Pest insects infesting lettuce crops Contents 5 Aphids 9 Caterpillars 14 Minor pest – thrips 15 Minor pest – wireworms 17 Minor pest – leatherjackets 19 Chemical methods of control 20 Whole crop IPM strategy The information in this booklet was compiled by: Rosemary Collier, University of Warwick David Norman, Fresh Produce Consultancy Ltd Photography: Potato aphid – Whitney Cranshaw, Colorado State University, Bugwood.org; thrips – Whitney Cranshaw, Colorado State University, Bugwood.org; wireworm – Iowa State University Extension and Outreach – Adam Sisson, and Fera Science Ltd. – Thrips, M.J. at German Wikipedia Introduction In lettuce crops, the scale of losses due to pests varies considerably depending on the crop and type of infestation, but whole crops may be rejected even if a relatively small proportion of the plants are infested with caterpillars or aphids. Virus transmission by aphids may also lead to considerable losses in yield. As they are cold-blooded organisms, the rate at which pest insects complete their life cycle is driven by the temperature of their surroundings and they will develop Rosemary Collier more rapidly when the weather is warm. University of Warwick This means they become active earlier following a warm spring and they are usually active earlier in the south of the UK than the north. In addition, weather conditions can have a considerable impact on the abundance of most pest insects; for example, heavy rainfall can lead to high levels of mortality in aphids and cutworms. There is no such thing as a ‘typical’ year when considering the complexity of pests that infest lettuce and their natural enemies. Overall, the life cycles of the various pests David Norman of lettuce are very different but there are Fresh Produce Consultancy Ltd periods when more than one species is active and so, consequently, control programmes should be planned to take this into account. 3 Key messages Lettuce crops can be infested by several transmission of plant viruses and species of pest insect, the principal pests contamination. This manual does not being aphids (Figure 1) and caterpillars. cover slugs; for slug control advice, visit Pests can reduce yield and crop quality ahdb.org.uk/projects/slugcontrol.aspx through direct feeding damage, Figure 1. Lettuce infested with currant-lettuce aphids • All pest species have natural enemies AHDB Pest Bulletin, AHDB Aphid that regulate their numbers to some News and other sources extent. Where feasible, selective • Adult males of the silver Y moth pesticides and/or methods of and turnip moth can be captured application should be used to protect in specific pheromone traps and beneficial species captures of male moths indicate • Be aware of the potential for when female moths are likely to be resistance to insecticides, particularly laying eggs in the peach-potato aphid and thrips • Some insecticide treatments have • To aid treatment timing, information activity against more than one pest on the timing of activity of most of species and programmes should be these pests can be obtained from the planned to take this into account 4 Aphids Life cycles eggs) on its summer hosts. Young aphids Lettuce crops are infested principally by are, therefore, genetically identical to their four species of aphid: currant-lettuce aphid mothers. In the autumn, winged individuals (Nasonovia ribisnigri, Figure 2), peach- fly to currant, males and females are potato aphid (Myzus persicae, Figure 3), produced, and males mate with the potato aphid (Macrosiphum euphorbiae, females which subsequently lay Figure 4) and lettuce-root aphid overwintering eggs. (Pemphigus bursarius, Figure 5). The first three species feed on the foliage, while the lettuce-root aphid feeds on the roots. The currant-lettuce aphid and lettuce-root aphid feed only on lettuce crops, while the other species have a wider host range. Peach-potato aphid and potato aphid In the UK, the peach-potato aphid and potato aphid overwinter as mobile aphids on a range of cultivated and wild host plants. In the spring, aphid development becomes more rapid and winged forms are produced that fly to new hosts, from which Figure 2. Currant-lettuce aphids (Nasonovia infestations of wingless aphids may ribisnigri) develop. Further winged forms are produced later in the summer and may infest new crops. These species reproduce without mating, producing live aphids rather than eggs, and the young aphids are genetically identical to their mother. The peach-potato aphid has a particularly wide range of hosts from different plant families (eg potato, sugar beet, lettuce, brassicas, pepper and a wide range of wild hosts) and, interestingly, lettuce is one of the less-preferred host species for peach- potato aphids. Currant-lettuce aphid Figure 3. Peach-potato aphid (Myzus persicae) The currant-lettuce aphid overwinters principally as a cold-resistant egg on Towards the south of the UK a proportion members of the currant family (Ribes spp.). of the population may overwinter as mobile The eggs hatch in early spring and the aphids on wild hosts. In the AHDB young aphids develop into adults which Horticulture-funded research project CP colonise the currant bush, leading to 067, currant-lettuce aphids survived the individuals that then disperse to colonise winter as nymphs and adults on wall lettuce and wild host plants. During the speedwell (Veronica arvensis) in the summer, this species reproduces without Midlands. Other weed species were suitable mating, producing live aphids (rather than hosts in the laboratory: Cichorium intybus 5 Aphids (chicory), Crepis capillaris (smooth available in the reports for AHDB hawksbeard), Lapsana communis (common Horticulture projects FV 407 and FV 407a. nipplewort), Hieracium aurantiacum (orange hawkweed), Hieracium pilosella (mouse-ear Damage hawkweed), Veronica spicata (spiked All species of aphid cause direct damage if speedwell) and Veronica officinalis sufficiently abundant, particularly the (common speedwell). In the spring, the lettuce-root aphid which may kill plants development of such overwintering aphids through desiccation if a sufficiently large becomes more rapid as temperatures rise part of the root system is damaged. Large and colonies produce winged forms that fly infestations of foliage-feeding aphids will to lettuce and other summer hosts. lead to distorted foliage. All foliage-feeding Lettuce-root aphid aphids are contaminants and produce honeydew that can support sooty moulds, The lettuce-root aphid has a similar life which also become a contaminant. cycle to the currant-lettuce aphid but overwinters on poplar, as a cold-resistant egg. The eggs hatch in early spring and the young aphids develop into adults which colonise the poplar tree, leading to female winged aphids that then disperse to colonise lettuce. While on lettuce during the summer, the female aphids reproduce without mating, producing live young. In the autumn, winged individuals fly to poplar, males and females are produced, males mate with the females and subsequently lay Figure 5. Lettuce-root aphids feeding on the root overwintering eggs. It is not known what system proportion of the population spend the Although the peach-potato aphid is likely summer on wild herbaceous hosts (wild to be very important for the transmission of relatives of lettuce) and whether some plant viruses, it is currently unclear which remain there to overwinter as mobile aphids. species of aphid (including species not Other species of aphid, such as the black named in this manual) and which viruses bean aphid (Aphis fabae) may also be found constitute the greatest threat to lettuce in lettuce crops from time to time. More crops in the UK (AHDB Horticulture project information on the black bean aphid is FV 427). Monitoring and forecasting Monitoring All four species of aphid are captured in the network of suction traps run by the Rothamsted Insect Survey (Figure 6). However, only peach-potato aphids and potato aphids are captured in relatively large numbers. Lettuce-root aphids cannot be easily distinguished from other species of Pemphigus and so are not recorded. Figure 4. Potato aphid (Macrosiphum euphorbiae) 6 Currant-lettuce aphids are only ever caught Currant-lettuce and lettuce root aphids in low numbers in both suction traps and Cold-blooded animals such as insects water traps and appear to be ‘trap-shy’; the require a certain amount of heat to develop best way to monitor infestations of currant- from one stage in their life cycle to another. lettuce aphids is to look at lettuce plants. The amount of heat required to complete a given stage of development is fixed and the combination of temperature (between upper and lower thresholds) and time will always be the same. This is often expressed in units called day degrees (D°). Day-degree forecasts have been developed for the two species of pest aphid that overwinter as eggs; currant- lettuce aphid and lettuce-root aphid. The forecasts for both species are based on accumulated D° from 1 February (lower threshold 4.4°C). The information was collected in projects funded by AHDB, Defra and LINK. Figure 6. Rothamsted suction trap at Wellesbourne In the study on the currant-lettuce aphid, the mean numbers of day degrees During the spring and summer, information accumulated until the first aphid was found on the capture of aphids by the suction and until peak numbers of aphids were traps is updated weekly, and bulletins are found were 507D° and 935D°, respectively. released on the Rothamsted Insect Survey Comparisons between observed and website every Friday. These contain predicted dates showed that this forecast information on captures for the week is likely to be accurate to within a 2–3 week ending on the previous Sunday. Bespoke period. monitoring services using yellow water In the study on the lettuce-root aphid, the traps are available. Regular crop walking is start of the migration of winged aphids important to identify developing problems. from poplar to lettuce occurred after 672D° Forecasts had been accumulated since 1 February.