Many species of bug are strongly associated with either single plant species, or a small number of related species. Targeted searching of these plants, by beating or sweeping, can be a productive way of finding a range of species. This is not a definitive list of all the species that are likely to be found on each plant, there are a large number of generalist species which may be found on a wide range of trees and other plants. Beating of evergreen trees or shrubs in winter may produce many over-wintering species which are not-associated with the specific plant.
Trees
Alder: Blepharidopterus angulatus, Elasmucha grisea (Parent Bug), Kleidocerys privignus, Orthotylus flavinervis, Oxycarenus modestus, Pantilius tunicatus, Psallus ambiguus, Psallus salicis
Apple: Atractotomus mali, Campylomma verbasci, Physatocheila smreczynskii, Phytocoris dimidiatus, Phytocoris reuteri, Temnostethus pusillus
Ash: Brachynotocoris puncticornis, Campyloneura virgula, Orthotylus tenellus, Psallus flavellus, Psallus lepidus, Pseudoloxops coccineus, Temnostethus pusillus, Tropidosteptes pacificus
Aspen: Brachyarthrum limitatum, Orthotylus bilineatus
Birch: Elasmostethus interstinctus (Birch Shieldbug), Blepharidopterus angulatus, Elasmucha grisea (Parent Bug), Kleidocerys resedae, Neolygus contaminatus, Psallus betuleti, Psallus falleni, Psallus montanus
Box: Anthocoris butleri, Gonocerus acuteangulatus (Box Bug)
Creeping Willow: Globiceps fulvicollis, Pilophorus confusus
Cupressaceae (Cypresses, Junipers & Redwoods): Cyphostethus tristriatus (Juniper Shieldbug), Dichrooscytus gustavi (Juniper only), Orthotylus caprai, Orthotylus junipericola, Eremocoris fenestratus, Orsillus depressus, Zygimus nigriceps (Juniper only)
Elm: Anthocoris gallarum-ulmi, Orthotylus viridinervis, Psallodema fieberi
Field Maple: Psallus assimilis
Hawthorn: Acanthosoma haemorrhoidale (Hawthorn Shieldbug), Atractotomus mali, Deraeocoris olivaceus, Miris striatus, Physatocheila dumetorum/confinis
Hazel: Compsidolon salicellum, Malacocoris chlorizans, Pantilius tunicatus, Phylus coryli
Junipers: Cyphostethus tristriatus (Juniper Shieldbug), Dichroocytus gustavi, Zygimus nigriceps
Larch: Parapsallus vitellinus, Psallus luridus
Lime: Deraeocoris lutescens, Neolygus viridis, Orthotylus nassatus, Phytocoris tiliae, Pinalitus cervinus, Reuteria marqueti
London Plane: Arocatus longiceps, Belonochilus numenius, Corythuca ciliata
Norway Spruce: Atractotomus magnicornis, Gastrodes abietum, Parapsallus vitellinus, Pinalitus atomarius, Pinalitus rubricatus
Oak: Cyllecoris histrionius, Deraeocoris lutescens, Dryophilocoris flavoquadrimaculatus, Harpocera thoracica, Megacoelum infusum, Orthotylus tenellus, Phylus melanocephalus, Pilophorus perplexus, Psallus albicinctus, Psallus confusus, Psallus mollis, Psallus perrisi, Psallus quercus, Psallus variabilis, Psallus varians, Rhabdomiris striatellus
Poplars: Anthocoris minki, Neolygus populi, Orthotylus flavinervis, Phytocoris populi, Sthenarus rotermundi
Scots Pine: Alloetomus gothicus, Aradus cinnamomeus, Camptozygum aequale, Dichroocytus rufipennis, Gastrodes grossipes, Leptoglossus occidentalis, Megacoelum beckeri, Orthotylus fuscescens, Phoenicocoris obscurellus, Pilophorus cinnamopterus, Plesiodema pinetella
Sycamore: Corythuca ciliata, Psallus pseudoplatani, Physatocheila harwoodi
Tamarisk: Tuponia brevirostris, Tuponia hippophaes, Tuponia mixticolor
Turkey Oak: Psallus anaemicus, Psallus helenae, Psallus lucanicus, Psallus pardalis
Willows: Anthocoris limbatus, Closterotomus fulvomaculatus, Lygocoris rugicollis, Monosynamma bohemanni, Monosynamma maritimum, Orthotylus marginalis, Orthotylus virens (Salix pentandra), Pilophorus clavatus, Pilophorus confusus, Psallus haematodes
Willows (narrow-leaved species only): Agnocoris rubicundus, Blepharidopterus diaphanus, Campylomma annulicorne, Salicarus roseri
Other Plants
Bastard Toadflax: Canthophorus impressus (Down Shieldbug)
Black Horehound: Macrotylus horvathi, Raglius alboacuminatus, Tritomegas bicolor (Pied Shieldbug), Tritomegas sexmaculatus (Rambur's Pied Shieldbug)
Black Medick: Bathysolen nubilus
Broom: Anthocoris sarothamni, Asciodema obsoleta, Dictyonota fuliginosa, Dictyonota strichnocera, Heterogaster tibialis, Orthotylus adenocarpi, Orthotylus concolor, Orthotylus virescens, Piezodorus lituratus (Gorse Shieldbug)
Buck's-horn Plantain: Henestaris laticeps
Bugle: Tingis reticulata
Bugloss: Graptopeltus lynceus, Sehirus morio
Bulrush: Chilacis typhae
Cleavers: Polymerus nigrita
Climbing Corydalis: Aphanus rolandri
Common Cornsalad: Acompus pallipes
Common Cow-wheat: Adomerus biguttatus (Cow-wheat Shieldbug), Charagochilus weberi
Common Rock-rose: Macroplax preyssleri, Strongylocoris leucocephalus, Tinicephalus hortulanus
Common Storksbill: Arenocoris falleni, Arenocoris waltlii, Emblethis denticollis, Emblethis griseus, Megalonotus sabulicola, Odonotoscelis fuliginosa, Odonotoscelis lineola, Peritrechus lundii, Pionosomus varius
Creeping Thistle: Tingis ampliata
Cudweeds: Amblytylus delicatus
Dyer's Greenweed: Coptomsoma scutellatum, Heterogaster genistae
Ferns: Bryocoris pteridis, Monalocoris filicis
Field Scabious: Placochilus seladonicus
Forget-me-nots: Sehirus luctuosus
Foxglove: Dicyphus pallicornis, Horvathiolus superbus
Garlic Mustard: Eurydema oleracea
Gorse: Dictyonota strichnocera, Piezodorus lituratus (Gorse Shieldbug), Platycranus bicolor
Great Willowherb: Dicyphus tamaninii
Hairy Willowherb: Dicyphus epilobii
Heather (Calluna): Nabis ericetorum, Orthotylus ericetorum, Rhacognathus punctatus (Heather Shieldbug), Scolopostethus decoratus
Heather (Erica): Kleidocerys ericae, Nabis ericetorum, Nysius helveticus, Orthotylus ericetorum
Hedge Woundwort: Dicyphus constrictus, Dicyphus pallidus, Dicyphus stachydis, Eysarcoris venustissimus (Woundwort Shieldbug), Macrolophus pygmaeus, Macrotylus solitarius
Ivy: Derephysia foliacea
Knapweed: Oncotylus viridiflavus
Lady's Bedstraw: Charagochilus gyllenhalii, Geotomus petiti, Geotomus punctulatus, Legnotus limbosus, Legnosus picipes, Polymerus unifasciatus, Polymerus vulneratus
Large Bird's-foot Trefoil: Adelphocoris setipennis, Adelphocoris ticensis, Capsodes gothicus
Marsh Bedstraw: Polymerus palustris
Marsh Cinquefoil: Rhopalus maculatus
Marsh Valerian: Acompus rufipes
Mayweeds: Conostethus venustus
Mistletoe: Anthocoris visci, Hypsoloecus visci, Pinalitus viscicola
Mouse-ear Hawkweed: Hoplomachus thunbergii, Sciocoris cursitans (Sandrunner)
Mugwort: Europiella artemisiae
Mullein: Campylomma verbasci
Ox-eye Daisy: Calocoris roseomaculatus, Catoplatus fabricii, Trapezonotus ullrichi
Pieris: Stephanitis takeyai
Purple Small-reed: Capsus wagneri
Red Campion: Dicyphus globulifer
Restharrow: Dicyphus annulatus, Gampsocoris punctipes, Macrotylus paykullii
Rhododendron: Stephanitis rhododendri, Stephanitis takeyai, Tupiocoris rhododendri
Salicornia: Orthotylus rubidus
Sand Couch-grass: Trigonotylus psammaecolor
Sea Purslane: Henestaris halophilus, Orthotylus moncreaffi, Parapiesma quadratum
Sea Wormwood: Europiella decolor
Sheep's Sorrel: Spathocera dalmani
Snapdragon: Dicyphus escalerae
Spear Thistle: Tingis cardui
Stinging Nettle: Adelphocoris quadripunctatus, Apolygus spinolae, Calocoris affinis, Calocoris alpestris, Deraeocoris ruber, Heterogaster urticae, Liocoris tripustulatus, Heterotoma planicornis, Orthonotus rufifrons, Plagiognathus arbustorum
Tansy: Megacoleus tanaceti
Violets: Thyreocoris scarabaeoides (Scarab Shieldbug)
Viper's Bugloss: Dictyla echium, Graptopeltus lynceus
Water Forget-me-not: Dictyla convergens
White Campion: Dicyphus globulifer
Wild Thyme: Heterogaster artemisiae, Lasiacantha capucina
Yarrow: Megacoleus molliculus