Including lianas rooted within 2 m of the tree increased this percentage to 78%. Using both aboveand below-ground measures may provide a better estimate of liana competition than either measure alone. 6. Liana infestation is increasing on BCI. Lianas were present in the crowns of 73.6% of canopy trees (d.b.h. ‡20 cm). Liana canopy infestation was 57% higher than in 1980 and 65% higher than in 1967, which is consistent with reported increases in liana abundance, biomass, and leaf and flower production. 7. Synthesis. We used one of the largest studies ever conducted on lianas to confirm the negative effects of lianas on tree growth and survival over 10 years. Liana infestation of trees was widespread, dynamic and increasing on BCI.

Pathogens and parasites can induce changes in host or vector behavior that enhance their transmission. In plant systems, such effects are largely restricted to vectors, because they are mobile and may exhibit preferences dependent upon plant host infection status. Here we report the first evidence that acquisition of a plant virus directly alters host selection behavior by its insect vector. We show that the aphid Rhopalosiphum padi, after acquiring Barley yellow dwarf virus (BYDV) during in vitro feeding, prefers noninfected wheat plants, while noninfective aphids also fed in vitro prefer BYDV-infected plants. This behavioral change should promote pathogen spread since noninfective vector preference for infected plants will promote acquisition, while infective vector preference for noninfected hosts will promote transmission. We propose the “Vector Manipulation Hypothesis” to explain the evolution of strategies in plant pathogens to enhance their spread to new hosts. Our findings have implications for disease and vector management.

Vectors of several economically important plant viruses have been shown to feed or settle preferentially on either infected or noninfected host plants. Recent research has revealed that the feeding or settling preferences of insect vectors can depend on whether a vector is inoculative (carries the virus). To explore the implications of such changes in vector preference for the spread of the pathogen, we create a basic model of disease spread, incorporating vector preferences for infected and noninfected plants dependent on whether the vector is inoculative. Previous modeling work assumed that vector preferences remain unchanged with vector infection status and showed that vector preference for infected host plants promotes disease spread when infected hosts are rare, whereas preference for noninfected hosts promotes spread once infected hosts become abundant. In contrast, our model shows that a change in preference following acquisition of the pathogen can increase pathogen spread throughout the epidemic if noninoculative vectors prefer infected plants and inoculative vectors prefer noninfected plants, as has been detected experimentally in two pathosystems. Our results show that conditional vector preference can substantially influence plant pathogen spread, with implications for agricultural and natural systems. Conditional preference as a component of virus manipulation of vector behavior is potentially more important for the understanding of plant disease spread than previously recognized.

Threats to native forests from non-native insects and pathogens (pests) are generally addressed with methods such as quarantine, eradication, biological control, and development of resistant stock through hybridization and breeding. In conjunction with such efforts, it may be useful to have citizen scientists locate rare surviving trees that may be naturally pest resistant or tolerant. The degree of resistance of individual trees identified in this way can be tested under controlled conditions, and the most resistant individuals can be integrated into plant breeding programs aimed at developing pest-resistant native stock. Involving citizen scientists in programs aimed at identifying rare trees that survive colonization by pests provides a low-cost means of maximizing search efforts across wide geographic regions and may provide an effective supplement to existing management approaches.