The box tree caterpillar has come top of the list of gardeners' concerns for the third year in a row.
The Royal Horticultural Society (RHS) said it was the number one pest inquiry last year, as it continues to spread across the UK.
The alder leaf beetle, which feeds on the leaves of trees, entered the top 10 for only the second time.
The beetle has recently become re-established in some parts of England.
Honey fungus was the top concern in terms of diseases, while other types of fungi took advantage of the warm wet weather to attack fruit trees.
The RHS has analysed thousands of gardener enquiries for its latest tally of top 10 diseases and pests. The charity is conducting research into controls for the box tree caterpillar, focusing on the use of nematodes.
Meanwhile, a research project is under way to identify different slug species and what tempts them into gardens.
Matthew Cromey, principal scientist at the RHS, said the research will help increase biosecurity and provide best practice.
"Pests and diseases are among the main challenges we face as climate change affects our gardens and horticulture more widely," he said.
"We want to develop a nation of gardeners equipped and motivated to deal with the challenges of our changing world."
The RHS's top 10 pests for 2019 in terms of calls for advice were:
1. Box tree caterpillar
2. Vine weevil
4. Fuchsia gall mite
5. Alder leaf beetle
6. Woolly aphid
7= Rosy apple aphid
7= Viburnum beetle
9. Glasshouse red spider mite
10. Plum leaf-curling aphid.
The top 10 diseases for 2019 were:
1. Honey fungus
2. Phytophthora root rot
3. Box blight
4. Pear rust
5. Brown rot of fruit
6. Leaf spot and canker of Prunus
7. Apple and pear scab
8. Rose black spot
9= Blossom wilt of fruit trees
9= Powdery mildew of Prunus.