creates the ‘shot- mosaic virus. Diamondback moths In the previous 5-10 years, new insect pests have emerged as serious problems of UK Brassica crops, due mainly to insecticide resistance and warmer summers. The most damaging of these is the diamondback moth, which is now well established in southern England and is reported increasingly frequently throughout the UK. Adult diamondback moths can be recognised by the characteristic diamond pattern on their backs when their wings are folded, which has given the pest its name (Figure 3). The pest has four developmental stag- es (instars) with average development times of 4.5, 4, 4 and 5 days, respec- 3 A diamondback moth adult resting on a tatsoi leaf (left) - the caterpillar is on the right. tively. The caterpillars (Figure 3) are ac- tive and if disturbed, wriggle violently, move backwards and spin down from the plant on silk strands. One of the major difficulties with controlling this pest is that populations can become resistant to insecticides, thereby mak- ing insecticide sprays ineffective. Host plants This insect has a strong preference for Cruciferous species including broccoli, sprouts, cabbage, Chinese cabbage, cauliflower, collard, kale, kohlrabi, mustard, radish, turnip, and watercress.

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