IPM OF VECTOR APHIDS Hajimu Takada Laboratory of Entomology, Faculty of Agriculture Kyoto Prefectural University, Kyoto, Japan ABSTRACT The following tactics are described for control of aphids as virus vectors; chemical con- trol (repellents, feeding deterrents, alarm pheromone, insecticides), physical control (mulch, fleece, netting) and biological control (insect parasitoids and predators). No tactic is able to protect plants completely against virus infections when used alone, except for certain fleeces. Multiple tactics must be used to build integrated pest management programs (IPM). INTRODUCTION Azadirachtin This paper reviews recent work on tactics This triterpenoid, isolated from the neem for controlling vector aphids to protect plants against tree (Azadirachta indica), reduced probing activity virus infection. I hope it will contribute to the by R. padi and Sitobion avenae on winter barley development of an effective strategy against the treated with concentrations of <500 ppm. The effect viruses, which cause serious damage to tropical lasted for at least four days after application (West crops. Such viruses include banana bunchy top virus and Mordue 1992). The reduction in probing activ- (BBTV) and papaya ringspot virus (PRSV). ity would diminish the probability of BYDV trans- mission by these aphids. CHEMICAL CONTROL Alarm Pheromone (-)-Polygodial Applying the alarm pheromone (E)-beta- This compound, isolated from water-pep- farnesene has not been effective, because it makes per (Polygonum hydropiper), is a sesquiterpenoid the aphids more active, and might increase transmis- which is highly repellent to aphids (Asakawa et al. sion of viruses. However, a spray application of 1988, Griffiths et al. 1989). Extraction of (-)- carbamate or organic phosphate, at one-tenth of the polygodial from the plant yielded material for a field recommended dose, in combination with the phero- trial to test the level of protection against barley mone, was successful in controlling aphids on ice- yellow dwarf virus (BYDV), transmitted by the berg lettuce (Ester et al. 1993). It thus seems to be aphid Rhopalosiphum padi. Under conditions of possible to diminish virus transmission by aphids by high pest and disease pressure, three two-weekly using an insecticide in combination with the phero- treatments of this compound at 50 g/ha increased the mone. yield by over 1 mt/ha. This is similar to the control Compounds derived from the alarm phero- achieved by the conventional broad-spectrum insec- mone can affect aphid settling and feeding, and ticide cypermethrin (Pickett et al. 1992). decrease virus spread both in the laboratory and in Keywords: Alarm pheromone, aphids, aphid parasitoid, feeding deterrent, fleece, IPM, mulch, repellent, virus vectors 1 the field (Dawson et al. 1988, Griffiths et al. 1989). systemic aphicides or sodium oleate, should be used. An alternative way of using alarm phero- mone is to disrupt effective alarm communication PHYSICAL CONTROL between aphids, by the presence of a high back- ground level of synthetic alarm pheromone. Under Mulch these conditions, predators may become more effec- tive in feeding on aphids. However, this hypothesis Kring (1964) first reported that short-wave- was not supported by the experimental results of E1- length radiation repelled aphids after a dispersal Agamy and Haynes (1992) when they worked with flight. Reflective (silver) mulch reduced aphid popu- the predator Navis americoferus. The potential role lations and virus incidence, and increased the yields of habituation or sensory adaptation is well worth of crops (Pinese et al. 1994). A new plastic mulch further investigation. with a reflective mirror-like surface, called “mirror mulch”, has recently been developed in Japan. When Mineral Oils mirror mulch was laid between rows in tobacco fields, populations of alate aphids in water traps were Certain mineral oils are known to reduce significantly lower, and a lower proportion of plants aphid colonization on plants, and thus the transmis- were diseased by potato virus Y-T strain (PVY-T), sion of virus disease (Simons and Zitter 1980). compared to treatments of silver mulch under the Vandenveken (1977) suggested two hypotheses for plants (Matsuzawa 1995). the effect of oils on aphid transmission of plant viruses. One is that oil might modify the charge of Fleece and Netting the stylets, thus impeding adsorption or elusion of virus particles. The second is that the inhibitory Various types of plant covers have been properties of oils might result from their electric suggested as a promising way of reducing virus insulating properties, which would hamper the ex- transmission by aphids. Even though the aphid change of charges between virus particles, aphid cannot pass through these covers, the question still mouthparts and plant cells. remains of whether penetration of leaves by the aphid’s stylets may occur if the leaves are in close Insecticides contact with the cover. One successful penetration per plant is sufficient for virus transmission by an The pyrethroid deltamethrin inhibits virus aphid. Field experiments with potato showed that transmission by the green peach aphid Myzus persicae one particular fleece (Lutrasil) completely protected by causing rapid knock-down and prolonged inca- the plants against virus transmission (PVY and pacitation of the insect (Gibson et al. 1982). In PLRV). The use of a second type of fleece laboratory tests with moderately pyrethroid-resis- (Agrifleece) and netting (Rantai, 19 mesh) resulted tant (R1) M. persicae, more of the aphids walked or in a small proportion of plants being infected flew from excised leaves pre-treated with deltamethrin (Harrewijn et al. 1991). A field of tobacco covered than from untreated leaves. R1 alatae and apterae with a fleece (possibly Lutrasil) for 50 days after which had dispersed from deltamethrin-treated leaves transplantation was fully protected against virus rarely transmitted potato virus Y (PVY). However, transmission (Tairako 1989). transmission by the more pyrethroid-resistant R2 alatae was only halved, although transmission by R2 BIOLOGICAL CONTROL apterae was undiminished. Treatment with deltamethrin decreased the spread of PVY both in a Insect Natural Enemies of Vector flight chamber, and in field experiments (Rice et al. Aphids 1983). Conventional insecticide treatments are not It is difficult to utilize insect natural en- necessarily effective in preventing the introduction emies of aphids for virus disease control. This is and subsequent field spread of non-persistent vi- particularly true when the pathogenic virus is trans- ruses. However, if naturally beneficial insects can- mitted in a non-persistent manner by several species, not suppress the population density of the vector as is the case with papaya ringspot virus (PRSV). aphid, the use of insecticides cannot be avoided. In Vector aphids of PRSV include the polyphagous such cases, insecticides with highly selective activity species Myzus persicae and Aphis gossypii, both of on the aphid and its natural enemies, such as certain which occur on many species of plants, not only in 2 the open field but also in scrub, orchards and gar- as mentioned above. L. scutellaris and other poten- dens. On the other hand, insect natural enemies can tial parasitoids may be useful. contribute to the control of a virus disease in cases where the pathogenic virus is transmitted in a semi- Predators of the Banana Aphid persistent or persistent manner by a particular aphid species. Banana bunchy-top virus (BBTV) is such a A total of about ten species belonging to case. This virus is transmitted in a semi-persistent Coccinellidae, Syrphidae, Chrysopidae and manner only by the banana aphid, Pentalonia Hemerobiidae have been recorded as predators of P. nigronervosa. nigronervosa (Tao 1990; Carver et al. 1993). Among these predators I think hemerobiids are the most Parasitoids of the Banana Aphid promising agents for the biological control of the aphid. This is because larvae of hemerobiids tend to Three programs of biological control of P. search and stay on concealed parts of the plant. nigronervosa have been attempted. In Western Small colonies of this aphid thus feed mainly on Samoa, two species of coccinellid predators were concealed parts such as the areas between the leaf used in a single program (Waterhause and Norris sheaths and the pseudostem of the banana plant. 1987), while in Tonga, two aphidiid parasitoids were used in succession. The first was Lysiphlebus Strategies for Vector Aphid Control testaceipes in 1987 (Stechmann and Voelkl 1988), and the second was Aphidius colemani in 1990-91 From the viewpoint of virus disease con- (Wellings et al. 1994). The first two programs have trol, there are two strategies for vector aphid con- produced no evidence that any of the introduced trol. agents have become established. The third program has not provided any evidence that the parasitoid is Direct Strategies: to prevent virus acqui- attacking P. nigronervosa in the field. However, A. sition and transmission by vector aphids colemani was recovered from Aphis gossypii on taro • To prevent vector aphids from landing about nine months after the last release. In August (repellents, mulches) 1992, the dissection of A. gossypii collected from • To prevent landing aphids from probing two taro sites revealed parasitism rates of 47% and (insecticides, feeding deterrents, fleece, 39%, respectively. P. nigronervosa collected from nets) banana growing on the margins of