11.2.2. Plant mining and boring
A range of insect larvae reside within and feed on the internal tissues of living plants. Leaf-mining species live between the two epidermal layers of a leaf and their presence can be detected externally after the area that they have fed upon dies, often leaving a thin layer of dry epidermis. This leaf damage appears as tunnels, blotches, or blisters (Fig. 11.2). Tunnels may be straight (linear) to convoluted and often widen throughout their course (Fig. 11.2a), as a result of larval growth during development. Generally, larvae that live in the confined space between the upper and lower leaf epidermis are flattened. Their excretory material, frass, is left in the mine as black or brown pellets (Fig. 11.2a,b,c,e) or lines (Fig. 11.2f).
The leaf-mining habit has evolved independently in only four holometabolous orders of insects: the Diptera, Lepidoptera, Coleoptera, and Hymenoptera. The commonest types of leaf miners are larval flies and moths. Some of the most prominent leaf mines result from the larval feeding of agromyzid flies (Fig. 11.2a-d). Agromyzids are virtually ubiquitous; there are about 2500 species, all of which are exclusively phytophagous. Most are leaf miners, although some mine stems and a few occur in roots or flower heads. Some anthomyiids and a few other fly species also mine leaves. Lepidopteran leaf miners (Fig. 11.2e-g) mostly belong to the families Gracillariidae, Gelechiidae, Incurvariidae, Lyonetiidae, Nepticulidae, and Tisheriidae. The habits of leaf-mining moth larvae are diverse, with many variations in types of mines, methods of feeding, frass disposal, and larval morphology. Generally, the larvae are more specialized than those of other leaf-mining orders and are very dissimilar to their non-mining relatives. A number of moth species have habits that intergrade with gall inducing and leaf rolling. Leaf-mining Hymenoptera principally belong to the sawfly superfamily Tenthredinoidea, with most leaf-mining species forming blotch mines. Leaf-mining Coleoptera are represented by certain species of jewel beetles (Buprestidae), leaf beetles (Chrysomelidae), and weevils (Curculionoidea).
Leaf miners can cause economic damage by attacking the foliage of fruit trees, vegetables, ornamental plants, and forest trees. The spinach leaf miner (or mangold fly), Pegomya hyoscyami (Diptera: Anthomyiidae), causes commercial damage to the leaves of spinach and beet. The larvae of the birch leaf miner, Fenusa pusilla (Hymenoptera: Tenthredinidae), produce blotch mines in birch foliage in north-eastern North America, where this sawfly is considered a serious pest. In Australia, certain eucalypts are prone to the attacks of leaf miners, which can cause unsightly damage. The leaf blister sawflies (Hymenoptera: Pergidae: Phylacteophaga) tunnel in and blister the foliage of some species of Eucalyptus and related genera of Myrtaceae. The larvae of the jarrah leaf miner, Perthida glyphopa (Lepidoptera: Incurvariidae), feed in the leaves of jarrah, Eucalyptus marginata, causing blotch mines and then holes after the larvae have cut leaf discs for their pupal cases (Fig. 11.2g). Jarrah is an important timber tree in Western Australia and the feeding of these leaf miners can cause serious leaf damage in vast areas of eucalypt forest.
Mining sites are not restricted to leaves, and some insect taxa display a diversity of habits. For example, different species of Marmara (Lepidoptera: Gracillariidae) not only mine leaves but some burrow below the surface of stems, or in the joints of cacti, and a few even mine beneath the skin of fruit. One species that typically mines the cambium of twigs even extends its tunnels into leaves if conditions are crowded. Stem mining, or feeding in the superficial layer of twigs, branches, or tree trunks, can be distinguished from stem boring, in which the insect feeds deep in the plant tissues. Stem boring is just one form of plant boring, which includes a broad range of habits that can be subdivided according to the part of the plant eaten and whether the insects are feeding on living or dead and/or decaying plant tissues. The latter group of saprophytic insects is discussed in section 9.2 and is not dealt with further here. The former group includes larvae that feed in buds, fruits, nuts, seeds, roots, stalks, and wood. Stalk borers, such as the wheat stem sawflies (Hymenoptera: Cephidae: Cephus species) and the European corn borer (Lepidoptera: Pyralidae: Ostrinia nubilalis) (Fig. 11.3a), attack grasses and more succulent plants, whereas wood borers feed in the twigs, stems, and/or trunks of woody plants where they may eat the bark, phloem, sapwood, or heartwood. The wood-boring habit is typical of many Coleoptera, especially the larvae of jewel beetles (Buprestidae), longicorn (or longhorn) beetles (Cerambycidae), and weevils (Curculionoidea), and some Lepidoptera (e.g. Hepialidae and Cossidae; Fig. 1.3) and Hymenoptera.
The root-boring habit is well developed in the Lepidoptera, but many moth larvae do not differentiate between the wood of trunks, branches, or roots. Many species damage plant storage organs by boring into tubers, corms, and bulbs.
The reproductive output of many plants is reduced or destroyed by the feeding activities of larvae that bore into and eat the tissues of fruits, nuts, or seeds. Fruit borers include:
- Diptera (especially Tephritidae, such as the apple maggot, Rhagoletis pomonella, and the Mediterranean fruit fly, Ceratitis capitata);
- Lepidoptera (e.g. some tortricids, such as the oriental fruit moth, Grapholita molesta, and the codling moth, Cydia pomonella; Fig. 11.3b);
- Coleoptera (particularly certain weevils, such as the plum curculio, Conotrachelus nenuphar).
Weevil larvae also are common occupants of seeds and nuts and many species are pests of stored grain (section 11.2.5).
(a) linear-blotch mine of Agromyza aristata (Diptera: Agromyzidae) in leaf of an elm, Ulmus americana (Ulmaceae); (b) linear mine of Chromatomyia primulae (Agromyzidae) in leaf of a primula, Primula vulgaris (Primulaceae); (c) linear-blotch mine of Chromatomyia gentianella (Agromyzidae) in leaf of a gentian, Gentiana acaulis (Gentianaceae); (d) linear mine of Phytomyza senecionis (Agromyzidae) in leaf of a ragwort, Senecio nemorensis (Asteraceae); (e) blotch mines of the apple leaf miner, Lyonetia speculella (Lepidoptera: Lyonetiidae), in leaf of apple, Malus sp. (Rosaceae); (f ) linear mine of Phyllocnistis populiella (Lepidoptera: Gracillariidae) in leaf of poplar, Populus (Salicaceae); (g) blotch mines of jarrah leaf miner, Perthida glyphopa (Lepidoptera: Incurvariidae), in leaf of jarrah, Eucalyptus marginata (Myrtaceae). ((a, e-f ) After Frost 1959; (b-d) after Spencer 1990)
(a) larvae of the European corn borer, Ostrinia nubilalis (Lepidoptera: Pyralidae), tunneling in a corn stalk; (b) a larva of the codling moth, Cydia pomonella (Lepidoptera: Tortricidae), inside an apple. (After Frost 1959)
(After Cherikoff & Isaacs 1989)