Study On Olfactory Preferences Of Monophagous Agasicles Hygrophila To Host Plants

Abstract-Alligatorweed host plants, Alternanthera philoxeroides (Mart.) Griseb (Amaranthaceae), is originating from South America and now widely spread distribution in six continents of tropical and warm temperate wetland areas, except Antarctica.it is located in the riparian zone, lakes, marsh edge with the enough light, but in some forest edge and sparse shrub also distribution, it becomes a worldwide serious weed.Agasicles hygrophila Selman & Vogt (Coleoptera:Chrysomelidae, Aliticinae) is a highly specialized monophagous flea beetle and an effective biocontrol agent for the internationally notorious alligatorweed Alternanthera philoxeroides (Mart.) Griseb (Amaranthaceae). Olfactory cues can determine the host preferences of herbivorous insects. We show that the monophagous alligator weed flea beetle-an important biocontrol agent of invasive aligatorweed-uses the specific ratios of two common plant volatiles, D and Z, to discriminate between its host plant and non-host plants in nature. The results show:1.The host plant AP was significantly preferred by A. hygrophila over non-host plant species, while within the non-host plant species, there were two main categories of reduced preference. Alternanthera sessilis (AS) (the relatives of A. philoxeroides) was second most preferred to the host plant, while, interestingly, Beta vulgaris (BV), B. vulgaris variety cicla (BVV) and Cucumis sativus (CS) were significantly less preferred than AS. Overall, BP, AR, PO, LE, SM, VF were all roughly half as likely as the host plant AP to be beetles' first choice. The non-host plants CA and AM were least preferred as the rates of beetles' first choice were around 5% relative to the choice rate to host plant AP. Remarkably, we found that although the females prefer to feed the leaves of A. philoxeroides (feeding areas:59.50±4.62 mm2) they can consume non-host plants with the largest feeding areas of 46.50±3.69 mm2 on AS plants compared with that consumed on other non-host plants. Similarly, we found that mated females can lay eggs on the leaves of AS (12.33±1.20 per plant), but less than on AP leaves (194.00±10.58 per plant).Other plants were free from egg laying by females in the field. Moreover, the egg hatching rate on non-target AS is about 50% and the mortality of 2nd-instar larvae is about 17%.In Y-tube olfactometers, adult A. hygrophila females significantly preferred the odors of AP, AS, or BV plants over controls, whereas they did not prefer the odor of AM to the control.Single female beetles did not prefer the odor of mechanically damaged AP plants to the odor of intact AP plants, but they preferred the odor of AP plants damaged by conspecifics over the odor of intact AP plants.2.Volatile blends of the healthy, mechanically damaged and insect-fed treatments were significantly different. Host plant volatiles mainly include D?T?B?C? In the model of healthy plants,6 compounds had a VIP value higher than 1 and D, with a value of 1.39, was the most important compound for group separation. In the model of artificially damaged plants, the most important compound for separation was Z (1.25). The green leaf volatiles, such as hexanal,2-hexanol, (Z)-3-hexenyl acetate, and Z were dominant compounds in the artificially damaged non-host plants BV and AM, while they were not detected in the volatile blend of the host plant AP and non-host AS. In the model of insect-damaged host plants, D was released in the highest amount in the various treatments and had the highest VIP value of 1.56 for separation of groups. Absolute amounts of volatiles released from the insect fed plants also were much higher than those from intact or artificially damaged plants.3.The gas-chromatography (GC) coupled with electroantennographic detection (EAD) studies revealed that A. hygrophila female antennae responded strongly to the homoterpenes D and T in healthy plants (HP), artificially damaged plants (CLP), A. hygrophila-damaged plants (FP and LP) and to a mixture of the 6 green leaf volatiles with the greatest responses to (Z)-3-hexenyl acetate, (E)-3-hexenol and Z. Female A. hygrophila adults significantly preferred D over the hexane control at or above 20 ng. In contrast, Z was repellent at or above 10 ng. Therefore, among the compounds eliciting a behavioral response, D was the most attractive and Z the most repellent.4. The host plant AP was more attractive to A. hygrophila females than the non-host plant AM in the Y-tube olfactometer. However, when synthetic Z (40 ng/lO?L hexane) was added to the arm with AP odor, A. hygrophila females showed a significant preference for the other arm in which hexane was added to AP odor. For the non-host plant AM, synthetic D (40 ng/l0?L) addition increased the preferences of A. hygrophila versus the control plant odor with hexane.In two compound mixture consisting of D and Z, when D was treated as variable fractions, high proportion of D (D:Z= 99:1) was significantly preferred by adult female A. hygrophila over control. But a ratio of D:Z=1:1 becomes repellence for female beetles. When we switched Z as variable fraction, the mixtures are repellent to female beetles in all ratios tested.In field trials, the preferences of naive female A. hygrophila to intact AP with synthetic Z added at different dosages negatively correlated with selection of non-host plants by A. hygrophila females, especially AS, BV, BVV and CS plants. Conversely, the preferences of naive female A. hygrophila for non-host AM plants with synthetic D added at different dosages showed a gradually increased trend that also negatively associated with visitation of other non-host plants.We further observed the mated females'oviposition preference and hatching rate among 6 plant species when two plant volatiles are manipulated. With the increase of Z concentration on the leaves of A philoxeroides, the oviposition preference to A.sessilis is gradually raised, however, oviposition preference of A. philoxeroides is gradually reduced. The trend of hatchability is consistent with the oviposition preference.